complex.The contaminationof the processingenvironmentappearsoftento be due
toL. monocytogenesstrainsalreadypresentin the plantenvironment(Hoffmanet
al., 2003),and not to continuousingressofL. monocytogenes. Therehaveto be,
therefore,otherfactorscausingcontaminationof the processingenvironment.
A veryparticular characteristicofL. monocytogenescontaminationin food
processing environments is the persistence of the contamination (R ̆rviket al.,
1995;Unnerstadet al., 1996;Miettinenet al., 1999; Lunde¬net al., 2003a;Autio
et al., 2003).L. monocytogeneshas beenobservedto occupyfoodprocessing
plantsfor monthsor evenyears.Even if some of the persistentcontaminations
can be explainedby continualingress, not all can. The persistentcontamination
has thus not beeneliminated withroutinesanitationmeasures. Moleculartyping
ofL. monocytogenesisolateshas shown that all or some of the genotypesfound
in raw materials are not seenin the post-heat treatmentareaor finalproducts
(R ̆rviket al., 1995; Nesbakkenet al., 1996;Autioet al., 1999; Tka¬cœikova¬et al.,
2000;Lunde¬net al., 2002;Nortonet al., 2001;Hoffmanet al., 2003). This
indicatesthat the persistentstrainsare true persisters, whichsurvivethe elimi-
nationproceduresin the foodprocessingplant.Therefore,two subpopulations
existin the food processing environment: persistent and non-persistent(or
transient) strains.This fact has led researchers to seekfurtherunderstanding of
the development of persistentstrains.Somefactorsassociated withpersistentL.
monocytogenesstrains are presentedin Table33.2.One veryimportantfactoris
the abilityto adhere to food contactsurfaces. This abilityis enhanced in
persistentstrains (Norwood and Gilmour, 1999; Lunde¬net al., 2000).
33.3.2 Listeria monocytogenescontaminationof products
The contamination of products withL. monocytogenesoccursmost typically
duringpost-processing(Fig.33.1).The processing environmentprovides several
harbourages forL. monocytogenes, which in turn serve as the source of
contaminationof finishedproducts (Autioet al., 1999;Nortonet al., 2001;
Lunde¬net al., 2002). Several contamination siteshave been identified in
molecular contamination analyses.
Table33.2 PropertiesofListeriamonocytogenesassociatedwithpersistentcontamination
Property Reference
Enhancedadherenceto food Norwoodand Gilmour(1999),Lunde¬net al.(2000)
contactsurfaces
Disinfectantresistance Aaseet al.(2000),Lunde¬net al.(2003b)
Cadmiumresistance Harveyand Gilmour(2001)
MonocinE production Harveyand Gilmour(2001)
Acidresistance Lunde¬n (unpublished)
Heatresistance Lunde¬n (unpublished)
Contaminationroutes and analysis in foodprocessing environments 545