ABSTRACT
Listeria monocytogenes is a facultative intracellular bacterium that has the capacity to enter, survive, and multiply not only in phagocytic but also in nonphagocytic cells, and to cross a host’s intestinal, blood-brain and fetoplacental barriers.1,2 Being tolerant of external stresses (including extreme pH, temperature, and osmolarity), this bacterium endures many food processing procedures. In line with the current trend toward an increased consumption of convenient, ready-to-eat or heat-and-eat food products nowadays, L. monocytogenes has thus become an important source of human foodborne infections. While all human population groups are susceptible to L. monocytogenes, infants, pregnant women, and elderly and immunocompromised individuals are especially vulnerable to listeriosis due to their generally weakened immune status. The initial clinical symptoms of human listeriosis are often mild and nonspecic (e.g., chills, fatigue, headache, muscular and joint pain, and gastroenteritis); however, failure to undertake prompt antibiotic treatment of the infection may have severe consequences, with septicemia, meningitis, abortions, and, occasionally, death being the usual outcomes.2,3
For a considerable period, L. monocytogenes has been regarded as pathogenic at the species level, with a generally accepted belief that all L. monocytogenes isolates are potentially virulent and capable of causing diseases. However, from the experimental data collected over the recent years, it becomes clear that L. monocytogenes demonstrates enormous serotype/strain variation in virulence and pathogenicity. Whereas many epidemic strains are unquestionably highly infective and sometimes deadly, others (especially those from food and environmental specimens) show limited capability to establish infection and are relatively avirulent.4-7 Of the 12 L. monocytogenes serotypes (i.e., 1/2a, 1/2b, 1/2c, 3a, 3b, 3c, 4a, 4b, 4c, 4d, 4e, and 7), only 3 (i.e., 1/2a, 1/2b, and 4b) are implicated in human listeriosis since these serotypes frequently account for over 96% of L. monocytogenes isolations from human clinical cases. Other serotypes (especially 4a and 4c)
8.1 Introduction ......................................................................................................................... 241 8.2 Virulence-Associated Genes and Proteins .......................................................................... 243 8.3 Laboratory Determination of Virulence .............................................................................246
