<?xml version="1.0" encoding="us-ascii"?>
<!-- RRD ActiveDisclosure(SM) XBRL Schema Document - http://www.financial.rrd.com/ -->
<!-- Creation Date      : 2016-03-14T18:11:00.9804608+00:00 -->
<!-- Version            : 2.1.1.0 -->
<!-- Package ID         : 0F10B9350DD8DFAEBDCBAA9040FAFEA7-0001609809-001511 -->
<!-- Copyright (c) 2016 R. R. Donnelley & Sons Company. All Rights Reserved. -->
<schema xmlns:nonnum="http://www.xbrl.org/dtr/type/non-numeric" xmlns:num="http://www.xbrl.org/dtr/type/numeric" xmlns:us-types="http://fasb.org/us-types/2015-01-31" xmlns:stpr="http://xbrl.sec.gov/stpr/2011-01-31" xmlns:naics="http://xbrl.sec.gov/naics/2011-01-31" xmlns:mcrb="http://serestherapeutics.com/20151231" xmlns:sic="http://xbrl.sec.gov/sic/2011-01-31" xmlns:invest="http://xbrl.sec.gov/invest/2013-01-31" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xmlns:us-gaap="http://fasb.org/us-gaap/2015-01-31" xmlns:dei="http://xbrl.sec.gov/dei/2014-01-31" xmlns:xbrli="http://www.xbrl.org/2003/instance" xmlns:country="http://xbrl.sec.gov/country/2013-01-31" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:currency="http://xbrl.sec.gov/currency/2014-01-31" xmlns:exch="http://xbrl.sec.gov/exch/2015-01-31" attributeFormDefault="unqualified" elementFormDefault="qualified" targetNamespace="http://serestherapeutics.com/20151231" xmlns="http://www.w3.org/2001/XMLSchema">
  <import schemaLocation="http://www.xbrl.org/2003/xbrl-instance-2003-12-31.xsd" namespace="http://www.xbrl.org/2003/instance" />
  <import schemaLocation="http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd" namespace="http://www.xbrl.org/2003/linkbase" />
  <import schemaLocation="http://www.xbrl.org/2005/xbrldt-2005.xsd" namespace="http://xbrl.org/2005/xbrldt" />
  <import schemaLocation="http://www.xbrl.org/dtr/type/numeric-2009-12-16.xsd" namespace="http://www.xbrl.org/dtr/type/numeric" />
  <import schemaLocation="http://www.xbrl.org/dtr/type/nonNumeric-2009-12-16.xsd" namespace="http://www.xbrl.org/dtr/type/non-numeric" />
  <import schemaLocation="http://xbrl.fasb.org/us-gaap/2015/elts/us-types-2015-01-31.xsd" namespace="http://fasb.org/us-types/2015-01-31" />
  <import schemaLocation="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd" namespace="http://fasb.org/us-gaap/2015-01-31" />
  <import schemaLocation="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd" namespace="http://xbrl.sec.gov/dei/2014-01-31" />
  <import schemaLocation="http://xbrl.sec.gov/country/2013/country-2013-01-31.xsd" namespace="http://xbrl.sec.gov/country/2013-01-31" />
  <import schemaLocation="http://xbrl.sec.gov/currency/2014/currency-2014-01-31.xsd" namespace="http://xbrl.sec.gov/currency/2014-01-31" />
  <import schemaLocation="http://xbrl.sec.gov/exch/2015/exch-2015-01-31.xsd" namespace="http://xbrl.sec.gov/exch/2015-01-31" />
  <import schemaLocation="http://xbrl.sec.gov/invest/2013/invest-2013-01-31.xsd" namespace="http://xbrl.sec.gov/invest/2013-01-31" />
  <import schemaLocation="http://xbrl.sec.gov/naics/2011/naics-2011-01-31.xsd" namespace="http://xbrl.sec.gov/naics/2011-01-31" />
  <import schemaLocation="http://xbrl.sec.gov/sic/2011/sic-2011-01-31.xsd" namespace="http://xbrl.sec.gov/sic/2011-01-31" />
  <import schemaLocation="http://xbrl.sec.gov/stpr/2011/stpr-2011-01-31.xsd" namespace="http://xbrl.sec.gov/stpr/2011-01-31" />
  <annotation>
    <appinfo>
      <link:roleType id="TemplateLink" roleURI="http://serestherapeutics.com/20151231/role/TemplateLink" xmlns:link="http://www.xbrl.org/2003/linkbase">
        <link:definition>00000 - Document - Template Link</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:linkbaseRef xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="mcrb-20151231_cal.xml" xlink:role="http://www.xbrl.org/2003/role/calculationLinkbaseRef" xlink:title="Calculation Links, all" xlink:type="simple" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" />
      <link:linkbaseRef xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="mcrb-20151231_def.xml" xlink:role="http://www.xbrl.org/2003/role/definitionLinkbaseRef" xlink:title="Definition Links, all" xlink:type="simple" />
      <link:linkbaseRef xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="mcrb-20151231_lab.xml" xlink:role="http://www.xbrl.org/2003/role/labelLinkbaseRef" xlink:title="Label Links, all" xlink:type="simple" />
      <link:linkbaseRef xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="mcrb-20151231_pre.xml" xlink:role="http://www.xbrl.org/2003/role/presentationLinkbaseRef" xlink:title="Presentation Links, all" xlink:type="simple" />
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DocumentDocumentAndEntityInformation" id="Role_DocumentDocumentAndEntityInformation">
        <link:definition>100000 - Document - Document and Entity Information</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/StatementConsolidatedBalanceSheets" id="Role_StatementConsolidatedBalanceSheets">
