o1 h up to 11 h) suggest an intoxication, though no toxin has been
isolated and described as yet.
7.2.4 Isolation and Identification
In an outbreak ofBacillus cereusfood poisoning, implicated foods will
contain large numbers ( 4105 g^1 ) of organisms so enrichment tech-
niques are not needed. The same is true of faecal or vomitus specimens
and a non-selective medium such as blood agar (sometimes with the
addition of polymyxin as a selective agent to suppress Gram-negatives) is
commonly used.B. cereuscan be identified after 24 h incubation at 37 1 C
by its characteristic colonial morphology of large (3–7 mm diameter), flat
or slightly raised, grey-green colonies with a characterisitc granular or
ground-glass texture and a surrounding zone ofaorbhaemolysis. To
confirm the identity of a blood agar isolate or to isolate smaller numbers
ofB. cereusfrom foods, a more selective diagnostic agar is necessary.
Several of these have been proposed which have a number of common
features. Polymyxin/pyruvate/eggyolk/mannitol/bromothymol blue agar
(PEMBA) is one widely used example. It includes polymyxin as a
selective agent and where yeasts and moulds are likely to be a problem
actidione may also be included. On PEMBA,B. cereusproduces typical
Figure 7.1 Cereulide, the emetic toxin ofB. cereus
188 Bacterial Agents of Foodborne Illness