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23 February 2004

NEW CHROMOGENIC BACILLUS CEREUS AGAR MORE SENSITIVE THAN TRADITIONAL MEDIA

In a recent study conducted by Burton’s Foods (Moreton, Wirral, Merseyside, UK) using naturally contaminated and spiked food samples, Oxoid Chromogenic Bacillus cereus Agar was shown to be more sensitive and selective than traditional Bacillus media. Performance of the Oxoid Medium was also shown to be superior to that of another commercially produced chromogenic medium.

Bacillus cereus associated gasteroenteritis results from the ingestion of two distinct toxins (emetic toxin and enterotoxin) produced during the vegetative stage of growth, in foods that have been poorly refrigerated following cooking. Intoxication has been associated with the consumption of rice (especially precooked rice dishes), cereals, pasta, vegetables, dairy products, meats and spices.

Two types of illness are caused by the two toxins. The diarrhoeal type of illness is caused by a large molecular weight protein or enterotoxin. Onset is usually within 6-15 hours of ingestion of contaminated food. The vomiting (emetic) type of illness is believed to be caused by a low molecular weight, heat-stable peptide and symptoms can start to occur within 0.5-6 hours of ingestion.

Confirmation of B. cereus as the etiologic agent in a foodborne outbreak requires isolation of the same strain from the suspect food and faeces or vomitus of the patient. However, the rapid onset time to symptoms in the emetic form of the disease, coupled with isolation of the organism from food, is often sufficient to diagnose this type of food poisoning.

The new Oxoid Chromogenic Bacillus cereus Agar (CM1036 & SR0230) incorporates the chromogenic substrate 5-Bromo-4-chloro-3-indolyl- β-glucopyranoside, which is cleaved by the enzyme β- glucosidase expressed by B. cereus resulting in the formation of blue colonies. The medium also incorporates selective agents to inhibit the growth of non-Bacillus species which are capable of utilising the chromogenic substrate.