encounteredin biofilms), probablynot activelymultiplyingat this solid±air
interface due to lackof moistureand nutrient.The opportunityfor biofilms to
develop on surfaceswouldbe minimal.The termbiofilm' is usedfrequently, and oftenincorrectly, in this context. Biofilms are defined, in simplestterms,asmatrix-enclosedbacterialpopula-
tionsadherentto eachotherand/or to surfaces or interfaces'(Verran,2002).The
interface most commonlyassociated withbiofilmformation is the solid±liquid
interface, withthe liquidflowingoverthe substratum (Costertonet al., 1995).In
foodprocessing,the presenceof a solid±liquid interface,withan elementof
liquidflowacross the surface, wouldbe limited, for example to production of
liquidfoods, and waterdistribution (anddisposal)systems,in predominantly
closed pipework(Holah and Gibson, 2000 ; Verran and Jones, 2000).It wouldbe
surprising, and undesirable, in the food production/processing context to
encounter a stagnant(non-flowing) supply of a liquidwhichcouldfunction as
microbial nutrient.Thus the testingof hygienic food contactsurfaces for
propensityto support biofilmand biofilmdevelopment,or for evaluationof
biofilm removalby cleaningagents mightnot be particularlyappropriate ±
depending on the environment of concern.
On many foodcontactsurfaces, the most commoninterfacepresentedwould
be solid±air: an opensystem. The typicalsolid±liquid interfacebiofilm couldnot
develop. Cellswouldbe immobilised on a hygienicsurface, unableto multiply
in the assumedrelative absence of food and water. Of course, in a food
processingplant,bothof theseare present in abundance,hencethe needto
continuallycleanand sanitizesurfaces.(Thedynamic accumulationof micro-
organismsat a solid±air interfaceis encounteredin otherenvironments where
the substratum itselfis not an inerthygienicmaterial, but providesan element of
nutrient.Humidity has a significantimpacton the survivaland multiplicationof
these attachedmicroorganisms.Biodegradationof artwork and textiles is a
consequence of these `sub-aerial' biofilms; Dornieden et al., 2000.) The
isolation of viable bacteriafromsuchsurfaces in the absence of growth indicates
theirability to survive.Most species, withthe exclusion of spores, die on
prolongedexposure on dry surfaces (McEldowney and Fletcher, 1988).Again,
organic material mayaid survivalby protecting organismsfromdesiccation,
physical removal, or cleaning/sanitizingagents(Verranet al., 2002),evenif it
doesnot providenutrientfor the microorganisms present.
Solid±liquid±air interfaces could be encountered frequently, as foodparticles,
or cleaning solutionspassacrossthe surfaces on whichmicroorganismsmaybe
attached.The accumulationof cellsat theseinterfaces is a well-recognized
phenomenon,and wouldprovide additionalconfoundingfactors of variability in
maintenance of hygienic status.
34.2.2 Testspecies
In the literature, foulingand cleanability testingfocuseson a relatively limited
rangeof bacteria:for example,Listeriamonocytogenes, in the meat and fish
558 Handbookof hygiene controlin the foodindustry