        <link:definition>100010 - Statement - Consolidated Balance Sheets</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/StatementConsolidatedBalanceSheetsParenthetical" id="Role_StatementConsolidatedBalanceSheetsParenthetical">
        <link:definition>100020 - Statement - Consolidated Balance Sheets (Parenthetical)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/StatementConsolidatedStatementsOfOperationsAndComprehensiveLoss" id="Role_StatementConsolidatedStatementsOfOperationsAndComprehensiveLoss">
        <link:definition>100030 - Statement - Consolidated Statements of Operations and Comprehensive Loss</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/StatementConsolidatedStatementsOfOperationsAndComprehensiveLossParenthetical" id="Role_StatementConsolidatedStatementsOfOperationsAndComprehensiveLossParenthetical">
        <link:definition>100040 - Statement - Consolidated Statements of Operations and Comprehensive Loss (Parenthetical)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/StatementConsolidatedStatementsOfConvertiblePreferredStockAndStockholdersEquityDeficit" id="Role_StatementConsolidatedStatementsOfConvertiblePreferredStockAndStockholdersEquityDeficit">
        <link:definition>100050 - Statement - Consolidated Statements of Convertible Preferred Stock and Stockholders&apos; Equity (Deficit)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/StatementConsolidatedStatementsOfConvertiblePreferredStockAndStockholdersEquityDeficitParenthetical" id="Role_StatementConsolidatedStatementsOfConvertiblePreferredStockAndStockholdersEquityDeficitParenthetical">
        <link:definition>100060 - Statement - Consolidated Statements of Convertible Preferred Stock and Stockholders&apos; Equity (Deficit) (Parenthetical)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/StatementConsolidatedStatementsOfCashFlows" id="Role_StatementConsolidatedStatementsOfCashFlows">
        <link:definition>100070 - Statement - Consolidated Statements of Cash Flows</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureNatureOfBusinessAndBasisOfPresentation" id="Role_DisclosureNatureOfBusinessAndBasisOfPresentation">
        <link:definition>100080 - Disclosure - Nature of the Business and Basis of Presentation</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPolicies" id="Role_DisclosureSummaryOfSignificantAccountingPolicies">
        <link:definition>100090 - Disclosure - Summary of Significant Accounting Policies</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureFairValueOfFinancialAssetsAndLiabilities" id="Role_DisclosureFairValueOfFinancialAssetsAndLiabilities">
        <link:definition>100100 - Disclosure - Fair Value of Financial Assets and Liabilities</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureInvestments" id="Role_DisclosureInvestments">
        <link:definition>100110 - Disclosure - Investments</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosurePropertyAndEquipmentNet" id="Role_DisclosurePropertyAndEquipmentNet">
        <link:definition>100120 - Disclosure - Property and Equipment, Net</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureAccruedExpensesAndOtherCurrentLiabilities" id="Role_DisclosureAccruedExpensesAndOtherCurrentLiabilities">
        <link:definition>100130 - Disclosure - Accrued Expenses and Other Current Liabilities</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureNotesPayable" id="Role_DisclosureNotesPayable">
        <link:definition>100140 - Disclosure - Notes Payable</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosurePreferredStockWarrantLiability" id="Role_DisclosurePreferredStockWarrantLiability">
        <link:definition>100150 - Disclosure - Preferred Stock Warrant Liability</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureConvertiblePreferredStock" id="Role_DisclosureConvertiblePreferredStock">
        <link:definition>100160 - Disclosure - Convertible Preferred Stock</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureStockholdersEquityDeficitCommonStock" id="Role_DisclosureStockholdersEquityDeficitCommonStock">
        <link:definition>100170 - Disclosure - Stockholders&#8217; Equity (Deficit) Common Stock</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureNetLossPerShare" id="Role_DisclosureNetLossPerShare">
        <link:definition>100180 - Disclosure -  Net Loss per Share</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureCommitmentsAndContingencies" id="Role_DisclosureCommitmentsAndContingencies">
        <link:definition>100190 - Disclosure - Commitments and Contingencies</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureIncomeTaxes" id="Role_DisclosureIncomeTaxes">
        <link:definition>100200 - Disclosure - Income Taxes</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureSelectedQuarterlyFinancialDataUnaudited" id="Role_DisclosureSelectedQuarterlyFinancialDataUnaudited">
