Mobile genetic elements related to carbapenem resistance in Acinetobacter baumannii

h tt p : / / w w w . b j m i c r o b i o l . c o m . b r /

Review

Mobile

genetic

elements

related

to

carbapenem

resistance

in

Acinetobacter

baumannii

Mariana

Pagano

a,c

_,

_Andreza

_Francisco

_Martins

b,c,∗

_,

_Afonso

_Luis

_Barth

a,c

a_{UniversidadeFederaldoRioGrandedoSul(UFRGS),FaculdadedeFarmácia,ProgramadePós-Graduc¸ãoemCiênciasFarmacêuticas,} PortoAlegre,Brazil

b_{UniversidadeFederaldoRioGrandedoSul(UFRGS),InstitutodeCiênciasBásicasdaSaúde,PortoAlegre,Brazil} c_{HospitaldeClínicasdePortoAlegre(HCPA),PortoAlegre,Brazil}

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t

i

c

l

e

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n

f

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Articlehistory:

Received5November2014 Accepted11February2016 Availableonline4July2016 AssociateEditor:JorgeLuizMello Sampaio

Keywords:

Acinetobacterbaumannii Antimicrobialresistance Mobilegeneticelements Carbapenemresistance

a

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Acinetobacterbaumanniiiswidelyrecognizedasanimportantpathogenassociatedwith noso-comialinfections.Thetreatmentoftheseinfectionsisoftendifficultduetotheacquisition ofresistancegenes.A.baumanniipresentsahighgeneticplasticitywhichallowsthe accu-mulationoftheseresistancedeterminantsleadingtomultidrugresistance.Itishighlighted theimportanceofthehorizontaltransferofresistancegenes,throughmobilegenetic ele-mentsanditsrelationshipwithincreasedincidenceofmultidrugresistantA.baumanniiin hospitals.Consideringthatresistancetocarbapenemsisveryimportantfromtheclinical andepidemiologicalpointofview,theaimofthisarticleistopresentanoverviewofthe currentknowledgeaboutgeneticelementsrelatedtocarbapenemresistanceinA.baumannii suchasintegrons,transposons,resistanceislandsandinsertionsequences.

©2016SociedadeBrasileiradeMicrobiologia.PublishedbyElsevierEditoraLtda.Thisis anopenaccessarticleundertheCCBY-NC-NDlicense(http://creativecommons.org/

licenses/by-nc-nd/4.0/).

Introduction

The Acinetobacter baumannii-calcoaceticus (Abc) complex has emergedasanimportantnosocomialpathogen.Amongthe membersofthiscomplex,A.baumannii,A.pittii,andA. noso-comialis are the three most common Acinetobacter species isolatedinclinicalsettings.1A.baumanniihasbeenextensively studiedduetoitsassociationwithinfectionsofhighmortality

∗ _{Correspondingauthorat:Av.SarmentoLeite,500/Prédio12101,BairroFarroupilha,PortoAlegreCEP90050-170,Brazil.} E-mail:andrezafm20@gmail.com(A.F.Martins).

rates.A.pittiiandA.nosocomialisareincreasinglyidentifiedas causativeagentsofnosocomialinfections.2

A. baumannii is considered an important nosocomial pathogen, causing a wide range of infections, including ventilator-associatedpneumonia,bloodstreaminfections, uri-narytractinfectionsandmeningitis.Thisspeciesisnaturally highly resistant to a number of antimicrobials commonly usedintheclinicalpractice,suchasfirstandsecond genera-tioncephalosporins,aminopenicillins,andchloramphenicol.

http://dx.doi.org/10.1016/j.bjm.2016.06.005

1517-8382/©2016SociedadeBrasileiradeMicrobiologia.PublishedbyElsevierEditoraLtda.ThisisanopenaccessarticleundertheCC BY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).

