Virulence factors are those components of an organism whose loss (usually by mutation or shedding of an unstable genetic element) specifically impairs pathogenicity but not viability.' The two major classes of virulence factors are toxins and surface molecules. Virulence factors are generally highly polygenic traits that are often affected by aspects of cellular physiology such as nutritional requirements, growth rate, temperature, and response to host factors. With certain exceptions, rarely does the virulence of an organism depend on a single gene product but the capacity to cause disease usually involves a balanced genome whereby many traits contribute to the virulence.

The Clostridia produce more toxins than any other bacterial genus,2'3 and pathogenic Clostridia are usually identified on the basis of the characteristic toxins they produce. More than 20 toxins and other extracellular proteins contributing to virulence such as spreading factors and proteolytic enzymes have been identified in Clostridium spp4 Two of the toxins, botulinum and tetanus toxins, are the most powerful poisons known - lethal doses of botulinum and

The Clostridia: Molecular Biology and Pathogenesis ISBN 0-12-595020-9

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Table 3.1. Virulence factors associated with extrachromosomal genetic factors or mobile genetic elements in Clostridia

Clostridium species

Virulence factor

Extrachromosomal genetic element


C botulinum types

C3 toxin

Nonintegrative lysogenic

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