Growth kinetics of Escherichia coli O157:H7 on the epicarp of fresh vegetables and fruits

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

Food

Microbiology

Growth

kinetics

of

Escherichia

coli

O157:H7

on

the

epicarp

of

fresh

vegetables

and

fruits

Mariel

Gullian-Klanian

∗

,

Maria

José

Sánchez-Solis

UniversityMaristofMérida,ExperimentalResearchUnit,Yucatán,Mexico

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received17September2015 Accepted20October2016 Availableonline30August2017 AssociateEditor:BeatrizErnestina CabilioGuth Keywords: E.coliO157:H7 Adhesion Predictivegrowth Lowselectivity Fruitandvegetables

a

b

s

t

r

a

c

t

Despitetheincreasingreportsontheincidenceoffreshvegetablesandfruitsasapossible vehicleforhumanpathogens,thereiscurrentlylimitedknowledgeonthegrowthpotential ofEscherichiacoliO157:H7ondifferentplantsubstrates.Thisstudyanalyzedtheselective adhesionandgrowthofE.coliO157:H7onchilihabanero(CapsicumchinenseL.),cucumber (Cucumissativus),radish(Raphanussativus),tomato(Lycopersiconesculentum),beet(Beta vul-garissubsp.vulgaris),andonion(AlliumcepaL.)underlaboratoryconditions.TheGompertz parameterswereusedtodeterminethegrowthkinetics.Scanningelectronmicroscopywas usedtovisualizetheadhesionofE.coliO157:H7ontheepicarpofthesamples.Predictive modelswereconstructedtocomparethegrowthofE.coliO157:H7onthesampleswith dif-ferentintrinsicfactorsandtodemonstratethelowselectivityofthepathogen.Nosignificant differencewasobservedinthelag-phaseduration(LPD),generationtime(GT),and expo-nentialgrowthrate(EGR)ofthepathogenadheredtothesamples.Theinteractionbetween themicroorganismandthesubstratewaslesssupportivetothegrowthofE.coliO157:H7 foronion,whereasfortomatoandcucumber,thetimeforthemicroorganismtoattainthe maximumgrowthrate(M)wassignificantlylongerthanthatrecordedforothersamples.

©2017SociedadeBrasileiradeMicrobiologia.PublishedbyElsevierEditoraLtda.Thisis anopenaccessarticleundertheCCBY-NC-NDlicense(http://creativecommons.org/

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

Introduction

EscherichiacoliO157:H7isafoodbornezoonoticpathogenthat serving as the causative agent of hemorrhagic colitis and hemolyticuremicsyndrome(HUS)inhumans.This microor-ganismpossessesaremarkablecapabilitytogettransmitted

∗ _{Correspondingauthor.}

E-mail:mgullian@marista.edu.mx(M.Gullian-Klanian).

fromanimalreservoirs(cattle,goats,andsheep)tohumans through avarietyoffood vehiclesthatplays acritical role initspathogenicity.E.coliO157:H7employsmultiple mech-anisms for colonizing vegetables through adhesion to cell surfaces.Themicroorganismadheresstronglytotomatoskin, spinach leavesand alfalfasproutrootsbycurli,1,2 _whichis mediatedbythefilamentoustypeIIIsecretionsystem com-posedofEspAfilaments.3_{Flagellaalsoplayakeyroleinthe} leafattachmentprocessinE.coliO157:H7.4_The internaliza-tionofE.coliO157:H7hasbeendemonstratedundercontrolled environmental conditions in the seedlings of temperate

https://doi.org/10.1016/j.bjm.2017.08.001

1517-8382/©2017SociedadeBrasileiradeMicrobiologia.PublishedbyElsevierEditoraLtda.ThisisanopenaccessarticleundertheCC BY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).

vegetablecropssuchaslettuce,tomato,cress,radish,alfalfa, andmungbean,5–8 _{andtheadhesionandinternalizationof}

E.coliO157:H7inmatureleafyvegetableshavebeen demon-stratedbothexperimentallyandinfieldconditions.9–13

Despite the increasing incidence of fresh produce as a vehicleforhumanpathogens,thereisadearthof informa-tionon thegrowth potentialofE.coliO157:H7 ondifferent plantsubstrates.Althoughmostoftheoutbreaksinvolvingthe occurrenceofE.coliO157:H7onleafyvegetableshavebeen associatedwithcross-contamination14–17 _{thereareveryfew} studies thathave investigatedthe adhesionand growth of E.coliO157:H7inothermaturevegetables.18–22 Interestingly, manyofthesevegetables possessavarietyofintrinsic fac-torsthat canconsiderably reducethe growth offoodborne microorganisms;however,E.coliO157:H7managesto success-fullysurviveunderthesevaryingconditions.

Assumingthatthedegreeofmutualinteractionbetween bacterial strainand the specific typeof plantaffects their association,thisstudy aimedtoinvestigatethe capacityof E.coliO157:H7tocolonizefreshlyconsumedvegetablesand fruits under experimental conditions. The growth of the pathogen on different substrates was compared using the Gompertz equation to obtain bacterial growth curves and linearregression-derivedparameters suchasthelag-phase duration,maximummicrobialpopulation,andspecific micro-bialgrowth.