A.baumanniicontainsanintrinsicAmpC␤-lactamase(blaADC) and OXA-51 serine-type oxacilinase (blaOXA-51), which con-tributetothenaturalresistanceto␤-lactams.3_{Moreover,this}

organismpresentsagreatcapacitytoacquirenewresistance mechanisms, including those responsible for carbapenem resistance.4

Carbapenemresistance in A. baumanniiinvolves mainly thecarbapenem-hydrolysingclassD␤-lactamases(CHDLs– AmblerclassD)andlessfrequently,themetallo-␤-lactamases (MBLs–AmblerclassB).Carbapenemresistancemayalsobe causedbyothermechanismssuchas,productionofother car-bapenemases,porinmodificationorloss,orbymodificationof thepenicillin-bindingproteins.1,5

SeveralacquiredclassDOXA-type␤-lactamaseshavebeen identified asa sourceofcarbapenem resistancein A. bau-mannii.Five maingroupsofCHDLs havebeen described in A.baumannii,correspondingtoOXA-23-like,OXA-24/40-like, OXA-58-like,OXA-143-likeandOXA-235-likeenzymes.6

OXA-23-like enzymes are the most widespread in A. baumannii worldwideandhavebeenidentifiedinallcontinents.6

InBrazil,OXA-23-like-producingA.baumanniiis dissemi-natedinmanystatesanditisresponsibleforhighendemic levelsofmultidrug-resistance.7,8_Thebla

OXA-143genehasthus farbeendetectedonlyinA.baumanniiisolatesfromBraziland isthesecondmostfrequentCHDLencodinggene.9–11

TheblaOXA-143 geneisfrequentlyfoundintheSoutheast region of Brazil, especially in the state of São Paulo. It is importanttonotethattwo newvariantsofthis genewere recentlydescribed.ThevariantsblaOXA-235andblaOXA-231were describedinMinasGeraisandParanástates,respectively.12,13

Thisdatademonstratesthedetectionofthesenewvariants ofblaOXA-143 inBrazilisacauseofgreatconcernandshows thepotentialofthesenewCHDLstospreadtootherBrazilian regions.

AlthoughblaOXA-24/40-like gene isdisseminatedinA. bau-manniiinEurope,inBrazil,thisgene isstillrare, withonly averyfewreportsofablaOXA-72(blaOXA-24/40-likevariant)inSão Paulo,9_Recife,14_{PortoAlegreandCuritiba.}

DespiteMBLsarelesscommonlyidentifiedinA.baumannii thantheOXA-typecarbapenemases,theirhydrolytic activi-tiestocarbapenemsaresignificantlymorepotent.FourMBLs havebeenidentifiedinA.baumannii:IMP,VIM,SIMand,more recently, NDM.15 _{It is important to note that MBL genes,}

suchasNDMandIMP-1,havebeendescribedin Acinetobac-ternon-baumanniispecies,whichdemonstratesthecapacityof theseresistancegenestospreadamongdifferentAcinetobacter species.16,17

MostofAmblerclassAESBLspossessactivityagainst peni-cillinsandbroad-spectrumcephalosporins.However,specific GESvariantshavebeenshowntopossesstheabilityto com-promisetheefficacyofcarbapenems.AmongA.baumannii,the variantsGES-11andGES-14possessspecificresidues enlarg-ingtheirhydrolysisspectrum(Table1).18,19

TheelevatedgeneticplasticitypresentedbyA.baumannii hasallowedtheaccumulationofmanyresistance determi-nants,whichcontributedtothehighincidenceofA.baumannii multirresistanttoantibiotics.Inthisreview,wepresentand discussthecharacteristicsofthedifferentmobilegenetic ele-mentsinvolvedinthetransferofresistancedeterminantsin A.baumannii.

