7.2.2 The Organism and its Characteristics
Members of the genusBacillusare Gram-positive, aerobic, spore-form-
ing rods, though they do, on occasion, display a Gram-negative or
variable reaction. Their taxonomy is quite complex and has been subject
to considerable revision in recent years. The genus still contains about 80
species, those causing food poisoning beingBacillus cereus, a number of
species very closely related toB. cereusandBacillus subtilis.
Bacillus cereusis facultatively anaerobic with large vegetative cells,
typically 1.0mm by 3.0–5.0mm in chains. It grows over a temperature
range from 8 to 55 1 C, optimally around 28–35 1 C, and does not have any
marked tolerance for low pH (min. 5.0–6.0, depending on the acidulant)
or water activity (min.B0.95).
Spores are central, ellipsoidal in shape and do not cause swelling in the
sporangium. As a spore former,B. cereusis widely distributed in the
environment and can be isolated from soil, water and vegetation. This
ubiquity means that it is also a common component of the transient gut
flora in humans. The spores show a variable heat resistance; recorded D
values at 95 1 C in phosphate buffer range between around 1 min up to
36 min. Resistance appears to vary with serovar.
In the UK, a serotyping scheme based on the flagellar (H) antigen
has been devised, based on a set of 29 agglutinating antisera raised
against outbreak and non-outbreak strains isolated from foods. In about
90% of outbreaks it is possible to serotype the causative organism,
although only about half of environmental isolates are typable. There
does not appear to be a strong association between the two different
types ofB. cereusfood poisoning and particular serotypes. Some have
been associated with both types of syndrome, although in a study of
200 outbreaks of the emetic syndrome from around the world, serotype
1, which possesses markedly greater heat resistance than other sero-
types, was isolated from implicated foods, faeces or vomitus in 63.5% of
cases.
7.2.3 Pathogenesis and Clinical Features
Symptoms of the diarrhoeal syndrome resemble those ofClostridium
perfringensfood poisoning. The onset of illness is about 8–16 h after
consumption of the food, lasts for between 12 and 24 h, and is charac-
terized by abdominal pain, profuse watery diarrhoea and rectal ten-
esmus. Nausea and vomiting are less frequent.
The emetic syndrome resembles the illness caused byStaphylococcus
aureus. It has a shorter incubation period than the diarrhoeal syndrome,
typically 0.5–5 h, and nausea and vomiting, lasting between 6 and 24 h,
are the dominant feature.
186 Bacterial Agents of Foodborne Illness