Materials

and

methods

Microorganismandpreparationofinoculums

Escherichia coli O157:H7 strain (SS11) used in this study was isolated from the feces of pigs from Santa Elena in Yucatán, Mexico.23 _{The strain was isolated after} pre-enrichment with EC broth (Difco, Detroit, MI, USA) using the immunomagnetic separation procedure (IMS) employ-ing anti-E. coli O157 Dynabeads (Dynal Biotech, Norway) asdescribedbythemanufacturer.TheIMS-enriched bacte-rial culture (50␮L) was streaked onto Chromocultcoliform agar(Merck,Darmstadt, Germany)and SorbitolMacConkey agar(CM813; OxoidLtd,USA)supplementedwith cefixime-potassiumtellurite(SR172;OxoidLtd.).Thephenotypeofthe microorganismwas characterizedwiththe API 20Esystem (BioMérieux, France). Strain serotype O157:H7 was identi-fiedwiththeimmunocapturesystemandlatexagglutination (TecraDiagnostics, NSW,Australia). Polymerasechain reac-tion(PCR)wasusedtoconfirmthepresenceofrfbO157and stx1 genes in the strain SS11 and to confirm the absence of stx2 verotoxin gene. For this purpose, three pairs of oligonucleotideprimerswere synthesizedforthe amplifica-tion of rfbO157(420bp),stx2 (584bp) and stx1 (306bp).24–26 The PCR products were detected by 2% agarose gel-electrophoresisat100Vandstainingwith10␮g/mLethidium bromide.

ThesuspensionsofE.coli(104_{CFU/mL)werepreparedfor}

theinoculationofthevegetablesandfruits.Asinglecolony of E. coli O157:H7 from a streak plate was suspended in 20mLofECselectiveenrichmentbroth(Difco)andincubated overnightat37◦C(16–18h)inashakingincubatorat150rpm

(Thermo Scientific,USA). The overnight culture(1mL) was inoculatedinto99mLofECbrothandincubatedat37◦Ctill itattainedtheexponentialphase(105_{CFU/mL).Thebacterial}

cellconcentrationswereestimatedbycomparingthe trans-mittanceofthecellsuspensionwithapreviouslydetermined standardcurveat540nmusingaUV–visible spectrophotome-ter.Thefinalconcentration of104_{CFU/mLwas obtainedby}

diluting the bacterial suspensionwith 400mL ofEC broth, and the concentration of the inoculum was further con-firmedbyplating 0.1mLofanappropriatelydilutedculture onTSA.

Selectionandpreparationofvegetablesandfruits

ThepreparedE. coliO157:H7 suspensionwas inoculatedin triplicateonthreevegetablesviz.,radish(Raphanussativus), beet(Betavulgarissubsp.vulgaris),andonion(AlliumcepaL.), andonthreefruitsviz.riperedtomato(Lycopersicon esculen-tumL.),cucumber(Cucumissativus),andgreenhabanerochili (CapsicumchinenseL.).Freshandhealthy-lookingfruitand veg-etablesampleswerepurchasednomorethanonedayprior to the inoculationfrom alocal supermarket and storedat 10◦Ctilluse.Thesampleswithvisibledefectsonthesurface suchasbruises,cuts,or abrasionswere excludedfrom the experiment.Thevegetablesweredecontaminatedby immers-ingin200mg/Lofchlorinesolution(6.5mg/Loffreechlorine) for2minbeforeinoculation.Thedecontaminationprocedures werepreviouslystandardizedbasedontheabsenceofgrowth ofEnterobacteriaceaeonMacConkeyagar.Thesampleswere thenrinsedwithsteriledeionizedwaterfor1minandairdried withpapertowelsinabiosafetycabinet(Esco,Infinity®Class II,Australia).

GrowthofE.coliO157:H7ontheepicarplayeroffruits

andvegetables

Thesamplesweretreatedforfivedifferentperiods(0,3,5, 7and9h)ofincubationtodeterminetheadhesioncapacity of the E. coli O157:H7. The samples were completely sub-mergedinto400mLofE.coliO157:H7broth(104_CFU/mL)for

30minandgentlyshakenat100rpmat37◦C.Thenthe sam-pleswerekeptindividuallyinsterileairbubblepolyethylene bags(Stomacher®Bag,Seward,UK)forthedefinedperiodof timeforeachtreatment.Afterincubation,thesampleswere washedthricewithsteriledeionizedwaterandairdriedina biosafetycabinet.Inaddition,twocontrolsampleswere con-sidered. Thefirstonewasaninternalcontrolofinoculated samplesthatwerenotsubjectedtothefinalwashes,and sec-ondonewasanegativecontrolofinoculatedsamplesthat were decontaminatedwith200mg/Lofchlorinesolutionin thefinalwash.

TheadhesionofE.coliO157:H7totheepicarp of vegeta-blesandfruitswasdeterminedbyspreadingthesampleson MacConkeyagar.Threeslicesofthevegetableshell(10g)were cutoffasepticallyandsoakedin90mLofphosphatebuffered saline(PBS,pH7.1).Afterafive-foldserialdilution,0.1mLof the samplesolution wasinoculated onMacConkeyagar in duplicateandincubatedfor24hat35±1◦C.Thenumberof coloniesobservedintheplateswasusedtocalculatetheCFU/g oftissue.