AbaR-type

genomic

resistance

islands

Genomicislandscontainingresistancemarkersarereferred to as resistance islands. Resistance islands have been describedmainlyinProteobacteria,includingShigellaflexneri, Salmonella enterica, Vibriocholerae,Staphylococcus aureus, and morerecently,inA.baumannii.20,21_{A.baumanniiisolates}

har-borlargeclustersofhorizontallytransferredgenesconferring resistancetomultipleantibioticsandheavymetals,whichare integratedataspecificsiteinaparticularATPasegene.22

Fournieretal.describedforthefirsttimetheA.baumannii ResistantIsland(AbaR).AbaRisdefinedasaregionwhichhas transposedintoaspecificpositioninthechromosome, creat-inga5bpduplicationsite(ACCGC).21_{ThebackboneofAbaRis}

comprisedoffiveopenreadingframes(ORFs)–orf1,tniA,tniB, orf2,orf3–whichconstitutethetranspositionmodule,andtwo othergenesencodingtotheuniversalstressprotein(uspA)and asulfatepermease(sul).21–23

Several AbaRhave already been described containing a varietyofresistancegenes,includingtheblaOXA-23-like,which confersresistancetocarbapenems.24_{Theseresistanceislands}

havebeendescribedinA.baumanniiepidemicstrains belong-ingtotheimportantglobalclones,EuropeanCloneI(ECI)and EuropeanCloneII(ECII),knownfortheirincreasedcapacity tospreadworldwide.22

Severalothergenomicresistanceislandshavebeenfully characterized inA. baumannii. The majoritywere foundin strainsofECI,suchas,AbaR1,AbaR3,AbaR5,AbaR6,AbaR7, AbaR8,AbaR9,and AbaR10. TheseAbaRsshare astructure representedbya16.3kbbackbonetransposon(Tn6019) inter-rupted by a large compound transposon that contains a variable-resistanceregionboundedbydirectlyorientedcopies ofTn6018.ExceptionsareAbaR6andAbaR7,eachwithalarge deleted region.25 _{Muchlessisknown aboutAbaRsinECII.}

Theresistanceislandsharboredbythiscloneareintegrated atthesamesiteoftheATPasegeneasisknownforAbaRsin ECI.25

AbaR1 isthelargest resistanceislanddescribed todate. Thisislandcontains86kbandwasoriginallydescribedinthe epidemicA.baumanniistrainAYEbelongingtoECI.Thisstrain wasresponsibleforoutbreaksinFranceduring2004.21_A.

bau-manniiAYEstrainrevealedthepresenceofalargegenecluster, containingmanyresistancedeterminants,insertedinto the chromosome.21

Ofthe45resistancegenesdescribedinAbaR1resistance island,25wereassociatedwithresistancetoseveralclassesof antibiotics.Theseincludegenesthathadnotbeenpreviously described inAcinetobacter speciessuchasstrA, strB,aphA1, andaac69(encodingresistancetoaminoglycosides);putative tetracycline-resistancegenestetA(tetracyclineeffluxpump) andtetR(repressorprotein);dfrX(resistancetocotrimoxazole); and the chloramphenicol-resistance gene cmlA (chloram-phenicol efflux pump). Moreover, Fournier et al. (2006) describedthepresenceofgenesinAbaR1thatencodeVEB-1 and OXA-10 ␤-lactamases, the aminoglycoside acetyltrans-ferasegeneaac3,andtheaminoglycosideadenyltransferases aadA1/DA1/B; the cotrimoxazole resistance-associated dfrI; cmlA5andonecopyofthechloramphenicolacetyl-transferase cat;therifampinADP-ribosyltransferasegenearr-2;andfive

Table1–CharacterizationofthemainmobilegeneticelementsassociatedwithresistanceinAcinetobacterbaumannii.

Mobilegenetic

element

Resistancegenesassociated Resistanceprofile Geographicregion Reference

ISAba1 blaOXA-23

blaOXA-51 blaOXA-58 blaAmpC ␤-Lactams including carbapenems Worldwide disseminated Villalónetal.,2013 Mugnieretal.,2010 Mugnieretal.,2009

ISAba2 blaOXA-58

blaAmpC Carbapenems Cephalosporins France Italy Spain Villalónetal.,2013

FernándezCuencaetal.,2012

Marquéetal.,2005

ISAba3 blaOXA-58 Carbapenems China

Italy Taiwan Lebanon Villalónetal.,2013 Donnarummaetal.,2010 Zarrillietal.,2008 Fuetal.,2014

