Listeria monocytogenes--aspects of pathogenicity

The genus Listeria includes different species of ubiquitary present gram-positive rod shaped bacteria. The species Listeria monocytogenes causes severe diseases like meningitis and meningoencephalitis in humans. Additional groups of syndromes associated with this microorganism are the listeriosis of the pregnant woman, mostly appearing as an abortion or septic premature birth and the meningitis of the newborn. Transmission of Listeria may occur on the oral route due to infected food like raw milk and cheese, raw meat or soil contaminated food like prepacked salads, respectively. The understanding of Listeria virulence was improved by different investigations employing cell cultures and molecular methods like knockout of genes encoding potential virulence factors. Nowadays the procedure of infection of cells is divided in four different parts: internalisation, escape from intracellular vacuole, nucleation of actin filaments and cell-to-cell spread. So called Internalins are produced by Listeria and are obviously needed for cell invasion. Listeria could escape intracellular vacuoles producing a hemolysin, Listeriolysin O, and proliferate inside the host cells. The surface bounded protein Actin A mediates the contact to the actin filament system of the host cell. This is important for the intracellular spread of Listeria. In the next step a cell-to-cell spread supported by phospholipase and lecithinase occurs. Despite the high incidence of contaminated food only a little incidence of listeriosis is observed. This may be explained in an indirect vaccination due to less virulent strains. However, the immune response of patients even with well documented listeriosis may be poor and causes false negative serological results sometimes. In this paper the know virulence factors of the interesting species L. monocytogenes are demonstrated and the course of infection is discussed.