Proximateanalysis

AOAC (Association of Analytical Communities) official methods27 _{were used for analysis of moisture (Method} 925.09), total protein (Method 950.48), fat (Method 983.23), and ash contents ofthe samples(Method 930.05).pH val-ueswere measured byplacingtheflat electrode(Sensorex, Stanton, USA) directly against the surface of the exposed tissue at three different points. Proximal analysis of the epicarp was performed using the skin of the fruits and vegetables.

Scanningelectronmicroscopy(SEM)

TheadhesionofE. coliO157:H7 totheexocarp ofthe sam-ples was visualized using a scanning electron microscope (PhilipsmodelXL30SEMmicroscope)operatingatan accel-erating voltageof15kV.A finalpressure of4Torr (533.3Pa) was gradually generated in the chamber in order to pre-vent the samples from drying, prior to performing the analysis.28

Modelingofmicrobialgrowth

Alltheexperimentswereperformedintriplicate.The micro-bialcountsper gramofthesamplewereconvertedtolog10

valuesbeforethecalculations.Themathematicalmodeling ofthemicrobial growth (predicted data)wasperformed by employingthemodifiedGompertzequation:

L(t)=A+Ce−e(−B(t−M)),

where L(t) is the log of the number of bacteria at time ‘t’ (in hour). A is the asymptotic log of number of bacte-riaas‘t’decreasesindefinitely;Cistheasymptoticamount of growth (log number) that occurs as ‘t’ increases indef-initely; M is the time (in hour) at which the absolute growthrateisatmaximum;andBistherelativegrowthat time‘M’.29,30

The equation was fitted using a nonlinear regression routineoftheSYSTATstatisticalcomputer systempackage (SYSTATInc.,Evanston,IL,USA).Thealgorithmwasselected from the set of parameters with the lowest residual sum of squares with a 95% confidence interval. The Gompertz parameters (A, B, C, and M) were subsequently used to calculate the growth kinetics, including the exponential growth rate, EGR (logCFU/g/h)=BC/e; the lag-phase dura-tion,LPD(h)=M−1/B;thegenerationtime,GT(h)=log(2)e/BC; and the maximum population density, MPD (logCFU/g) =A+C.

Statisticalanalysis

Ananalysisofvariance(ANOVA)wasperformedtocompare thegrowthparametersofE.coliO157:H7obtainedfromthe growthcurves.TheANOVAandleastsignificancedifference (LSD)betweenthemeanswerecalculatedat95%confidence levelusingtheSYSTATsoftware.Themeansthatdifferedfrom the calculated LSDwere considered statisticallysignificant (˛=0.05).

Results

and

discussion

Thisstudy investigatedthe quantificationofthe growthof E.coliO157:H7onvegetablesandfruits.Althoughthemajor diseaseoutbreaksattributedtothispathogenhavebeen asso-ciatedwithleafyvegetables,theresultsofthisstudyshowed thatE.coliO157:H7isalsocapableofadheringtovegetables with different physicochemical characteristics (Fig. 1). The methodology used in this study forthe adhesion ofE. coli O157:H7wasvalidatedthroughinternalcontrols.The bacte-rial cellthatadhered tothe surfaceofthe samplesformed amatrixwiththeepidermallayer(biofilm),whichprovided them adequate support tocounter the effects ofwashing. E. coli O157:H7 strain possesses several fimbrial and non-fimbrial adhesinswhichusuallydonotplayany role inits pathogenesis,butareinvolvedwithitsadhesiontosurfaces.31 For example,curli-expressingthin-aggregative fimbriae are responsibleforbindingtoeukaryoticextracellularmatrix pro-teinsandpromotingtheformationofE.coliO157:H7biofilms oninertsurfaces.32,33_{AccordingtoTorresetal.,}31_the forma-tionofbiofilmsincreasestheprobabilityofsurvivalofE.coli O157:H7,andhence,facilitatesitsprotectionagainstadverse environmentalconditions.Thesamplesfromthevegetables andfruitsthatwerenotsubjectedtothefinalwashpriorto plating(internalcontrol)recordedhigherbacterial concentra-tion thanthosethat werewashed,suggestingthepresence ofboth adheredand freemicroorganism onthe surfaceof thesamples.TheSEMimagesofthesamplesobtainedafter thefinalwashingclearlyshowedalayerofcapsuleboundto thecellwallofthesamplesthatcouldpossiblybeassociated withtheexopolysaccharidessecretedbythemicroorganism toformthematrixstructure(Fig.1DandE).