ISAba4 blaOXA-23 Carbapenems France

Belgium

Bogaertsetal.,2008

Corvecetal.,2007

ISAba10 blaOXA-23 Carbapenems Korea Leeetal.,2011

ISAba125 blaNDM-1 blaNDM-2 blaAmpC aphA6 Carbapenems Cephalosporins Aminoglycosides India Switzerland Greece Australia

Bonninetal.,2012a

Kaaseetal.,2011

Mishraetal.,2013

Hamidianetal.,2012

IS18 blaOXA-58 Carbapenems Lebanon

Turkey

Villalónetal.,2013

Zarrillietal.,2008

Marquéetal.,2005

Tn2006 blaOXA-23 Carbapenems Spain

Tahiti France Turkey Vietnam Romania Lybia Australia Mugnieretal.,2010 Corvecetal.,2007

Tn2007 blaOXA-23 Carbapenems France

Algeria

Corvecetal.,2007

Tn2008 blaOXA-23 Carbapenems UnitedArab

Emirates Bahrain Mugnieretal.,2010 Int1 blaGES-11 blaGES-14 dfrA1 sat2 aadA1 orfX ybfA ybfB ␤-Lactams including carbapenems Aminoglycosides Europe (widespread) Korea Iran Brazil Bonninetal.,2011 Nemecetal.,2004 Leeetal.,2005 Japoni-Nejadetal.,2013 Mendesetal.,2007 Int2 dfrA1 sat2 aadA1 orfX ybfA ybfB ybgA Aminoglycosides Argentina Chile Brazil Paganoetal.,2012 Ramirezetal.,2012 Fonsecaetal.,2011

copiesofthesulfonamide-resistancegenesulIencoding dihy-dropteroatesynthetase,acomponentofclass1integrons.21,26

AbaR2wasdescribedintheepidemic,multidrug-resistant A.baumanniistrainnamedACICU.27_{ThisstrainbelongstoECII}

andcarriestheplasmid-mediatedblaOXA-58.A.baumanniiAYE andACICUbelongtodifferentclonalgroups(Europeanclones IandII,respectively),however,thepresenceofrelated resis-tanceislandsinbothlineagessuggeststhatAbaR1andAbaR2 derivedfrom an islandacquired bya common A. bauman-niiancestorbeforetheirdivergenceintotwodifferentclonal lineages.21,27

ThegenomicresistanceislandvariantAbaR3appearsto beanancestralofseveralAbaRvariantswhichhavearisen from AbaR3 by loss of segments of different lengths that include one or more of the antibiotic resistance genes.28

AbaR3containseightgenesassociatedwithantibiotic resis-tance.UniquesequencesinAbaR3includeablaTEMgenethat isassociatedwithaTn3transposonand asmallclusterof genes, including two that encode toa DNA topoisomerase and asingle-strand bindingproteinthatare similarto pro-teins from a broad-host-range plasmid.29 _{In addition, it is}

DNAtopoisomerasemaycontributetotheresistanceisland mobility.

Transposable

elements

Transposableelements havetheabilitytomovewithinthe bacterialgenome,beingabletotranslocatethemselvesfrom onesiteofthegenometoother sites.Thesetranspositions are considered one of the major causes of bacterial DNA rearrangements, which inturn can cause changes ingene expression.30 _{In A. baumannii, transposable elements, such}

astransposonsandinsertionsequenceshavebeen responsi-blefortheexpressionandspreadofantimicrobialresistance mechanisms.1

Insertion

sequences

Bacterialinsertionsequences(IS)aretheleastcomplextype oftransposableelements;theyrarelyexceed2kbinsizeand maybeassmallas0.5kb.Theseelementspossessan impor-tantroleinthespreadofresistancegenessincethepresence oftwocopiesofthesameISflankingaresistancegeneform acomplexstructurecalledcompositetransposon.Composite transposonsareabletomobilizeavarietyofresistancegenes, contributingtoantimicrobialresistancedissemination.31

