Fundamental Food Microbiology, Third Edition - Fuad Fathir

538 FUNDAMENTAL FOOD MICROBIOLOGY
a culture broth. These places, in turn, can be a constant source of undesirable
microorganisms to foods handled in the environment.
The concept and importance of microbial attachment and biofilm formation in
solid food, equipment, and food environments are now being recognized. 1–3 Limited
studies have shown that under suitable conditions, many of the microorganisms
important in food can form a biofilm. Several species and strains of Pseudomonas
were found to attach to stainless steel surfaces, some within 30 min at 25�C to 2 h
at 4�C. Listeria monocytogenes was found to attach to stainless steel, glass, and
rubber surfaces within 20 min of contact. Attachment of several pathogenic and
spoilage bacteria has also been demonstrated on meat and carcasses of poultry, beef,
pork, and lamb. The microorganisms found to attach to meat surfaces include Lis.
monocytogenes, Micrococcus spp., Staphylococcus spp., Clostridium spp., Bacillus
spp., Lactobacillus spp., Brochothrix thermosphacta, Salmonella spp., Escherichia
coli, Serratia spp., and Pseudomonas spp.. It is apparent from the limited data that
microbial attachment to solid food and food contact surfaces is quite wide and needs
to be considered in controlling the microbiological quality of food.
II. MECHANISMS OF ATTACHMENT
Several possible mechanisms by which microbial cells attach and form a biofilm on
solid surfaces have been suggested. 1–3 One suggestion is that the attachment occurs
in two stages. In the first stage, which is reversible, a cell is held to the surface by
weak forces (electrostatic and Van der Waals forces). In the second stage, a cell
produces complex polysaccharide molecules to attach its outer surface to the surface
of a food or equipment, and the process is irreversible. A three-step process that
includes adsorption, consolidation, and colonization has been suggested by others.
In the reversible adsorption stage, which can occur in 20 min, the cells attach loosely
to the surface. During the consolidation stage, the microorganisms produce threadlike
exopolysaccharides fimbriae and firmly attach the cells to the surface. At this stage,
the cells cannot be removed by rinsing. In the colonization stage, which is also
irreversible, the complex polysaccharides may bind to metal ions on equipment
surfaces and the cells may metabolize products that can damage the surfaces 1–3 (see
cover page).
III. INFLUENCING FACTORS
The level of attachment of microorganisms to food-processing equipment surfaces
is found to be directly related to contact time. As the contact time is prolonged,
more cells attach to the surface, the size of the microcolony increases, and attachment
between cells increases. Fimbriae formation by the cells occurs faster at optimum
temperature and pH of growth. Limited studies also showed that when microorganisms
such as Pseudomonas fragi and Lis. monocytogenes are grown together, they
form a more complex biofilm than when either is grown separately. 1–3