TheGompertzfunctionfitswellfortheobserveddatawith thesumofsquaresbeinglessthan1inallthecases(Fig.2). Despite of the different intrinsic factors of the vegetables andfruitsusedinthisstudythatcouldinfluencethegrowth ofhumanpathogenicmicroorganisms,nosignificant differ-encewasfoundinthelag-phaseduration(LPD=2.17±1.03h; mean±S.E.),generationtime(GT=0.47±0.07h),or exponen-tial growthrate(EGR=0.72±0.12logCFUmL/h)ofthe E.coli O157:H7adheredtothesamples(Table1).Additional informa-tionontheproximatecompositionofthevegetablesandfruits underinvestigation areshowninTable2.These valuesare comparabletothosereportedbyKimetal.,34_{forperillaleaves} inoculated with E. coli O157:H7 at 36◦C (LPD=2.40±0.32h; EGR=0.55±0.11CFU/mL/h).ThegrowthoftheE.coliO157:H7 strainontheepicarpofthevegetablesandfruitswasslower than thatobserved under controlledlaboratory conditions. ThegrowthmodelofBuchananandKlawitter,35_which con-sideredthesametemperatureandpHconditionsemployed inthis study (35±1◦C;pH6.0)displayed fasterkinetic val-uesthanthoseobservedinthisstudy(LPD=1.54h;GT=0.33h; andEGR=0.92logCFU/mL/h).Thiscouldbeanoutcomeofthe variationintheinteractionsthatoccurbetweenthesubstrate andpathogenwhichmightnotnecessarilyoccurinagrowth medium.Similarly,thedurationofthelagphaseisdependent onawidevarietyoffactors,includingtheinitialconcentration oftheinoculum,thetimerequiredtorecoverfromthephysical damageorshockduetothetransfer,andthetimerequiredfor

A

B

C

E

D

F

Fig.1–ScanningelectronmicrographsdemonstratingtheadhesionofEscherichiacoliO157:H7ontheepicarpofvegetables

andfruits,(A)beet,Betavulgarissubsp.vulgaris(1550×);(B)habaneroChili,CapsicumchinenseL.,(1810×);(C)tomato,

Lycopersiconesculentum(1500×);(D)radish,Raphanussativus(4499×);(E)cucumber,Cucumissativus(3200×);(F)onion,Allium

cepaL.(1635×).

thesynthesisofessentialcoenzymesordivisionfactorsand otherenzymesinvolvedinthemetabolismofthesubstrates presentinthemedium.36 _{Ingeneralwhenpathogens} colo-nizeplantsurfaces,theyarerequiredtoevadevariousdefense mechanismsofthehostplants.Plants possessavarietyof compleximmuneresponses,whichenablethemtodetectand initiatecountermeasurestocontrolundesiredproliferationof foreignsubstances.37_{Inaddition,ethylene,aplanthormone,} playsavitalroleinthe defenseagainstplantpathogens.38 Manymicroorganismsconsiderablyincreasetheexpressionof ethylene;howeverthetranscriptionlevelofethyleneinduced byE.coliO157wereten-foldlower,suggestingthateitherthe plantsdonot sensethe presenceof themicroorganism or thatthemicroorganismevadethedefensemechanismsofthe plantsconsequentlyinhibitingtheactivationofethylene.39

E. coli O157:H7 growth was lower in onion than other samples suggesting an interaction that is not completely

favorablebetweenthemicroorganismandtheepicarpofthe vegetable.Inthecaseoftomatoandcucumber,thetimefor the microorganismtoreach the maximumgrowth rate (M) wassignificantlylongerthanthatobservedforothersamples; themaximumpopulationdensities(MPD)forthesevegetables werealsothelowest(Table1).Thevariationinthesizeofthe bacterialpopulationsamongthedifferentspecieshasbeen attributedtoa rangeofplantcharacteristics,including the leafwatercontent,pH,andthepresenceofbacterialinhibitory compounds.Inaddition,itoccursonlywhenthewater pres-sureonthesurfaceoftheproductovercomestheinternalgas pressure,andalsoitdependsonthehydrophobicnatureof thesurface.40–42_{E.coligrowsinabroadpHrangeof4.4–10.0} withanoptimumpHof6–7.43_{Specifically,E.coliO157:H7can} tolerateharshacidicconditions,andfewotherstrainshave beenreportedtobeabletosurviveevenatpH2.5–3.0forover 4h.44_{ArnoldandKaspar,}45_{foundthatE.coliO157:H7becomes}

2.00 2.50 3.00 3.50 4.00 4.50 5.00 5.50 6.00 6.50 7.00 0 3 5 7 9 E . co li O 157 :H 7 ( Log CF U/mL ) Hours

A

2.00 2.50 3.00 3.50 4.00 4.50 5.00 5.50 6.00 6.50 7.00 0 3 5 7 9 E. c o li O 157 :H 7 ( Log CF U/mL ) Hours

B

2.00 2.50 3.00 3.50 4.00 4.50 5.00 5.50 6.00 6.50 7.00 0 3 5 7 9 E. c o li O 157 :H 7 ( Log CF U/mL ) Hours

C

2.00 2.50 3.00 3.50 4.00 4.50 5.00 5.50 6.00 6.50 7.00 0 3 5 7 9 E. c o li O 157 :H 7 ( Log CF U/m L ) Hours

D

2.00 2.50 3.00 3.50 4.00 4.50 5.00 5.50 6.00 6.50 7.00 0 3 5 7 9 E. c o li O 157 :H 7 ( Log CF U/mL ) Hours

E

2.00 2.50 3.00 3.50 4.00 4.50 5.00 5.50 6.00 6.50 7.00 0 3 5 7 9 E. c o li O 157 :H 7 ( Log CF U/mL ) Hours

F

Fig.2–ObservedandpredicteddataforEscherichiacoliO157:H7adheredtotheepicarpofvegetablesandfruits,(A)tomato;