Besidestheirtranspositionrole,someIShavebeenshown toactivateor toincreasetheexpressionofneighborgenes. Thiscapacitymaybeduetothepresenceofpromoterregions intheinsertionsequenceorbytheformationofnew promo-tersaftertheinsertionevent.31

SomeISelementshaveanimportantrole inA. bauman-niiantimicrobialresistance. ISAba1, ISAba2, ISAba3, ISAba4 and IS18 are commonly associatedwith the expression of carbapenemases genes inA. baumannii (Table 1).32 _Villalón

etal.(2013)investigatedthepresenceoftheseISelementsin 59multidrug-resistantA.baumanniiisolatesandobserveda prevalenceof93.2%,25.4%,20.3%and5.1%forISAba1,ISAba2, ISAba3andIS18,respectively.ISAba4wasnotdetectedinany oftheisolatesinthisstudy.32

Itisimportanttonote that ISelements suchas ISAba1 cancontributetothespreadofcarbapenemasegenesamong different Acinetobacter species. Poirel et al. (2008) hypoth-esized that blaOXA-23 was likely mobilized by the ISAba1 insertionsequencefromA.radioresistenstoA.baumannii.33_The

authorsdemonstratedthatA.radioresistensistheprogenitor oftheblaOXA-23-likegene,whichwasmobilizedtoA.baumannii throughISAba1insertionssequenceprovidedbyA. bauman-nii.Thishypothesis isbased on theidentification ofgenes encodingbothOXA-23-likeandATPase-likeenzymesontheA. radioresistenschromosomewithoutthepresenceofISAba1 ele-ments,thatisinvolvedinthemobilizationofblaOXA-23gene.33 TheISAba1elementbelongstotheIS4familyandhas 11-bpinvertedrepeatssequencesflankedby9-bpdirectrepeats ofthetargetsequence.Althoughthiselementisconsidered exclusivetoA.baumannii,Segaletal.(2005)identifiedISAba1in Acinetobacterlwoffiiisolates,demonstratingthehighmobility oftheseelementsandindicatingthattranspositioneventsof theISAba1occurfrequently.34

ISAba1 has been found upstream the blaOXA-23-like, blaOXA-51-like, blaOXA-58-like and blaAmpC genes inA. bauman-nii. This IS acts as a promoter sequence which increases the expression ofresistance genes. In fact, it was demon-stratedthatitisnecessarythepresenceofISAba1upstream blaOXA-23 and blaOXA-51 forthese genestoconfer resistance to carbapenems.35 _{Although several authors have}

demon-strated therelationshipbetweenISAba1upstreamblaOXA-51 andcarbapenemresistance,thismaynotbeenoughtoconfer resistance,asA.baumanniiisolatessusceptibleto carbapen-emswiththeassociationISAba1/blaOXA-51havealreadybeen described.36

TheISAba2,ISAba3and ISAba4elementshavealsobeen identifiedupstreamblaOXA-58-likeandblaOXA-23-likegenesinA. baumanniiisolates.37_{Giannoulietal.(2009)analyzedthe}

inser-tionsequencesof24A.baumanniiisolateswithblaOXA-58gene andidentifiedthepresenceISAba2,IS18orISAba1locatedat the5 end,whileat3 endallisolatespresentedtheISAba3 element.Ofnote,theISelementsat5endofblaOXA-58were evidencedinstrainsofdistinctPFGEprofilesandSTgroupsin thesamegeographicalarea.Itsuggeststhattheseelements mighthavebeenacquiredthroughhorizontalgenetransfer andconfirmstheirdisseminationcapacityamongA. bauman-niiisolates.38

Corvec et al.(2007) described the first A. baumannii iso-lateharboringanISAba4elementupstreamblaOXA-23gene,in France.Subsequently,itwasshownthatanisolatefrom Bel-giumcontainingtheassociationofISAba4andtheblaOXA-23 presentedthesamePFGEprofileastheisolatefromFrance. Thesefindingsdemonstratethepropensityofresistantstrains tospread,highlightingtheimportanceofepidemiological sur-veys to estimate the true prevalence of isolatesharboring ISAba4/blaOXA-23.39,40

