batchand 11 HTSTpasteurisationexperiments usingraw milkor ultra-heat
treatment (UHT)milksamples spiked withM. paratuberculosis.E. coliandM.
bovisBCGstrainswereusedas controls.No survivors weredetected fromany
of the slantsor brothscorrespondingto the 7 regularbatch, but survivors were
detected in 2 of the 11 HTSTexperiments.No survivors weredetectedafterheat
treatment for 15 min at 63 ÎC. TheseresultsindicatethatM. paratuberculosis
maysurviveHTSTpasteurisation (Gaoet al., 2002).
3.3 Biofilms and microbial contaminationin foodprocessing.
Prolongedor persistentcontamination of someListeriamonocytogenesstrains,
whichmeansthat they havecausedfoodplant contamination for longperiodsof
up to severalyears,has beenreportedin severalfoodindustryareas,e.g. meat,
poultry, fish,dairyand freshsauces (Miettinenet al., 1999;Boruckiet al., 2003;
R ̆rviket al., 2003; Lunde¬n, 2004).EscherichiacoliandSalmonellaisolates are
also known to be persistentin foodand fish feedfactories(Holahet al., 2004;
M ̆retr ̆et al., 2003b).PersistentL. monocytogenesplant contaminationappears
to be the result of the interactionof severaldifferentfactors. Propertiesthat
influence survival, includingenhanced adherenceto foodcontactsurfaces and
adaptation to disinfectants,in addition to suchpredisposingfactors in the
processing line as complex processingmachines and poorzoning mayleadto
persistentplant contamination(Lunde¬n, 2004).Theeradicationof persistent
contaminationofL. monocytogeneshas beenshown to be difficult but not
impossible.Targeted and improvedsanitation has led to successful eradication
(Miettinenet al., 1999).
In studiesperformedby Lunde¬n (2004),persistentL. monocytogenesstrains
wereobserved to adhereto stainlesssteelsurfaces in higher cell numbersthan
non-persistent strains after short contact times. Such enhanced adherence
increases the likelihoodof the survivalof the persistentstrainsdue to increased
resistance againstpreventionmethodsand may havean effect on the initiation of
persistentplantcontamination. If the adherenceperiodof strainswas prolonged
thenthe adherencelevelof non-persistent strainswas closeto the adherence
levelof persistentstrains (Lunde¬n, 2004).The initialresistance of persistentand
non-persistentL. monocytogenesstrains to disinfectantsvaried,and the increase
in resistance was similar for persistent and non-persistent strains. The
concentrationsof disinfectants usedat foodprocessingplantswerenot reached
in the studiesperformed by Lunde¬n (2004). AlsoHolahet al.(2002)reported in
theirstudies that the resistance of persistentstrainsofL. monocytogenesandE.
colifoundin the foodindustry to the mostcommonlyused disinfectantswere
not significantlydifferentfromthe laboratorycontrolstrain. A study carriedout
by Earnshaw and Lawrence (1998)concludedthat it is unlikelythat the strains
that persistedin the poultryprocessingenvironment did so by meansof plasmid-
mediatedresistance to the commercialdisinfectants used.
Biofilmrisks 57