(B)radish;(C)cucumber;(D)beet;(E)habanerochili;(F)onion(:internalcontrol;:predicteddata;:observeddata).

moretoleranttoacidwhenitisinthestationarygrowthphase orisstarvedduringthelog-phaseofgrowth.ThesystemicpH ofthevegetablesandfruitsusedinthisstudyisinasuitable rangesuitableforthegrowthofthepathogenic

microorgan-ism(Table2),tomatoandonionpossessthelowestpH(4.77

and5.40,respectively).ThissuggeststhatthepHintomatoes mighthaveinfluencedthegrowthrateofE.coliO157:H7 dur-ingthelog-phase,asthepathogentooklongertoreachthe maximumgrowthrate(M)intomatoes(Fig.2,Table1).This findingisinagreementwithastudybyBeuchat46_withripe tomatoes,whichreportedthatthepHrangeof3.9–4.4present intheripetomatoespreventsorretardsbacterialgrowth.Del RosarioandBeuchat47_{alsosuggestedthatvegetablesandfruit} juiceswithapHvaluelessthan4.0are,ingeneral,notgood substratestosupportthegrowthofE.coliO157:H7.

Other researchers have shown that MPD decreases sig-nificantlywhentheconcentrationofpreservativesincreases orwhen naturalmicrobial compoundsare present.48 Asin the caseof cucumberthe volatile oilsand the methanolic

and dichloromethaneextracted from the peel ofC. sativus exhibitedantimicrobialactivity.49_Choetal.,50_reportedthat thehighcontentofthevolatilessuchas2,6-nonadienaland 2-nonenal show strong activity against E. coli O157:H7. In contrast to this, there are no reports on the presence of antimicrobial compoundsagainsthumanpathogens in the skinoftomatoand onion.Althoughnotspecificallyagainst E.coliO157:H7,butstillsomestudieshavereported antimicro-bialactivityofonionaqueousextractsagainstgram-negative microorganisms.51 _{However, Rounds et al.,}52 _{showed that} powderedonioninducedthegrowthofE.coliby1.2logCFU/g, indicatingthatonion,likeothervegetables,providesnutrients thatenhancethegrowthofbacteria.Nevertheless,thereisno conclusiveexplanationonthedelayofthegrowthrateofE. coliO157:H7onthosevegetables.

In conclusion, the predictive models presented in this studyofferagoodcomparativeaccountofthegrowthofE.coli O157:H7onsampleswithdifferentintrinsicfactors demon-stratinglowselectivityofthepathogen.Thedatasuggestthat

Table1–MathematicalmodelingofthegrowthofE.coliO157:H7adheredtotheepicarpofvegetablesandfruits.The Gompertz(A,B,C,M)andderivedparameters(GT,generationtime;EGR,exponentialgrowthrate;LPD,lag-phase duration;MPD,maximumpopulationdensity)areshown.Dataarepresentedasmean±standarderror(n=3).Theletters betweenthecolumnsindicatesignificantdifferences(p<0.05).

A(logCFU/mL) Bgrowth(h) C(logCFU/mL) M(h) GT(h) EGR(log

CFU/mL/h)

LPD(h) MPD(log CFU/mL) Mean SE Mean SE Mean SE Mean SE Mean SE Mean SE Mean S.E Mean S.E Tomato 2.819 0.12 a 1.498 0.60 a 2.060 0.59 a 3.631 0.03 a 0.398 0.11 a 0.881 0.23 a 2.895 1.53 a 4.279 0.54 a Beet 2.139 0.12 b 1.684 1.30 a 2.729 1.20 a 2.009 0.38 b 0.562 0.03 a 0.540 0.03 a 2.230 1.50 a 4.868 0.27 b Radish 2.507 0.15 a 1.431 0.63 a 2.537 0.80 a 2.032 0.13 b 0.336 0.07 a 0.968 0.18 a 1.026 0.37 a 5.044 0.69 b Chili 2.573 0.15 a 1.344 0.92 a 2.351 1.00 a 2.066 0.55 b 0.632 0.07 a 0.491 0.06 a 2.709 1.30 a 4.924 1.05 b Cucumber 2.742 0.05 a 1.790 0.84 a 1.971 0.79 a 3.351 0.17 a 0.413 0.10 a 0.806 0.16 a 1.966 0.69 a 4.314 0.85 a Onion 2.804 0.03 a 1.286 0.59 a 1.921 0.61 a 2.961 0.06 a 0.480 0.06 a 0.647 0.08 a 2.193 0.74 a 4.125 0.63 a F-value 5.193 0.053 0.147 3.766 1.953 1.847 0.349 4.052 p-value 0.009 0.998 0.977 0.045 0.159 0.178 0.873 0.048

Table2–Epicarpproximalcomposition(%)ofvegetables andfruits(n=5).