Lee et al. (2011) identified a novel 1203bp insertion sequence, named ISAba10. This element was found to be insertedintotheISAba1elementupstreamblaOXA-23genein an A. baumannii presenting high minimum inhibitory con-centrations(MICs)tocarbapenems(≥32␮g/mL).Inaddition, isolateswithouttheinsertionofthis elementshowedMICs between 8and 16␮g/mL. The authors suggested that this sequencemayincrease2–5-foldtheblaOXA-23geneexpression. Basedontheseresults,theysuggestedthattheISAba10 ele-mentmayplayanimportantroleincarbapenemresistance byprovidinganadditionalpromotersequencetotheblaOXA-23 gene.41

IS elements have also been associated to metallo- ␤-lactamases such as blaNDM, which have been increasingly reported inAcinetobacterbaumanniiand inother Acinetobac-ter species such as A. johnsonnii, A. pittii, A. junii and A. lwoffii.16,42–44 _Thebla

NDM can belocatedeitheron the plas-midorchromosomeinAcinetobacterspecies.44_{However,itwas}

evidencedthatthespreadoftheblaNDMgenewasnot associ-atedwithclonaldissemination,buthorizontalspreadofthe geneticstructure.42

Several studies reported that blaNDM gene is located betweentwocopiesoftheISAba125element,forminga com-positetransposonnamedTn125.ISAba125elementprovides the-35sequenceofthehybridpromoterresponsibleforthe expressionofthe blaNDM gene.45 Curiously, thisIS element has been originallyidentified from an A. baumanniiisolate

withoutanyassociationwiththeblaNDMgene.Bycontrast,this IShasbeenidentifiedinEnterobacteriaceaeandP.aeruginosaas aremnantoftheTn125andhasneverbeenidentifiedalone inthesespecies.ThisobservationsuggeststhatA.baumannii isalikelyreservoirofISAba125.Findingslikethesehighlight thateventhoughA.baumanniiisusuallyrecognizedasafinal acceptorforresistancegenes,itmayacquireseveralresistance determinantsandthentransferthemtoEnterobacteriaceaeand Pseudomonasspp.

Recently,astudydemonstratedthatTn125hasbeen dis-ruptedbyIS26inA.baumanniiNDM-producingisolatesfrom India.ThisnewrearrangementhasresultedinblaNDM-1being withinan IS26 composite transposon, which might poten-tiallymobilize blaNDM-1 and contributetothespreadofthe carbapenemasegene.46

Robledoet al.(2010) described the first report ofblaKPC geneinA.baumanniiisolatesfromPuertoRico.Inthatstudy, fourvariantsofblaKPCwereidentified:KPC-2,-3,and-4and a novelvariant, KPC-10. The integrationof these genes in theA.baumanniichromosomewasrelatedtoatransposition eventmediatedbythetransposaseofISEcp1.31,47_Thiselement

islikelytoberesponsibleformobilizingnumerousblaCTX-M genesandseveralotherresistancegenessuchasqnrB19,rmtC, blaACC-1 andblaCMY-2g7,16,21.48 Inaddition,itwasresponsible forthemobilizationofblaCTX-M-5 fromanarrowrange plas-midtothechromosomeofA.baumannii,eventsimilartowhat Martinezetal.observedwithblaKPCgene.47

As described above, ISs can cause insertion mutations, genome rearrangements and enhance the spread of resis-tanceandvirulencedeterminantswithinpathogenicspecies. Besides being involved in the expression and spread of carbapenemases,IS elementssuch asISAba1, ISAba10and ISAba825areinvolvedonthedisruptionofcarOgene,which codesforanimportantoutermembranechannel.Theabsence of this outer membrane protein has been correlated with reducedsusceptibilitytocarbapenems.41,49,50

