Bacillus cereus

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Bacillus cereus spore stain

Bacillus cereus has been recognized as an agent of food poisoning since 1955. There are only a few outbreaks a year reported by CDC. Between 1972 and 1986, 52 outbreaks of food-borne disease associated with B. cereus were reported to the CDC (in 2003, there were two), but this is thought to represent only 2% of the total cases which have occurred during these periods. It is not a reportable disease, and usually goes undiagnosed.

B. cereus causes two types of food-borne illnesses. One type is characterized by nausea and vomiting and abdominal cramps and has an incubation period of 1 to 6 hours. It resembles Staphylococcus aureus (staph) food poisoning in its symptoms and incubation period. This is the "short-incubation" or emetic form of the disease.

The second type is manifested primarily by abdominal cramps and diarrhea following an incubation period of 8 to 16 hours. Diarrhea may be a small volume or profuse and watery. This type is referred to as the "long-incubation" or diarrheal form of the disease, and it resembles food poisoning caused by Clostridium perfringens. In either type, the illness usually lasts less than 24 hours after onset. In a few patients symptoms may last longer.

The short-incubation form is caused by a preformed, heat-stable emetic toxin, ETE. The mechanism and site of action of this toxin are unknown, although the small molecule forms ion channels and holes in membranes. The long-incubation form of illness is mediated by the heat-labile diarrheagenic enterotoxin Nhe and/or hemolytic enterotoxin HBL, which cause intestinal fluid secretion, probably by several mechanisms, including pore formation and activation of adenylate cyclase enzymes.

TOXINS

Bacillus cereus produces one emetic toxin (ETE) and three different enterotoxins: HBL, Nhe,and EntK.

Two of the three enterotoxins are involved in food poisoning. They both consist of three different protein subunits that act together. One of these enterotoxins (HBL) is also a hemolysin; the second enterotoxin (Nhe) is not a hemolysin. The third enterotoxin (EntK) is a single component protein that has not been shown to be involved in food poisoning. All three enterotoxins are cytotoxic and cell membrane active toxins that will make holes or channels in membranes.

The emetic toxin (ETE) is a ring-shaped structure of three repeats of four amino acids with a molecular weight of 1.2 kDa. It is a K⁺ ionophoric channel, highly resistant to pH between 2 and 11, to heat, and to proteolytic cleavage.

The nonhemolytic enterotoxin (Nhe) is one of the three-component enterotoxins responsible for diarrhea in Bacillus cereus food poisoning. Nhe is composed of NheA, NheB and NheC. The three genes encoding the Nhe components constitute an operon. The nhe genes have been cloned separately, and expressed in either Bacillus subtilis or Escherichia coli. Separate expression showed that all three components are required for biological activity.

The hemolytic enterotoxin, HBL, is encoded by the hblCDA operon. The three protein components, L1, L2 and B, constitute a hemolysin. B is for binding; L1 and L2 are lytic components. This toxin also has dermonecrotic and vascular permeability activities, and it causes fluid accumulation in rabbit ileal loops.