Moisture Ash Crudefiber pH Mean SE Mean SE Mean SE Mean SE Tomato 95.4 0.23 0.74 0.04 0.57 0.22 4.77 0.08 Beet 81.6 0.25 1.69 0.13 1.11 0.24 6.44 0.07 Radish 90.0 0.76 1.34 0.37 0.65 0.08 7.03 0.04 Chili 92.3 0.35 0.76 0.01 2.98 0.19 6.30 0.40 Cucumber 94.9 0.17 0.64 0.03 1.25 0.07 7.08 0.11 Onion 95.1 0.27 0.27 0.03 0.47 0.04 5.40 0.60

selectivityofthebacterium,E.coliO157:H7strainSS11slightly dependsontheintrinsicfactorsofthevegetablesandfruits understudy,withtheclearexceptionoftomato,cucumberand onion,whereinthedelayinthegrowthofthepathogenwas significant.However,itisimportanttonotethatthegrowth

modelsconstructedunder controlledenvironmental

condi-tions offer limited insightsand may vary widely from the

observedprofileof vegetables and fruitsinthe field, since thisstudywasdesignedtoinoculatethesampleswithE.coli

O157:H7withoutthe presenceofcompetitive microfloraon

thesurface.Alowerconcentrationofthemicroorganismcould alsobeexpectedinthefieldthanthatwasemployedinthe study inthe laboratory. Theresultsreported byBrackett,52 suggestedthatreducingthenativemicrobialpopulationsby washingandsanitizingorbycontrolledatmospherestorage, enablesproliferationofhumanpathogensonthesurfaceof vegetables.Thereductioninthesurfacepopulationsreduces competitionforspaceandnutrientstherebyprovidinggrowth potentialfor pathogenic contaminants. Intheory, this sce-nariocanresultinanunspoiledproductthat isunsafefor consumption.Futureresearchinthisfieldshouldaimto ver-ify the potentialfor adhesionand internalization of E. coli O157:H7 in the tissues of vegetables under realistic field conditions.

Conflicts

of

interest

Theauthorsdeclarenoconflictsofinterest.

Acknowledgement

The authors are grateful to Ana Ruth Christopher Ramos from CINVESTAV-Mérida (México) for technical assistance withthescanningelectronmicroscopy.WealsothankDaniela A.AcevedoOjedafromUMMforassistancewithPCRanalysis. ThisprojectwassupportedbyFOMIX-CONACYT–Gobierno delEstadodeYucatán(México),grantno.108103.

r

e

f

e

r

e

n

c

e

s

1.JeterC,MatthysseAG.Characterizationofthebindingof

diarrhea-genicstrainsofE.colitoplantsurfacesandtherole

ofcurliintheinteractionofthebacteriawithalfalfasprouts.

MolPlantMicrobeIntract.2005;18:1235–1242.

2.MacarisiD,PatelJ,BauchanG,GironJA,SharmaVK.Roleof

curliandcelluloseexpressioninadherenceofEscherichiacoli

O157:H7tospinachleaves.FoodbornePathogDis.

2012;9:160–167.

3.KnuttonS.Electronmicroscopicalmethodsinadhesion.

MethEnzymol.1995;253:145–158.

4.Xicohtencatl-CortesJ,Sánchez-ChacónE,SaldanaZ,FreerE,

GironJA.InteractionofEscherichiacoliO157:H7withleafy

greenproduce.JFoodProtect.2009;72:1531–1537.

5.JohannessenGS,BengtssonGB,HeierBT,BredholtS,

WastesonY,RorvikLM.PotentialuptakeofEscherichiacoli

O157:H7fromorganicmanureintocrispheadlettuce.Appl

EnvironMicrobiol.2005;71:2221–2225.

6.FranzE,VisserAA,vanDiepeningenAD,KlerksMM,

TermorshuizenAJ,vanBruggenAH.Quantificationof

contaminationoflettucebyGFP-expressingEscherichiacoli

O157:H7andSalmonellaentericaserovarTyphimurium.Food

Microbiol.2007;24:106–112.

7.GrantJ,WendelboeAM,WendelA,etal.Spinach-associated

EscherichiacoliO157:H7outbreak,UtahandNewMéxico.

EmergInfectDis.2008;14:1633–1636.

8.HarapasD,PremierR,TomkinsB,FranzP,AjilouniS.

PersistenceofEscherichiacolioninjuredvegetableplants.IntJ

FoodMicrobiol.2009;138:232–237.

9.VijayakumarC,Wolf-HallCE.Evaluationofhousehold

sanitizersforreducinglevelsofEscherichiacolioniceberg

10.SinghPK,ParsekMR,GreenbergEP,WelshMJ.Acomponent

ofinnateimmunitypreventsbacterialbiofilmdevelopment.

Nature.2002;417:552–555.

11.BoyerRR,SumnerSS,WilliamsRC,PiersonMD,PophamDL,

KnielKE.InfluenceofcurliexpressionbyEscherichiacoli

O157:H7onthecell’soverallhydrophobicity,charge,and

abilitytoattachtolettuce.JFoodProtect.2007;70:1339–1345.

12.ChangJM,FangTJ.SurvivalofEscherichiacoliO157:H7and

SalmonellaentericaserovarsTyphimuriuminiceberglettuce

andtheantimicrobialeffectofricevinegaragainstE.coli

O157:H7.FoodMicrobiol.2007;24:745–751.

13.McEvoyJL,LuoY,ConwayW,ZhouB,FengH.Potentialof

EscherichiacoliO157:H7togrowonfield-coredlettuceas

impactedbypostharveststoragetimeandtemperature.IntJ

FoodMicrobiol.2009;128:506–509.

