Fundamental Food Microbiology, Third Edition - Fuad Fathir

562 FUNDAMENTAL FOOD MICROBIOLOGY
ganisms will not grow. A small amount (@0.01 ml) of the enrichment broth is then
usually streaked on the surface of a prepoured selective-differential agar medium
plate, which is then incubated for specified time for the colonies to develop. From
the differential colony characteristics, the presence of a specific pathogen can be
tentatively established.
This is generally considered a presumptive test. For confirmation, the cells from
the characteristic colonies are purified and examined by the recommended methods
for biochemical reaction profiles and serological reaction against a specific antibody.
Isolation of a pathogen using the conventional methods can take 10 to 12 d, depending
on a particular species. 1–3
VII. TEST FOR BACTERIAL TOXINS IN FOODS
Sta. aureus and Clo. botulinum strains and several others (e.g., Bac. cereus, Vib.
parahaemolyticus), while growing in foods, can produce toxins and cause intoxication
or food poisoning among consumers. Specific methods have been developed to
test the presence of toxins in the suspected foods. 1–3
To detect Sta. aureus enterotoxin in foods, the recommended method is to
initially extract the toxin from 100 g food and then to concentrate the toxin in 0.2
ml sterile water or 0.2 M saline. The presence of toxin in the concentrated extract
is then assayed against specific antibody by the microslide precipitation method.
This method is sensitive at 0.05 mg enterotoxin/ml of the extract. (Food poisoning
can probably occur from consuming less than 1.0 mg of Sta. aureus enterotoxin A.)
Methods such as radioimmunoassay (RIA), enzyme-linked immunosorbent assay
(ELISA), and reverse passive latex agglutination assay (RPLA) can also be used in
place of the microslide precipitation method.
To detect botulin, the toxin of Clo. botulinum strains, the recommended procedure
is used to extract toxin from a food. To activate toxin of nonproteolytic types
(B and E), a portion of the extract is then treated with trypsin. Both trypsinized and
nontrypsinized (for proteolytic types A and B) extracts in 0.5-ml portions are then
injected into mice intraperitoneally (two mice for each sample). Heated (100�C for
10 min) extracts are injected similarly (control). The mice are then observed up to
48 h for botulism symptoms and death. Typical botulism symptoms in mice, in
sequence, are ruffling of fur, rapid and gasping breathing, weakness of limbs, and
death due to respiratory failure.
Specific methods for testing toxins of some other pathogenic bacteria and molds
have also been developed and listed in the recommended books as well as available
from commercial producers. 1–3
VIII. RAPID METHODS AND AUTOMATION
The traditional or conventional methods used for the quantitative or qualitative
detection of microorganisms in foods and toxins take a relatively long time and are
labor intensive. To overcome these difficulties, different rapid methods, many of