The species B. cereus is a typical member of the B. cereus-Group of the genus Bacillus. B. cereus strains are motile, facultatively anaerobic Gram-positive spore-bearing bacilli (1.0 µm wide or wider). The central to sub-terminal spores do not distend the cells. They are haemolytic and not sensitive to penicillin. They form white colonies on bacteriological media. The chromosomal DNA of B. cereus has a GC content of 35.7-36.2mol%.

They are lecithinase positive. They are motile, ferment a few carbohydrates producing acid from glucose but not gas. They do not grow above 50°C but grow in up to 7% NaCl. They produce catalase, give a positive Voges-Proskauer reaction and reduce nitrate to nitrite. They hydrolyse casein and starch.

The genetic aspects of virulent strains of B. cereus have been especially studied. The genetic organisations for the three enterotoxin genes: Haemolysin BL (hbl), Non-haemolytic enterotoxin (nhe) and Cytotoxin K (cytK) have been extensively studied. All three proteins of hbl are transcribed from one operon. Similarly, the nhe operon contains three open reading frames, but the function of one is still unknown. The cytK operon is generally present on strains of B. cereus, which do not produce any enterotoxins. The cytK gene product is also haemolytic.

Different serotypes of B. cereus are not recognized.

Enterotoxins:

B. cereus produces a number of enterotoxins. The Emetic toxin causes vomitting, while a number of other enterotoxins cause diarrhoeal diseases.

The Emetic Toxin:

The emetic toxin (also named cereulide) has a molecualr weight of 1.2 kDa and consists of a ring structure comprising three repeats of four amino and/or oxy acids. High concentrations of cereulide can also cause liver damage.

The Enterotoxins:

B. cereus can produce up to five different enterotoxins, three of which are likely to be involved in food-borne illnesses. About 60% of B. cereus strains produce the multicomponent Haemolysin BL (HBl)and nearly all strains produce the Non-haemolytic enterotoxin (Nhe). Their relative importance in diarrhoeal diseases is unknown. Enterotoxin T, present in about 30% of B. cereus isolates is only released when cells lyse. Little is known about this enterotoxin or enterotoxin FM. The cytotoxin K has been found to be closely related to the b-toxin of Clostridium perfringens and has been involved in cases of potentially fatal bloody diarrhoea.

Extraintestinal Infections:

There do not appear to be extraintestinal infections associated with B. cereus.

Animal Infections:

There do not appear to be animal infections associated with B. cereus.

B. cereus strains can be grown very easily on most microbiological media. There are special commercial kits available for some of the toxins and others are in development. The organisms can be identified on the basis of their biochemical reactions including ability to grow anaerobically, at 7% NaCl, their production of catalase, hydrolysis of starch, casein and production of lecithinase. They reduce nitrate to nitrite, produce acid but no gas from glucose and give a positive Voges-Proskauer reaction.

B. cereus survives well in soil and growing plants. Traditionally, it was considered that B. cereus cannot grow below 10°C, however strains appear to have evolved which can grow at refrigeration temperatures (4-6°C). Foods such as rice, pasta, spices and dairy products are the most common food vehicles which have been associated with B. cereus food poisoning cases.

There are no specific industrial uses of B. cereus.

There are no B. cereus vaccines.