extracellularmatrices, whichmainly consistof polysaccharidesand glyco-
proteins, and are called biofilms (Wirtanen, 1995).The microcolonyformation
is the first stagein biofilm formation, whichoccursundersuitableconditionson
any surface± both inertand living. Microbes can startup this formation when
thereis wateror moisture available(Bryers, 2000).Physical parameterssuchas
fluidflowrate,charge, hydrophobicityand micro-topography of the surface
material affectthe attachmentof cellsto the surface.Cellsmustovercome the
energy-intensive repulsion barrier, which affects the particlesurfaces (van
Loosdrechtet al., 1989).Bacteria withpili could conceivablyovercome this
barrier to achieve micro-colonisation and biofilm formation (Zottola &
Sasahara, 1994). It has beenfoundthattemperaturesbelow 50 ÎC promote
biofilmformation (Miller& Bott, 1982).
In the foodindustry, equipment designplaysthe mostimportantrolein
combating biofilm formations. The choice of materials and their surface
treatments as wellas roughness, e.g. grindingand polishing, are important
factors for inhibiting the formation of biofilm and making surfaceseasierto
clean.Treating surfacematerials so that theyrejectbiofilms can be performed
actively to removeor passively to retardbiofilmreoccurrence.The cleanliness
of surfaces, trainingof personneland goodmanufacturing and designpractices
are the mostimportanttoolsin combatingbiofilmproblems in the foodindustry
(Holah & Timperley,1999;Wirtanen,2002).
3.1.2 Biofilm formation on foodprocessingsurfaces
It is alsoimportantto rememberthat about85±96% of a biofilmconsistsof
water,whichmeans that only2±5%of the totalbiofilmvolumeis detectable on
dry surfaces(Costertonet al., 1981).Biofilm can generally be produced by any
microbes undersuitable conditions,althoughsomemicrobesnaturallyhavea
highertendencyto producebiofilmthanothers. A biofilmconsistsof microbial
cell clusters witha network of internalchannelsor voidsin the extracellular
polysaccharideand glycoprotein matrix(Carpentier& Cerf, 1993).Thisallows
nutrients and oxygento be transported fromthe bulkliquidto the cells(Stoodley
et al., 1994;Kostya¬l, 1998).
It has beensuggested thatthe mechanisms of microbialattachmentand
biofilmbuild-up occurin two-,three-, five-and eight-step processes (Wirtanen,
1995;Bryers, 2000). The two-step process is dividedinto reversible and
irreversiblebiofilm formation. The reversiblephaseinvolvesthe association of
cellsnear to but not in contact withthe surface. Cellsassociated withthe surface
synthesise exopolymers, which irreversibly bind the cells to the surface.
Characklis (1981)describedbiofilm build-up using the followingfivesteps:
transportationof cells to a wetted surface, absorption of the cells intoa
conditioningfilm,adhesion of microbial cellsto the wettedsurface, reactionof
the cells in the biofilmand detachmentof biofilm fromthe surface. Bryers and
Weightman(1995)dividedthe biofilm build-up into the following eightsteps:
preconditioningof the surfaceby macromolecules, transport of cells to the
Biofilmrisks 47