Gram stain of Bacillus cereus showing Gram-positive (purple/black) long rod shaped bacilli, often forming filaments that look like 'tram-cars'. This is undistinguishable by microscopy from the highly dangerous related Bacillus anthracis that causes anthrax.
Bacillus cereus is a Gram-positive, facultatively aerobic sporeformer whose cells are large rods. These and other characteristics, including biochemical features, are used to differentiate and confirm the presence B. cereus, although these characteristics are shared with B. cereus var. mycoides, B. thuringiensis and B. anthracis. Differentiation of these organisms depends upon determination of motility (most B. cereus are motile), presence of toxin crystals (B. thuringiensis), haemolytic activity (B. cereus and others are beta haemolytic whereas B. anthracis is usually non-haemolytic), and rhizoid growth which is characteristic of B. cereus var. mycoides.
B. cereus food poisoning is the general description, although two recognised types of illness are caused by two distinct metabolites. The diarrheal type of illness is caused by a large molecular weight protein, while the vomiting (emetic) type of illness is believed to be caused by a low molecular weight, heat-stable peptide.
The symptoms of B. cereus diarrheal type food poisoning mimic those of Clostridium perfringens food poisoning. The onset of watery diarrhoea, abdominal cramps, and pain occurs 6–15 hours after consumption of contaminated food. Nausea may accompany diarrhoea, but vomiting (emesis) rarely occurs. Symptoms persist for 24 hours in most instances. The emetic type of food poisoning is characterised by nausea and vomiting within 0.5–6 hours after consumption of contaminated foods. Occasionally, abdominal cramps and/or diarrhoea may also occur. Duration of symptoms is generally less than 24 hours. The symptoms of this type of food poisoning parallel those caused by Staphylococcus aureus foodborne intoxication.
The presence of large numbers of B. cereus (greater than 106organisms/g) in a food is indicative of active growth and proliferation of the organism and is consistent with a potential hazard to health.