Transposons

Transposonssequencesmayvaryinsizefrom3to40kb,in somecasescontainingdozensofgenes.Theseelementsare into two main classes: composite transposons or complex transposons.Compositetransposonshaveresistancegenesin itscentralregion;furthermore,theseelementsareflankedby aninsertionsequence(IS)ateachend.Complextransposons haveamorecomplicatedgeneticstructurethanISelements orcompositetransposons.Theclassiccomplextransposonis Tn3,whichisderivedfromresistanceplasmidR1.51

InA.baumannii,transposonshavebeencharacterizedas geneticstructuresharboringimportantresistantgenes,such asblaOXA-23.ThreetransposonshavebeenrelatedtoblaOXA-23: Tn2006,Tn2007andTn2008.InTn2006,theblaOXA-23geneis flankedbytwocopiesoftheinsertionsequenceISAba1,which islocatedinoppositedirections.Tn2008issimilartoTn2006 butlacksthe secondcopy ofISAba1. Finally,inTn2007the blaOXA-23 geneisassociatedwithonecopyofISAba4located upstreamtothisgene.52_{Severalstudieshavedemonstrated}

thatTn2006iscurrentlythemostcommon determinantof

carbapenemresistance,withagreatabilitytospreadamong A.baumanniiisolates.53

Integrons

Theseelementsarenaturalcloningandexpressionsystems thatincorporateORFsbysite-specificrecombinationand con-vert them to functional genes due to the presence of a promotersequence(Rowe-Magnusetal.,2001).Itisnowwell establishedthatthesemobileelementsconstitutethemajor vectorsofantibioticmultiresistanceinGram-negativeand,to alesserextent,inGram-positivebacteria.54

Five different classes of mobile integrons have been defined to date, based on the sequence of the encoded integrases.54,55 _{Itisknownthatthree(classes1,2and3)of}

these classes have animportantrole inthe dissemination ofantimicrobialresistancegenes.55,56_{Theseclassesarewell}

describedintheliteratureandareassociatedtomultiresistant phenotypes.54,55

Several studieshave demonstrateda highprevalenceof class1integronsinA.baumanniiisolatesinEurope,Asiaand UnitedStates.57_{Duetoitsgreaterspreadcapacity,class1}

inte-grons are the mainexperimental modelof integrons.This class is usuallyassociated to functional or non functional transposonsderivedfromTn402whichmaybeinsertedinto largertransposonsasTn21.Class1integronshavebeen asso-ciated to a variety of insertion sequences, including IS26, IS1999,IS2000eIS6100.58

MostacquiredMBLgenesinA.baumanniihavebeenfound withinclass1integrons,oftencontaininganarrayof resis-tancegenecassettes.1,6_{Mendesetal.(2007)describedseven}

blaIMP-1 harboringAcinetobacterspp.isolatesrecoveredfrom Brazilian inpatients. All isolates possessed a class1 integ-ron, named In86,carrying the same cassettearray: blaIMP, aac(6)-31, andaadA1,which wasplasmid-locatedinfiveof theisolates(Mendesetal.,2007).Thisgenecassettecontained a aminoglycoside resistancegene – aac(6)-31 – that might becapableofconferringresistancetoallclinicallyavailable aminoglycosides.Thisgenewasabletodisseminateamong unrelatedA.baumanniiclinicalisolatesfromaBrazilian hos-pital(Mendesetal.,2007).Recently,Cayôetal.,reportedanew structureofclass1integron,In990,harboringtheblaIMP-10in A.baumanniiisolatesfromBrazil.Thecassettearrangement ofIn990wasverysimilartothatofIn86describedbyMendes etal.59,60

Class 2 integrons are included in the Tn7 family of transposons, and consistof an integrasegene followed by genecassettes.Tn7areidentifiedasasophisticatedmobile geneticelement containingatransposition moduleformed byfivetransposition genes,tnsA,tnsB, tnsC,tnsD, andtnsE, ratherthantheoneortwoseeninmanyothertransposable elements.56_{Class3integronsarelessprevalentthanclass2}

andarealsolocatedintransposons.54

Despitereportsofahigherprevalenceofclass1integronsin A.baumannii,studiesconductedinLatinAmericancountries suchasChile,Argentina andBrazildemonstrated agreater distributionofclass2integronsamongisolatesofA. bauman-niiinthese regions.36,61 _{Fonsecaet al.(2011) demonstrated}

wereinsertedintoTn7transposon,besideshavingthegene cassettecontainingthearrangementofgenesdfrA1 (trimetho-prim resistance),sat2 (streptothricin resistance) and aadA1 (spectinomycinandstreptomycinresistance).62

