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The pathogenicity of Listeria monocytogenes: a public health perspective

Within the genus Listeria, Listeria monocytogenes is the major pathogenic species. In humans the disease from this bacterium (listeriosis) generally presents as an extremely severe systemic infection, although subclinical infection may also occur as mild gastrointestinal or influenza-like illness. It is generally believed that the consumption of contaminated food is the principal route of transmission. Epidemiological data indicate that foodborne listeriosis can have an extremely variable incubation period (1->90 days), and the attack rate for the development of serious systemic disease is very low. L. monocytogenes infects the human host through the gastrointestinal tract, although the exact site of invasion is not known. Animal- and laboratory-based models of infection are available, and the application of molecular biological techniques to these models has revealed a partial understanding of listeriosis at the cellular level. The pathogenicity of this bacterium is multifactorial, resulting from the activity of at least nine gene products. Although some differences have been shown in the ability of wild-type L. monocytogenes strains to cause disease in these models, the majority of strains tested are capable of causing disease using experimental animals and in vitro models. However, the animal-and laboratory-based models differ markedly from the natural route of infection and hence are not satisfactory for extrapolation to the entire process of human foodborne disease. Laboratory methods are available to subtype L. monocytogenes, and a large number of subtypes can be distinguished. Subtyping data, together with epidemiological evidence, may indicate that some strains are more pathogenic than others for humans. However, none of the subtyping methods can be used to identify non-pathogenic or less virulent strains. It is the opinion of the author that, in the interests of public safety and for considerations for public health purposes, all L. monocytogenes, including those recovered from food, should be regarded as potentially pathogenic.