        <link:definition>100210 - Disclosure - Selected Quarterly Financial Data (Unaudited)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureRelatedPartyTransactions" id="Role_DisclosureRelatedPartyTransactions">
        <link:definition>100220 - Disclosure - Related Party Transactions</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/Disclosure401KSavingsPlan" id="Role_Disclosure401KSavingsPlan">
        <link:definition>100230 - Disclosure - 401(k) Savings Plan</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureSubsequentEvents" id="Role_DisclosureSubsequentEvents">
        <link:definition>100240 - Disclosure - Subsequent Events</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesPolicies" id="Role_DisclosureSummaryOfSignificantAccountingPoliciesPolicies">
        <link:definition>100250 - Disclosure - Summary of Significant Accounting Policies (Policies)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureFairValueOfFinancialAssetsAndLiabilitiesTables" id="Role_DisclosureFairValueOfFinancialAssetsAndLiabilitiesTables">
        <link:definition>100260 - Disclosure - Fair Value of Financial Assets and Liabilities (Tables)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureInvestmentsTables" id="Role_DisclosureInvestmentsTables">
        <link:definition>100270 - Disclosure - Investments (Tables)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosurePropertyAndEquipmentNetTables" id="Role_DisclosurePropertyAndEquipmentNetTables">
        <link:definition>100280 - Disclosure - Property and Equipment, Net (Tables)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureAccruedExpensesAndOtherCurrentLiabilitiesTables" id="Role_DisclosureAccruedExpensesAndOtherCurrentLiabilitiesTables">
        <link:definition>100290 - Disclosure - Accrued Expenses and Other Current Liabilities (Tables)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosurePreferredStockWarrantLiabilityTables" id="Role_DisclosurePreferredStockWarrantLiabilityTables">
        <link:definition>100300 - Disclosure - Preferred Stock Warrant Liability (Tables)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureStockholdersEquityDeficitCommonStockTables" id="Role_DisclosureStockholdersEquityDeficitCommonStockTables">
        <link:definition>100310 - Disclosure - Stockholders&#8217; Equity (Deficit) Common Stock (Tables)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureNetLossPerShareTables" id="Role_DisclosureNetLossPerShareTables">
        <link:definition>100320 - Disclosure -  Net Loss per Share (Tables)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureCommitmentsAndContingenciesTables" id="Role_DisclosureCommitmentsAndContingenciesTables">
        <link:definition>100330 - Disclosure - Commitments and Contingencies (Tables)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureIncomeTaxesTables" id="Role_DisclosureIncomeTaxesTables">
        <link:definition>100340 - Disclosure - Income Taxes (Tables)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureSelectedQuarterlyFinancialDataUnauditedTables" id="Role_DisclosureSelectedQuarterlyFinancialDataUnauditedTables">
        <link:definition>100350 - Disclosure - Selected Quarterly Financial Data (Unaudited) (Tables)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureNatureOfBusinessAndBasisOfPresentationAdditionalInformationDetail" id="Role_DisclosureNatureOfBusinessAndBasisOfPresentationAdditionalInformationDetail">
        <link:definition>100360 - Disclosure - Nature of the Business and Basis of Presentation - Additional Information (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesAdditionalInformationDetail" id="Role_DisclosureSummaryOfSignificantAccountingPoliciesAdditionalInformationDetail">
        <link:definition>100370 - Disclosure - Summary of Significant Accounting Policies - Additional Information (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureFairValueOfFinancialAssetsAndLiabilitySummaryOfAssetsAndLiabilitiesMeasuredAtFairValueOnRecurringBasisDetail" id="Role_DisclosureFairValueOfFinancialAssetsAndLiabilitySummaryOfAssetsAndLiabilitiesMeasuredAtFairValueOnRecurringBasisDetail">
        <link:definition>100380 - Disclosure - Fair Value of Financial Assets and Liability - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureFairValueOfFinancialAssetsAndLiabilitiesAdditionalInformationDetail" id="Role_DisclosureFairValueOfFinancialAssetsAndLiabilitiesAdditionalInformationDetail">
        <link:definition>100390 - Disclosure - Fair Value of Financial Assets and Liabilities - Additional Information (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureInvestmentsScheduleOfFairValueOfAvailableForSaleInvestmentsByTypeOfSecurityDetail" id="Role_DisclosureInvestmentsScheduleOfFairValueOfAvailableForSaleInvestmentsByTypeOfSecurityDetail">
