Virulence plasmids of the pathogenic clostridia

Many clostridial species are ubiquitous in the environment and in the intestinal tracts of birds, fish, and mammals. Commensal species are often carried asymptomatically within a host. However, if the immune status of the host is compromised, due to either age, illness or a change in diet, disease can result from toxigenic strains. Alternatively, some clostridial species or strains don’t require predisposing factors. They can cause disease simply if they gain entry into the host either through damage to the skin or through the gastrointestinal tract, often via poorly prepared or incorrectly stored food. These clostridia then overgrow and cause cell and tissue damage. Diseases mediated by the clostridial species discussed in this review are predominantly mediated by potent protein toxins, many of which are located extrachromosomally. These toxins have diverse mechanisms of action and include pore-forming cytotoxins, phospholipases, metalloproteases, ADP-ribosyltransferases and large glycosyltransferases. This review focuses on these toxins and the elements that carry the toxin structural genes. For ease of discussion it has been structured on a bacterial species-specific basis.