Fundamental Food Microbiology, Third Edition - Fuad Fathir

396 FUNDAMENTAL FOOD MICROBIOLOGY
where Bac. cereus was recognized as a causative gastroenteritis agent in 1969.
Between 1973 and 1987, 58 outbreaks involving 1123 cases were reported. Both
the incidence of outbreaks and number of cases per outbreak were low, and no
fatalities were reported. However, as the symptoms are not severe and last for ca.
12 h, many cases may not be reported.
B. Characteristics
The cells are Gram-positive motile rods, which form endospores in the middle of
the cells. Cells are sensitive to pasteurization. Spores can survive high heat treatment
used in many cooking procedures. Bac. cereus is aerobic, but can also grow under
some degree of anaerobic environment. The cells can multiply in a temperature range
of 4 to 50�C, with the optimum ca. 35 to 40�C. Other parameters of growth are pH
of 4.9 to 9.3, A w of 0.95 and above, and NaCl concentration below 10%. 6–8
C. Habitat
Spores and cells of Bac. cereus are common in soil and dust and can be readily
isolated in small numbers from many foods, which include both raw and finished
products. Intestinal tracts of 10% of healthy adult humans have Bac. cereus under
normal conditions.
D. Toxins and Toxin Production
The strains produce at least two types (emetic and enteric) of enterotoxins, each
probably associated with specific types of symptoms. 3,6 The toxins are produced
during growth of cells at the growth temperature range and retained in the cells.
Only when the cells are lysed are the toxins released. They occur in the intestinal
tract but can also occur in foods. Thus, the cases can also be regarded as food
poisoning, as in staphylococcal food poisoning.
E. Disease and Symptoms
In general, a large number of cells (10 6–8 /g) need to be ingested to produce gastroenteritis.
The two types of enterotoxins produce two types of symptoms. The enterotoxin
associated with the diarrheal form is a heat-labile protein, and that associated
with the emetic form is a heat-stable protein. In the diarrheal form, symptoms occur
6 to 12 h following consumption of a food containing the viable cells. Symptoms
include abdominal pain, profuse watery diarrhea, and perhaps nausea, but no vomiting
or fever. Recovery is usually within 24 h. These symptoms in many respects
are similar to those produced by Clo. perfringens.
In the emetic form, the symptoms occur 1 to 5 h following ingestion of a food
containing the viable cells. As the toxin is heat stable, once the toxin forms in cells,
heating food containing a large number of cells before eating can produce the
symptoms. Symptoms are nausea and vomiting; abdominal pain and diarrhea may
also be present. Symptoms last for ca. 24 h. In some respects, the symptoms are
similar to those of staphylococcal gastroenteritis.