        <link:definition>100400 - Disclosure - Investments - Schedule of Fair Value of Available-for-Sale Investments by Type of Security (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureInvestmentsAdditionalInformationDetail" id="Role_DisclosureInvestmentsAdditionalInformationDetail">
        <link:definition>100410 - Disclosure - Investments - Additional Information (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosurePropertyAndEquipmentNetScheduleOfPropertyAndEquipmentNetDetail" id="Role_DisclosurePropertyAndEquipmentNetScheduleOfPropertyAndEquipmentNetDetail">
        <link:definition>100420 - Disclosure - Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosurePropertyAndEquipmentNetAdditionalInformationDetail" id="Role_DisclosurePropertyAndEquipmentNetAdditionalInformationDetail">
        <link:definition>100430 - Disclosure - Property and Equipment, Net - Additional Information (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureAccruedExpensesAndOtherCurrentLiabilitiesSummaryOfAccruedExpensesAndOtherCurrentLiabilitiesDetail" id="Role_DisclosureAccruedExpensesAndOtherCurrentLiabilitiesSummaryOfAccruedExpensesAndOtherCurrentLiabilitiesDetail">
        <link:definition>100440 - Disclosure - Accrued Expenses and Other Current Liabilities - Summary of Accrued Expenses and Other Current Liabilities (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureNotesPayableAdditionalInformationDetail" id="Role_DisclosureNotesPayableAdditionalInformationDetail">
        <link:definition>100450 - Disclosure - Notes Payable - Additional Information (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosurePreferredStockWarrantLiabilityAdditionalInformationDetail" id="Role_DisclosurePreferredStockWarrantLiabilityAdditionalInformationDetail">
        <link:definition>100460 - Disclosure - Preferred Stock Warrant Liability - Additional Information (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosurePreferredStockWarrantLiabilitySummaryOfAssumptionsAndInputsUsedInDeterminingFairValueOfPreferredStockWarrantLiabilityValuedUsingBlackScholesOptionPricingModelDetail" id="Role_DisclosurePreferredStockWarrantLiabilitySummaryOfAssumptionsAndInputsUsedInDeterminingFairValueOfPreferredStockWarrantLiabilityValuedUsingBlackScholesOptionPricingModelDetail">
        <link:definition>100470 - Disclosure - Preferred Stock Warrant Liability - Summary of Assumptions and Inputs Used in Determining Fair Value of Preferred Stock Warrant Liability Valued Using Black-Scholes Option-Pricing Model (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosurePreferredStockWarrantLiabilitySummaryOfRollforwardOfFairValueOfPreferredStockWarrantLiabilityDetail" id="Role_DisclosurePreferredStockWarrantLiabilitySummaryOfRollforwardOfFairValueOfPreferredStockWarrantLiabilityDetail">
        <link:definition>100480 - Disclosure - Preferred Stock Warrant Liability - Summary of Rollforward of Fair Value of Preferred Stock Warrant Liability (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureConvertiblePreferredStockAdditionalInformationDetail" id="Role_DisclosureConvertiblePreferredStockAdditionalInformationDetail">
        <link:definition>100490 - Disclosure - Convertible Preferred Stock - Additional Information (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureStockholdersEquityDeficitCommonStockAdditionalInformationDetail" id="Role_DisclosureStockholdersEquityDeficitCommonStockAdditionalInformationDetail">
        <link:definition>100500 - Disclosure - Stockholders&apos; Equity (Deficit) Common Stock - Additional Information (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureStockholdersEquityDeficitCommonStockCommonStockWarrantsAdditionalInformationDetail" id="Role_DisclosureStockholdersEquityDeficitCommonStockCommonStockWarrantsAdditionalInformationDetail">
        <link:definition>100510 - Disclosure - Stockholders&apos; Equity (Deficit) Common Stock - Common Stock Warrants - Additional Information (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureStockholdersEquityDeficitCommonStock2012StockIncentivePlanAdditionalInformationDetail" id="Role_DisclosureStockholdersEquityDeficitCommonStock2012StockIncentivePlanAdditionalInformationDetail">
        <link:definition>100520 - Disclosure - Stockholders&apos; Equity (Deficit) Common Stock - 2012 Stock Incentive plan - Additional Information (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureStockholdersEquityDeficitCommonStock2015IncentiveAwardPlanAdditionalInformationDetail" id="Role_DisclosureStockholdersEquityDeficitCommonStock2015IncentiveAwardPlanAdditionalInformationDetail">
        <link:definition>100530 - Disclosure - Stockholders&apos; Equity (Deficit) Common Stock - 2015 Incentive Award Plan - Additional Information (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureStockholdersEquityDeficitCommonStock2015EmployeeStockPurchasePlanAdditionalInformationDetail" id="Role_DisclosureStockholdersEquityDeficitCommonStock2015EmployeeStockPurchasePlanAdditionalInformationDetail">
