food processing. Most bactericides are uti-
lized as a method of decontaminating food-
stuffs or as a sanitizer for cleaned equipment,
utensils, and rooms. (Sanitizers are discussed
in detail in Chapter 10.)
Interaction between Growth Factors
The effects that factors such as tempera-
ture, oxygen, pH, and Awhave on microbial
activity may be dependent on each other.
Microorganisms generally become more sen-
sitive to oxygen availability, pH, and Awat
temperatures near growth minima or max-
ima. For example, bacteria may require a
higher pH,Awand minimum temperature
for growth under anaerobic conditions than
when aerobic conditions prevail. Microor-
ganisms that grow at lower temperatures are
usually aerobic and generally have a high Aw
requirement. Lowering Awby adding salt or
excluding oxygen from foods (such as meat)
that have been held at a refrigerated temper-
ature dramatically reduces the rate of micro-
bial spoilage. Normally, some microbial
growth occurs when any one of the factors
that controls the growth rate is at a limiting
level. If more than one factor becomes limit-
ing, microbial growth is drastically curtailed
or completely stopped.
Role of Biofilms
Biofilms are microcolonies of bacteria
closely associated with an inert surface
attached by a matrix of complex polysaccha-
ride-like material in which other debris,
including nutrients and microorganisms,
may be trapped. A biofilm is a unique envi-
ronment that microorganisms generate for
themselves, enabling the establishment of a
“beachhead” on a surface resistant to intense
assaults by sanitizing agents. When a
microorganism lands on a surface, it attaches
itself with the aid of filaments or tendrils.
The organism produces a polysaccharide-
like material, a sticky substance that will
cement in a matter of hours the bacteria’s
position on the surface and act as a glue to
which nutrient material will adhere with
other bacteria and, sometimes, viruses. The
bacteria become entrenched on the surface,
clinging to it with the aid of numerous
appendages. Bacteria within a biofilm can be
up to 1,000 times more resistant to some san-
itizers than those freely dispersed in solution.
A biofilm builds upon itself, adding sev-
eral layers of the polysaccharide material
populated with microorganisms, such as Sal-
monella,Listeria,Pseudomonas, and others
common to the specific environment.
Increased time of organism contact with the
surface contributes to the size of the micro-
colonies formed, amount of attachment, and
difficulty of removal. The biofilm will even-
tually become a tough plastic that often can
be removed only by scraping. Although
cleaned surfaces may be sanitized, a firmly
established biofilm has layers of organisms
that may be protected from the sanitizer.
Biofilm buildup can be responsible for por-
tions of it being sheared off by the action of
food or liquid passing over the surface.
Because the shear force is greater than the
adherence force in the topmost layers of the
biofilm, chunks of the polysaccharide cement,
with the accompanying microbial population,
will be transferred to the product, with subse-
quent contamination.
There has been additional interest in
biofilms since the mid 1980s because it has
been demonstrated that Listeria monocyto-
geneswill adhere to stainless steel and form
a biofilm. Biofilms form in two stages. First,
an electrostatic attraction occurs between the
surface and the microbe. The process is
reversible at this state. The next phase occurs
when the microorganism exudes an extracel-
lular polysaccharide, which firmly attaches
the cell to the surface. The cells continue to
grow, forming microcolonies and, ultimately,
the biofilm.The Relationship of Microorganisms to Sanitation 33