14.SeltzerJ,RushJ,KinseyJ.NaturalSelection:2006E.coliRecallof FreshSpinach.ACaseStudybytheFoodIndustryCenter.The FoodIndustryCenter,UniversityofMinnesota;2015. Availablefrom:http://ageconsearch.umn.edu/bitstream/

54784/2/Natural%20Selection.pdf.Accessed30.08.09.

15.WendelAM,JohnsonDH,SharapovU,etal.Multistate

outbreakofEscherichiacoliO157:H7infectionassociatedwith

consumptionofpackagedspinach,August–September2006:

theWisconsininvestigation.ClinInfectDis.

2009;48:1079–1086.

16.EthelbergS,LisbyM,BottigerB,etal.Outbreaksof

gastroenteritislinkedtolettuce,Denmark,January2010.

EuroSurveill.2010;15:6.

17.CentersforDiseaseControlandPrevention(CDC).Multistate OutbreakofShigaToxin-producingE.coliO157:H7Infections LinkedtoReady-to-EatSalads;2015.Availablefrom:

http://www.cdc.gov/ecoli/2013/O157H7-11-13/index.html.

Accessed30.08.13.

18.HanY,ShermanDM,LintonRH,NielsenSS,NelsonPE.The

effectsofwashingandchlorinedioxidegasonsurvivaland

attachmentofEscherichiacoliO157:H7togreenpepper

surfaces.FoodMicrobiol.2000;17:521–533.

19.OngengD,MuyanjaC,GeeraerdAH,SpringaelD,RyckeboerJ.

SurvivalofEscherichiacoliO157:H7andSalmonellaenterica

serovTyphimuriuminmanureandmanure-amendedsoil

undertropicalclimaticconditionsinSub-SaharanAfrica.J

ApplMicrobiol.2011;110:1007–1022.

20.AbadiasM,AlegreI,OliveiraM,AltisentR,Vi ˜nasI.Growth

potentialofEscherichiacoliO157:H7onfresh-cutfruits(melon

andpineapple)andvegetables(carrotandescarole)stored

underdifferentconditions.FoodControl.2012;27:37–44.

21.ForslundA,EnsinkJHJ,MarkussenB,etal.Escherichiacoli

contaminationandhealthaspectsofsoilandtomatoes

(SolanumlycopersicumL.)subsurfacedripirrigatedwith

on-sitetreateddomesticwastewater.WaterRes.

2012;46:5917–5934.

22.ChangWS,Afsah-HejriL,RukayadiY,etal.Quantificationof

EscherichiacoliO157:H7inorganicvegetablesandchickens.

IntFoodResJ.2013;20:1023–1029.

23.GullianM,Duran-CasanovaG,Isla-EsquivelML, Suárez-WeganE,Alarcón-SánchezA.Estudiodefactores predisponentesdeenfermedaddiarreicaagudaenla comunidaddeSanSimón,Yucatánenbaseaunanálisisde vulnerabilidadnutricionalyambiental.PoblaciónySaluden Mesoamérica.2011;9(5).Availablefrom:http://revistas.

ucr.ac.cr/index.php/psm/article/view/733.

24.CebulaTA,PayneWL,FengP.Simultaneousidentificationof

strainsofEscherichiacoliserotypeO157:H7andtheir

Shiga-liketoxintypebymismatchamplificationmutation

assay-multiplexPCR.JClinMicrobiol.1995;33:248–250.

25.MaurerJJ,SchmidtD,PetroskoP,SanchezS,BoltonL,Lee

MD.DevelopmentofprimerstoO-antigenbiosynthesis

genesforspecificdetectionofEscherichiacoliO157byPCR.

ApplEnvironMicrobiol.1999;65:2954–2960.

26.SonI,BinetR,Maounounen-LaasriA,LinA,HammackTS,

KaseJA.DetectionoffiveShigatoxin-producing

EscherichiacoligeneswithmultiplexPCR.FoodMicrobiol.

2014;40:31–40.

27.AOAC.OfficialMethodsofAnalysisofAOACInternational.18th

ed.Gaithersburg,Maryland20877-2417,USA:AOAC

International;2005.

28.ElAbedS,Koraichi-IbnsoudaS,LatracheH,HamadiF. Scanningelectronmicroscopy(SEM)andenvironmental SEM:suitabletoolsforstudyofadhesionstageandbiofilm formation.In:KazmirukV,ed.ScanningElectronMicroscopy. 2012,http://dx.doi.org/10.5772/34990[chapter35].Available from: http://www.intechopen.com/books/scanning-electron- microscopy/scanning-electron-microscopy-sem-and-

environnmental-sem-suitable-tools-for-study-of-adhesion-stage-a.Accessed30.08.15.

29.GibsonAM,BratchellN,RobertsTA.Predictingmicrobial

growth:growthresponseofsalmonellaeinalaboratory

mediumasaffectedbypH,sodiumchlorideandstorage

temperature.IntApplMicrobiol.1988;6:155–178.

30.ZwieteringMH,JongenburgerI,RomboutsFM,van’tRictK.

Modellingofthebacterialgrowthcurve.ApplEnviron

Microbiol.1990;56:1875–1881.

31.TorresAG,JeterC,LangleyW,MatthysseAG.Differential

bindingofEscherichiacoliO157:H7toalfalfa,human

epithelialcells,andplasticismediatedbyavarietyof

surfacestructures.ApplEnvironMicrobiol.2005;71:8008–8015.