        <link:definition>100540 - Disclosure - Stockholders&apos; Equity (Deficit) Common Stock - 2015 Employee Stock Purchase Plan - Additional Information (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureStockholdersEquityDeficitCommonStockSummaryOfEstimatedFairValueOfEmployeeStockOptionsDetail" id="Role_DisclosureStockholdersEquityDeficitCommonStockSummaryOfEstimatedFairValueOfEmployeeStockOptionsDetail">
        <link:definition>100550 - Disclosure - Stockholders&apos; Equity (Deficit) Common Stock - Summary of Estimated Fair Value of Employee Stock Options (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureStockholdersEquityDeficitCommonStockSummaryOfStockOptionActivityDetail" id="Role_DisclosureStockholdersEquityDeficitCommonStockSummaryOfStockOptionActivityDetail">
        <link:definition>100560 - Disclosure - Stockholders&apos; Equity (Deficit) Common Stock - Summary of Stock Option Activity (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureStockholdersEquityDeficitCommonStockStockOptionsAdditionalInformationDetail" id="Role_DisclosureStockholdersEquityDeficitCommonStockStockOptionsAdditionalInformationDetail">
        <link:definition>100570 - Disclosure - Stockholders&apos; Equity (Deficit) Common Stock - Stock Options - Additional Information (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureStockholdersEquityDeficitCommonStockSummaryOfRestrictedStockActivityDetail" id="Role_DisclosureStockholdersEquityDeficitCommonStockSummaryOfRestrictedStockActivityDetail">
        <link:definition>100580 - Disclosure - Stockholders&apos; Equity (Deficit) Common Stock - Summary of Restricted Stock Activity (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureStockholdersEquityDeficitCommonStockRestrictedCommonStockAdditionalInformationDetail" id="Role_DisclosureStockholdersEquityDeficitCommonStockRestrictedCommonStockAdditionalInformationDetail">
        <link:definition>100590 - Disclosure - Stockholders&apos; Equity (Deficit) Common Stock - Restricted Common Stock - Additional Information (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureStockholdersEquityDeficitCommonStockSummaryOfStockBasedCompensationExpenseDetail" id="Role_DisclosureStockholdersEquityDeficitCommonStockSummaryOfStockBasedCompensationExpenseDetail">
        <link:definition>100600 - Disclosure - Stockholders&apos; Equity (Deficit) Common Stock - Summary of Stock Based Compensation Expense (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureStockholdersEquityDeficitCommonStockStockBasedCompensationAdditionalInformationDetail" id="Role_DisclosureStockholdersEquityDeficitCommonStockStockBasedCompensationAdditionalInformationDetail">
        <link:definition>100610 - Disclosure - Stockholders&apos; Equity (Deficit) Common Stock - Stock Based Compensation - Additional Information (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureNetLossPerShareBasicAndDilutedNetLossPerShareAttributableToCommonStockholdersDetail" id="Role_DisclosureNetLossPerShareBasicAndDilutedNetLossPerShareAttributableToCommonStockholdersDetail">
        <link:definition>100620 - Disclosure - Net Loss per Share - Basic and Diluted Net Loss per Share Attributable to Common Stockholders (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureNetLossPerShareSummaryOfPotentialCommonSharesExcludedFromCalculationOfDilutedNetLossPerShareDetail" id="Role_DisclosureNetLossPerShareSummaryOfPotentialCommonSharesExcludedFromCalculationOfDilutedNetLossPerShareDetail">
        <link:definition>100630 - Disclosure - Net Loss per Share - Summary of Potential Common Shares Excluded from Calculation of Diluted Net Loss per Share (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureCommitmentsAndContingenciesAdditionalInformationDetail" id="Role_DisclosureCommitmentsAndContingenciesAdditionalInformationDetail">
        <link:definition>100640 - Disclosure - Commitments and Contingencies - Additional Information (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureCommitmentsAndContingenciesSummaryOfFutureMinimumLeasePaymentsForOperatingLeasesDetail" id="Role_DisclosureCommitmentsAndContingenciesSummaryOfFutureMinimumLeasePaymentsForOperatingLeasesDetail">
        <link:definition>100650 - Disclosure - Commitments and Contingencies - Summary Of Future Minimum Lease Payments For Operating Leases (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureIncomeTaxesAdditionalInformationDetail" id="Role_DisclosureIncomeTaxesAdditionalInformationDetail">
        <link:definition>100660 - Disclosure - Income Taxes - Additional Information (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureIncomeTaxesScheduleOfReconciliationOfUSFederalStatutoryIncomeTaxRateToCompanySEffectiveIncomeTaxRateDetail" id="Role_DisclosureIncomeTaxesScheduleOfReconciliationOfUSFederalStatutoryIncomeTaxRateToCompanySEffectiveIncomeTaxRateDetail">