Martinsetal.(2015)investigatedtheassociationofclass 2integronsandgenecassetteswithclonallineagesofA. bau-mannii.Theyreportedtheassociationofclass1and2integrons withCC109/1(InternationalCloneI)andCC113/79A. bauman-niistrains,respectively.Theauthorshypothesizedthatclass2 integron,predominantinLatinAmerica,maybeaccountedfor thehighprevalenceoftheCC113/79type.Inthesamestudy, asimilarprevalencewasobservedforA.nosocomialis.63

Class1and2integronshavebeendescribedinA.baumannii isolatesrelatedtonosocomialinfectionoutbreaks.Inastudy publishedbyTurtonetal.(2005),itwasobservedthatallA. baumanniiisolatesassociatedwithoutbreakscontainedclass 1integrons,incontrast,nonesporadicisolatepresentedthis classofintegron.64

Morethan130differentgene cassettescontaining resis-tancegeneshavebeenidentifiedinintegrons.Distinctgenes are evidenced ingene cassettes,promotingresistanceto a varietyofantimicrobialclasses.Together,thesegenecassettes provide resistance to most classes of antibiotics including ␤-lactams,all aminoglycosides, chloramphenicol, trimetho-prim, streptothricin, rifampin, erythromycin, fosfomycin, lincomycin, quinolones, and antiseptics of the quaternary ammonium-compoundfamily.65_{Besidesthesegenes,several}

ORFs withunknown functionhave beenidentified ingene cassettes.66

InA.baumannii,genecassetteshavebeendescribed con-tainingseveralgenes,suchasaacA4responsibleforresistance toamikacin,netilmicinandtobramycin,thecatB8geneisan acetyltransferasewhichencodesresistanceto chlorampheni-col,aadA1isresponsibleforresistancetostreptomycinand spectinomycin,aac3responsibleforresistancetogentamicin and blaOXA-10 encodes resistance to ␤-lactams, except car-bapenemsandextended-spectrumcephalosporins.21

Final

remarks

Thisreviewhighlightedtherole ofresistancedeterminants inthecapacityofspreadinA.baumannii.Thisspeciesshows aconsiderable abilitytoacquireforeign DNA suchasdrug resistancegenes,whichprovideageneticdiversityand over-comestheantibioticselectionpressure.Itisimportanttonote that the maincarbapenem-resistance mechanism involved inA. baumannii(production ofoxacillinases)presents alow hydrolyticpowerwhenitisnotassociatedwithaninsertion sequence.Moreover,thecapacityofOXAgenestospreadis directlyrelatedtotheirassociationwithacomposite trans-poson(Tn2006). Thesefeatureshighlighttheimportanceof investigatingthe geneticcontext ofthesegenesinorderto definetheirrealclinicalsignificance.

Thecontinuousdescriptionofgenecassettesinintegrons, mainlythoseleadingtoresistanceto␤-lactamsand aminogly-cosides,hasbeenofgreatconcern.Furthermore,thenumber ofresistancegenesinsertedinthesameplasmid,eveninthe same integron,seems to beincreasing. This integrationof resistancedeterminantsinthesameplasmidmayfacilitate

thepersistenceintheenvironmentforlongperiodsbecause ofthephysicalassociationofintegronswithotherelements, allowingtheircontinuedselection.57

In this context,the knowledgeabout the genetic struc-tureofresistancedeterminantsisveryimportantinorderto understandthecapacityofresistancegenestospreadinA. baumannii.

Conflicts

of

interest

Theauthorsdeclarenoconflictsofinterest.

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