is commonin parts of the worldwheremilkis neitherboilednor pasteurised.It
occurs,but much less frequently, in developed countrieswhere the main
products implicated are pasteurisedmilk, powdered milkand certain cheeses
(Mead,1993). Formationof a biofilm by Salmonellaon various types of
surfaces usedin the foodprocessingindustryhas beenreported by several
groups(Mafuet al., 1990;Helkeet al., 1993;Ronner& Wong,1993;Josephet
al., 2001).Thesestudies haveshownthatSalmonellaspp. can formbiofilms on
foodcontactsurfaces and that the cells in biofilms are much moreresistantto
sanitisers compared to planktonic cells(Ronner& Wong,1993;Josephet al.,
2001;Stepanovicet al., 2003).Mokgatlaand co-workers(1998)studied the
resistance ofSalmonellasp. isolatedfroma poultryabattoir and foundout that it
will growin the presenceof in-useconcentrationsof hypochlorousacid. The
presenceofPseudomonasfluorescensin the biofilmresultedin the increased
resistance ofS.Typhimuriumto chlorine (Leriche& Carpentier, 1995).
3.2.2 Escherichiacolibiofilms
Escherichia coliis a Gram-negative,rod-shaped bacterium.Because many
microbes fromfaecesare pathogenicin animals and humans,the presenceof the
intestinal bacteriumE. coliin waterand foodsindicatesa potentialhygiene
hazard.MoststrainsofE. coliare harmless.However,a few strainswithwell-
characterised traits are known to be associated with pathogenicity
(Venkitanarayanan& Doyle,2003).Thoseof greatest concern in waterand
foodsare the intestinal pathogens,whichare classifiedinto five majorgroups:
the enterohaemorrhagicE. coli(EHEC), the enterotoxigenicE. coli(ETEC), the
enteroinvasiveE. coli(EIEC), the enteropathogenicE. coli(EPEC) and the
enteroaggregative E. coli (EAEC). Growthcan occur at 7±46ÎC with the
maximalgrowth rate at 35±37ÎC. The minimumawfor growthranges from0.94
and 0.97.The optimum pH for growthis approximately 7.0, witha minimum
and maximumpH for growthof 4.5 and 9.0. EHEChas beenshown to grow
poorlyat temperaturesof 44 ÎC (Venkitanarayanan& Doyle, 2003).
Escherichiacolihas beenisolatedfroma largenumber of foodsand drinks,
e.g. fermentedmeat sausage, dairyproducts, vegetables,meat, poultryand fish
products, waterand apple cider.Theseagents can causediarrhoealoutbreaks
(Junkins & Doyle,1993;Venkitanarayanan& Doyle,2003).Unpasteurisedmilk
is a commonvehicle ofE. coliO157:H7 transmissionto humans (Dontorouet
al., 2004).E. colican also survivefor extendedperiodsof timein several acidic
foods, e.g. cheese and yogurt. Acid-adapted E. coliO157:H7 has shown
enhanced survival and prevalence in biofilms on stainless steel surfaces
(Stopforthet al., 2003a,b; Venkitanarayanan & Doyle, 2003 ). In a hygiene
surveyperformed in the foodindustry by Holahet al.(2002),microbial strains,
e.g.E. coliandL. monocytogenes, were foundeitheron surfacesor in products
or in both,and someof thesestrainswerepersistent. Failleet al.(2002,2003)
foundout thatE. colicellswerepoorlyadheredto surfaces. The cells were
embeddedin the organic matrixof the biofilm, whichshowsthat the structureof
Biofilmrisks 51