The Relationship of Microorganisms to Sanitation 57swabbing is not done. A covered dish or rehy-
drated Petrifilm is opened, and the growth
medium (agar) is pressed against the area to
be sampled. The incubation process is the
same as for the plate count method. This
method is easy to conduct, and less chance
for error (including contamination) exists.
The greatest limitation of this technique is
that it can be used only for surfaces that are
lightly contaminated because dilution is not
possible. Press plates can be used to monitor
the effectiveness of a sanitation program.
The amount of growth on the media suggests
the amount of contamination.
Indicator and Dye Reduction Tests
Various microorganisms secrete enzymes as
a normal metabolic function of their growth,
which are capable of inducing reduction reac-
tions. Some indicator substances (such as
dyes) are used as the basis of these tests. The
rate of their reduction, which is indicated by a
color change, is proportional to the number
of microorganisms present. The time required
for the complete reduction of a standard
amount of the indicator is a measure of the
microbial load. In a modification of these
methods, a dye-impregnated filter paper is
applied directly onto a food sample or piece
of equipment. The time required for the filter
paper to change color is used to determine the
microbial load.
This method lacks utility because of biofilm
formation, incomplete detection of micro-
organism’s, and material cost. This technique
does not quantify the extent of contamina-
tion. However, it is quicker and easier to con-
duct than the plate count technique and has
become an acceptable tool for evaluation of
a sanitation program effectiveness.
Direct Microscopic Count
A known volume is dried and fixed to a
microscope slide and stained; then a number
of fields (frequently 50) are counted. Because
viable and nonviable bacteria are not dis-
tinguished by most staining techniques, this
method estimates the number of micro-
organisms. Sophisticated digital cameras
may be attached to microscopes, to capture
images using image analysis software. These
images may be analyzed to differentiate bac-
teria based on size and to enumerate organ-
isms/ field, thus eliminating human error.
Although this method provides morphologic
or specific staining information and the
slides may be retained for future reference, it
is not used much because analyst fatigue can
produce errors, and only a small quantity is
examined.Most Probable Number
This estimate of bacterial populations is
obtained by placing various dilutions of a
sample in replicate tubes containing a liquid
medium. The number of microorganisms is
determined by the number of tubes in each
replicate set of tubes in which growth
occurred (as evidenced by turbidity) and
referring this number to a standard most
probable number (MPN) table. This method
measures only viable bacteria, and it permits
further testing of the cultures for purposes
of identification.Petrifilm Plates
Petrifilm plates are manufactured with a
dehydrated nutrient medium on a film. This
self-contained, sample-ready approach has
been developed as an alternative method to
the standard aerobic plate count (SPC) and
coliform counts, as determined by violet red
bile (VRB) pour plates. The most commonly
used methodology for enumerating E. coli
from broiler chicken carcasses and ground
beef are rapid detection methods such as
Petrifilm (3M Co.) and SimPlate (Neogen).
These methods, which are available as