        <link:definition>100670 - Disclosure - Income Taxes - Schedule of Reconciliation of U.S Federal Statutory Income Tax Rate to Company&apos;s Effective Income Tax Rate (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureIncomeTaxesScheduleOfNetDeferredTaxAssetsDetail" id="Role_DisclosureIncomeTaxesScheduleOfNetDeferredTaxAssetsDetail">
        <link:definition>100680 - Disclosure - Income Taxes - Schedule of Net Deferred Tax Assets (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureIncomeTaxesSummaryOfChangesInValuationAllowanceForDeferredTaxAssetsDetail" id="Role_DisclosureIncomeTaxesSummaryOfChangesInValuationAllowanceForDeferredTaxAssetsDetail">
        <link:definition>100690 - Disclosure - Income Taxes - Summary of Changes In Valuation Allowance for Deferred Tax Assets (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureSelectedQuarterlyFinancialDataUnauditedSummaryOfQuarterlyFinancialInformationDetail" id="Role_DisclosureSelectedQuarterlyFinancialDataUnauditedSummaryOfQuarterlyFinancialInformationDetail">
        <link:definition>100700 - Disclosure - Selected Quarterly Financial Data (Unaudited) - Summary of Quarterly Financial Information (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureRelatedPartyTransactionsAdditionalInformationDetail" id="Role_DisclosureRelatedPartyTransactionsAdditionalInformationDetail">
        <link:definition>100710 - Disclosure - Related Party Transactions - Additional Information (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://serestherapeutics.com/20151231/taxonomy/role/DisclosureSubsequentEventsAdditionalInformationDetail" id="Role_DisclosureSubsequentEventsAdditionalInformationDetail">
        <link:definition>100720 - Disclosure - Subsequent Events - Additional Information (Detail)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
    </appinfo>
  </annotation>
  <element id="mcrb_DocumentAndEntityInformationAbstract" name="DocumentAndEntityInformationAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <element id="mcrb_DeferredOfferingCostsNonCurrent" name="DeferredOfferingCostsNonCurrent" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <element id="mcrb_LeaseIncentivePayableNoncurrent" name="LeaseIncentivePayableNoncurrent" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <element id="mcrb_PreferredStockWarrantLiability" name="PreferredStockWarrantLiability" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <element id="mcrb_TemporaryEquitySharesIssuedDuringPeriod" name="TemporaryEquitySharesIssuedDuringPeriod" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <element id="mcrb_StockIssuedDuringPeriodValueCommonStockWarrantsExercised" name="StockIssuedDuringPeriodValueCommonStockWarrantsExercised" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <element id="mcrb_StockIssuedDuringPeriodSharesCommonStockWarrantsExercised" name="StockIssuedDuringPeriodSharesCommonStockWarrantsExercised" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <element id="mcrb_TemporaryEquityIssuanceCost" name="TemporaryEquityIssuanceCost" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <element id="mcrb_SeriesBConvertiblePreferredStockMember" name="SeriesBConvertiblePreferredStockMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <element id="mcrb_SeriesCConvertiblePreferredStockMember" name="SeriesCConvertiblePreferredStockMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <element id="mcrb_SeriesDAndD1ConvertiblePreferredStockMember" name="SeriesDAndD1ConvertiblePreferredStockMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <element id="mcrb_SeriesDConvertiblePreferredStockMember" name="SeriesDConvertiblePreferredStockMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <element id="mcrb_PaymentOfLicenseFeesInCommonStockWarrant" name="PaymentOfLicenseFeesInCommonStockWarrant" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <element id="mcrb_ProceedsFromIssuanceOfConvertiblePreferredStockNetOfIssuanceCosts" name="ProceedsFromIssuanceOfConvertiblePreferredStockNetOfIssuanceCosts" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <element id="mcrb_ProceedsFromIssuanceOfNotesPayableAndPreferredStockWarrantsNetOfIssuanceCosts" name="ProceedsFromIssuanceOfNotesPayableAndPreferredStockWarrantsNetOfIssuanceCosts" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <element id="mcrb_ProceedsFromIssuanceOfCommonAndRestrictedStocks" name="ProceedsFromIssuanceOfCommonAndRestrictedStocks" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <element id="mcrb_IssuanceOfWarrantsInConnectionWithNotesPayable" name="IssuanceOfWarrantsInConnectionWithNotesPayable" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <element id="mcrb_PreferredStockWarrantsLiabilityDisclosureTextBlock" name="PreferredStockWarrantsLiabilityDisclosureTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <element id="mcrb_ResearchContractCostsAndAccrualsPolicyTextBlock" name="ResearchContractCostsAndAccrualsPolicyTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <element id="mcrb_PatentCostsPolicyTextBlock" name="PatentCostsPolicyTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <element id="mcrb_WarrantsToPurchaseConvertiblePreferredStockPolicyTextBlock" name="WarrantsToPurchaseConvertiblePreferredStockPolicyTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <element id="mcrb_ScheduleOfAccruedExpenseAndOtherCurrentLiabilitiesTableTextBlock" name="ScheduleOfAccruedExpenseAndOtherCurrentLiabilitiesTableTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <element id="mcrb_RedeemablePreferredStockAndWarrantsTextBlock" name="RedeemablePreferredStockAndWarrantsTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <element id="mcrb_NatureOfBusinessAndBasisOfPresentationLineItems" name="NatureOfBusinessAndBasisOfPresentationLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <element id="mcrb_NatureOfBusinessAndBasisOfPresentationTable" name="NatureOfBusinessAndBasisOfPresentationTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <element id="mcrb_UnderwritersOptionToPurchaseAdditionalSharesMember" name="UnderwritersOptionToPurchaseAdditionalSharesMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <element id="mcrb_CashEquivalentsOriginalMaturityMaximumPeriod" name="CashEquivalentsOriginalMaturityMaximumPeriod" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <element id="mcrb_AccountingPoliciesLineItems" name="AccountingPoliciesLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <element id="mcrb_AccountingPoliciesTable" name="AccountingPoliciesTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <element id="mcrb_LaboratoryEquipmentMember" name="LaboratoryEquipmentMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <element id="mcrb_FurnitureAndOfficeEquipmentMember" name="FurnitureAndOfficeEquipmentMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <element id="mcrb_ReclassificationOfDeferredOfferingCostsToAdditionalPaidInCapital" name="ReclassificationOfDeferredOfferingCostsToAdditionalPaidInCapital" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <element id="mcrb_MinimumPercentageOfLikelihoodOfTaxBenefitBeingRealizedUponSettlement" name="MinimumPercentageOfLikelihoodOfTaxBenefitBeingRealizedUponSettlement" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <element id="mcrb_RepurchaseAgreementsFairValueDisclosure" name="RepurchaseAgreementsFairValueDisclosure" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <element id="mcrb_PreferredStockWarrantLiabilityFairValueDisclosure" name="PreferredStockWarrantLiabilityFairValueDisclosure" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <element id="mcrb_CorporateBondsCommercialPaperGovernmentSecuritiesAndTreasuryBondsMember" name="CorporateBondsCommercialPaperGovernmentSecuritiesAndTreasuryBondsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <element id="mcrb_InvestmentsRemainingMaturityMaximumPeriod" name="InvestmentsRemainingMaturityMaximumPeriod" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <element id="mcrb_AccruedDevelopmentAndClinicalManufacturingCostsCurrent" name="AccruedDevelopmentAndClinicalManufacturingCostsCurrent" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <element id="mcrb_ComericaMember" name="ComericaMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <element id="mcrb_DebtInstrumentPaymentOfOutstandingPrincipalAmount" name="DebtInstrumentPaymentOfOutstandingPrincipalAmount" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <element id="mcrb_DebtInstrumentPaymentOfFeesAndAccruedInterest" name="DebtInstrumentPaymentOfFeesAndAccruedInterest" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <element id="mcrb_ClassOfWarrantOrRightWarrantsContractualTerm" name="ClassOfWarrantOrRightWarrantsContractualTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <element id="mcrb_SeriesA2ConvertiblePreferredStockMember" name="SeriesA2ConvertiblePreferredStockMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <element id="mcrb_IssuanceOfPreferredStockWarrant" name="IssuanceOfPreferredStockWarrant" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <element id="mcrb_ReclassificationOfWarrantLiabilitiesToEquity" name="ReclassificationOfWarrantLiabilitiesToEquity" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <element id="mcrb_MayoFoundationMember" name="MayoFoundationMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <element id="mcrb_PerformanceWarrantMember" name="PerformanceWarrantMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <element id="mcrb_WarrantExpense" name="WarrantExpense" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <element id="mcrb_SharebasedCompensationArrangementByShareBasedPaymentAwardStockOptionsTerm" name="SharebasedCompensationArrangementByShareBasedPaymentAwardStockOptionsTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <element id="mcrb_TwoThousandTwelveStockIncentivePlanMember" name="TwoThousandTwelveStockIncentivePlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <element id="mcrb_PercentageCommonStockOutstandingShares" name="PercentageCommonStockOutstandingShares" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <element id="mcrb_TwentyFifteenIncentivePlanMember" name="TwentyFifteenIncentivePlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <element id="mcrb_TwoThousandFifteenEmployeeStockPurchasePlanMember" name="TwoThousandFifteenEmployeeStockPurchasePlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <element id="mcrb_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentOtherThanOptionsExercisableInPeriodGross" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentOtherThanOptionsExercisableInPeriodGross" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <element id="mcrb_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceAndAdditionalDisclosuresAbstract" name="SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceAndAdditionalDisclosuresAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <element id="mcrb_StockOptionsAndRestrictedStockMember" name="StockOptionsAndRestrictedStockMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <element id="mcrb_UnvestedRestrictedCommonStockMember" name="UnvestedRestrictedCommonStockMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <element id="mcrb_WarrantsToPurchaseConvertiblePreferredStockMember" name="WarrantsToPurchaseConvertiblePreferredStockMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <element id="mcrb_WarrantsToPurchaseCommonStockMember" name="WarrantsToPurchaseCommonStockMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <element id="mcrb_OfficeSpaceMember" name="OfficeSpaceMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <element id="mcrb_OfficeAndLaboratorySpaceMember" name="OfficeAndLaboratorySpaceMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <element id="mcrb_BMRSidneyResearchCampusLimitedLiabilitiesCompanyMember" name="BMRSidneyResearchCampusLimitedLiabilitiesCompanyMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <element id="mcrb_OperatingLeasesGrossFloorAreaLeased" name="OperatingLeasesGrossFloorAreaLeased" type="num:areaItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <element id="mcrb_LeaseCommencementPeriod" name="LeaseCommencementPeriod" type="xbrli:gYearMonthItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <element id="mcrb_LeaseExpirationPeriod" name="LeaseExpirationPeriod" type="xbrli:gYearMonthItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <element id="mcrb_IncomeTaxDisclosureLineItems" name="IncomeTaxDisclosureLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <element id="mcrb_IncomeTaxDisclosureTable" name="IncomeTaxDisclosureTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <element id="mcrb_StockBasedCompensationTaxBenefitMember" name="StockBasedCompensationTaxBenefitMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <element id="mcrb_EffectiveIncomeTaxRateReconciliationRevaluationOfPreferredStockWarrantLiability" name="EffectiveIncomeTaxRateReconciliationRevaluationOfPreferredStockWarrantLiability" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <element id="mcrb_DeferredTaxAssetsCapitalizedOrganizationCosts" name="DeferredTaxAssetsCapitalizedOrganizationCosts" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <element id="mcrb_DeferredTaxAssetsDeferredExpenseCapitalizedResearchAndDevelopmentCosts" name="DeferredTaxAssetsDeferredExpenseCapitalizedResearchAndDevelopmentCosts" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <element id="mcrb_OperatingLossCarryforwardsExpirationYear" name="OperatingLossCarryforwardsExpirationYear" type="xbrli:gYearItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <element id="mcrb_TaxCreditCarryforwardExpirationYear" name="TaxCreditCarryforwardExpirationYear" type="xbrli:gYearItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <element id="mcrb_DecreaseInValuationAllowanceDeferredTaxAssetAmount" name="DecreaseInValuationAllowanceDeferredTaxAssetAmount" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <element id="mcrb_FlagshipVenturesManagementIncMember" name="FlagshipVenturesManagementIncMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <element id="mcrb_ServicesAgreementMember" name="ServicesAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <element id="mcrb_UpfrontPaymentInCash" name="UpfrontPaymentInCash" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <element id="mcrb_DevelopmentApprovalAndCommercialMilestones" name="DevelopmentApprovalAndCommercialMilestones" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <element id="mcrb_SalesBasedMilestones" name="SalesBasedMilestones" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <element id="mcrb_NestleHealthScienceMember" name="NestleHealthScienceMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
</schema>
