Virulence factors and pathogenesis of other Bacillus species

The state of knowledge on virulence factors of Bacillus species other than B. anthracis is less advanced owing to the relatively spasmodic association between these species and infections and to the absence of adequate models for analyzing the candidate factors and determining their actions in the laboratory.

Bacillus species characteristically produce a host of enzymes and other extracellular metabolites. Among those of B. cereus, four groups of what are broadly described as toxins have been distinguished and most of these are believed also to be virulence determinants in infections. These are:

1 Three phospholipases C (alias lecithinase, egg-yolk turbidity factor), phosphatidylcholine hydrolase (23 kDa), phosphatidylinositol hydrolase (phosphatasemic factor, 29 kDa) and sphingomyelinase (29 kDa). Although under normal in vivo circumstances they cannot reach their phospholipid substrates in the eukaryotic cell membrane, they may conceivably act secondarily after exposure of the phospholipids in wounds or other infections. Sphingomyelinase is also hemolytic.

2 Hemolysin I (alias cereolysin), a thiol-activated thermolabile protein cytolysin with a single 518 amino acid polypeptide chain of 55 kDa. The binding-site of the thiol-activated cytolysins on the eukaryotic cell membrane is cholesterol and binding results in pitting and micropuncturing of the cell membrane, leading to loss of control of ion exchange. The net flow of ions and water into the cell leads to swelling and rupture. However, cereolysin is inactivated by free cholesterol in the blood, which presumably limits its activities in natural infections. It is almost instantly lethal on intravenous injection in mice and causes necrosis if injected into skin.

3 Hemolysin II (HBL), consisting of a protein (component B, 36 kDa) which binds to or alters cells, allowing subsequent lysis by a second protein (component L, 45 kDa). This appears also to be the necrotic enterotoxin responsible for diarrheal-type B. cereus food poisoning and occasional severe wound, eye and other infections. It is almost instantly lethal if injected intravenously in mice; unlike cereolysin, this activity is not neutralized by cholesterol.

4 Emetic principle. This is a highly stable, small molecular weight entity, or group of entities, capable of surviving extremes of heat and pH and not susceptible to proteolytic enzymes. It may be associated with breakdown products from the implicated food, although it has recently been proposed that it is now identifiable as 'cereulide', a novel dodecadepsipeptide.

The protein cytotoxin enterotoxin identified by Japanese workers in 1991 may represent a fifth group. The different toxins appear to be produced to different extents by different strains, which may account for the varying degrees of seriousness of B. cereus infections.

Toxins have not been demonstrated in culture fluids or extracts of other Bacillus species periodically implicated in infections and, assuming they did play active roles in the infections from which they were isolated, the basis of their pathogenic activity is not clear.

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