32.CooksonAL,CooleyWA,WoodwardMJ.Theroleoftype1

andcurlifimbriaeofShigatoxin-producingEscherichiacoliin

adherencetoabioticsurfaces.IntJMedMicrobiol.

2002;292:195–205.

33.RyuJH,KimH,FrankJF,BeuchatLR.Attachmentandbiofilm

formationonstainlesssteelbyEscherichiacoliO157:H7as

affectedbycurliproduction.LettApplMicrobiol.

2004;39:359–362.

34.KimJ,RoE,YoonK.Comparisonofgrowthkineticsofvarious

pathogenicE.colionfreshperillaleaf.Foods.2013;2:364–373.

35.BuchananRL,KlawitterLA.Theeffectofincubation

temperature,initialpH,andsodiumchlorideonthegrowth

kineticsofEscherichiacoliO157:H7.FoodMicrobiol.

1992;9:185–196.

36.SchultzD,KishonyR.Optimizationandcontrolinbacterial lagphase.BMCBiol.2013;11:120.Availablefrom:

http://www.biomedcentral.com/1741-7007/11/120.Accessed

30.08.15.

37.BergerCN,SodhaSV,ShawRK,etal.Freshfruitand

vegetablesasvehiclesforthetransmissionofhuman

pathogens.EnvironMicrobiol.2010;12:2385–2397.

38.BroekaertWF,DelaureSL,DeBolleMF,CammueBP.Therole

ofethyleneinhost–pathogeninteractions.AnnuRevPlant

Physiol.2006;44:393–416.

39.ThilmonyR,UnderwoodW,HeSY.Genomewide

transcriptionalanalysisoftheArabidopsisthaliana

interactionwiththeplantpathogenPseudomonassyringaepv.

tomatoDC3000andthehumanpathogenEscherichiacoli

O157:H7.PlantJ.2006;43:34–53.

40.BurnettSL,ChenJ,BeuchatLR.AttachmentofEscherichiacoli

O157:H7tothesurfaceandinternalstructuresofapplesas

detectedbyconfocallasermicroscopy.ApplEnvironMicrobiol.

2000;66:4679–4687.

41.YadavV,MandhanR,DaburR,ChhillarAK,GuptaJ,Sharma

GL.AfractionfromEscherichiacoliwithanti-Aspergillus

properties.JMedMicrobiol.2004;54:375–379.

42.ZhuangRY,BeuchatLR,AnguloFJ.FateofSalmonella

montevideoonandinrawtomatoesasaffectedby

temperatureandtreatmentwithchlorine.ApplEnviron

Microbiol.1995;61:2127–2131.

43.DesmarchelierPM,FeganN.EnteropathogenicEscherichiacoli.

Significance.Sydney:AustralianInstituteofFoodSciandTech

(NSWBranch);2003:267–310[chapter9].

44.MolinaPM,ParmaAE,SanzME.Survivalinacidicmediumof Shigatoxin-producingEscherichiacoliO157:H7and

non-O157:H7isolatedinArgentina.BMCMicrobiol.2003;3:17. Availablefrom:

http://www.biomedcentral.com/1471-2180/3/17.Accessed30.08.15.

45.ArnoldKW,KasparCW.Starvation-and

stationary-phase-inducedacidtoleranceinEscherichiacoli

O157:H7.ApplEnvironMicrobiol.1995;61:2037–2039.

46.BeuchatLR.Ecologicalfactorsinfluencingsurvivaland

growthofhumanpathogensonrawfruitsandvegetables.

MicrobInfect2002.2002;4:413–423.

47.DelRosarioBA,BeuchatLR.Survivalandgrowthof

enterohemorrhagicEscherichiacoliO157:H7incantaloupe

andwatermelon.JFoodProt.1995;1:105–107.

48.AndrésSC,GiannuzziL,ZaritzkyNE.Theeffectof

temperatureonmicrobialgrowthinApplecubespackedin

filmandpreservedbyuseoforangejuice.IntJFoodSci

Technol.2004;39:927–933.

49.SotiroudisG,MelliouE,SotiroudisTG,ChinouI.Chemical

analysis,antioxidantandantimicrobialactivityofthree

Greekcucumber(Cucumissativus)cultivars.JFoodBiochem.

2010;34:61–78.

50.ChoIH,ChoiES,LimHG,LeeHH.Purificationand

characterizationofsixfibrinolyticserine-proteasesfrom

earthwormLumbricusrubellus.JBiochemMolBiol.

2004;37:199–205.

51.AzuN,OnyeagbaR.Antimicrobialpropertiesofextractsof Alliumcepa(Onions)andZingiberofficinale(Ginger)on Escherichiacoli,SalmonellatyphiandBacillussubtilis.JTropMed. 2006;3(2).Availablefrom:http://ispub.com/IJTM/3/2/11056. Accessed30.08.15.

52.RoundsJM,RigdonCE,MuhlLJ,etal.Non-O157Shiga

toxin-producingEscherichiacoliassociatedwithvenison.

EmergInfectDis.2012;18:279–282;

BrackettRE.Shelfstabilityandsafetyoffreshproduceas

influencedbysanitationanddisinfection.JFoodProt.