Listeria monocytogenes. A model system for the molecular study of intracellular parasitism.

Listeria monocytogenes is a facultative intracellular bacterium which, in its mammalian host, can infect enterocytes and mononuclear phagocytes. It is responsible for severe infections in humans and animals. Recovery from infection and resistance depends on the development of a T-cell response, antibodies not being protective. Several features of L. monocytogenes make it particularly suitable for the study of genetic and molecular aspects of invasion and intracellular parasitism. First, L. monocytogenes not only multiplies rapidly in bacterial broth but also easily infects macrophages and other cells in culture. Second, since it infects primarily immunocompromised individuals or pregnant women, its manipulation does not require extensive containment. Third, the genus Listeria includes several nonpathogenic species, facilitating the identification of species-specific genes and products required for pathogenicity. This identification is now possible due to the parallel development of powerful genus-specific genetic tools (transposons, plasmids, genetic transformation) and improvement of recombinant DNA techniques. Finally, the in vivo relevance of the putative virulence genes or gene products can be tested in the experimental murine infection, which has already proved invaluable in the study of the induction and expression of T-cell-mediated immune response. This review discusses current knowledge concerning these particular features, with an emphasis on listeriolysin O, a major virulence factor, and the only bacterial gene product known to be absolutely required for intracellular growth.