Legionella species of different human prevalence induce different rates ofapoptosis in human monocytic cells

Legionella species of different human prevalence were examined with respect to induction of apoptosis in the human monocytic cell line Mono Mac 6 (MM6 ). L. pneumophila serogroup 1 (Pontiac), L. pneumophila serogroup 1 (Philad elphia-1), L. longbeachae serogroup 1, L. gormanii, L. micdadei and L. stei geiwaltii were used to infect MM6 cells. Subsequent induction of apoptosis was investigated by enzyme-linked immunosorbent assay (ELISA), gel electrop horesis of cellular DNA extracts, and staining of cells with the DNA dye 4' , 6-diamidino-2-phenylindole (DAPI). Additionally, the concomitant occurren ce of infection and apoptosis was demonstrated by a combination of immunohi stochemistry with nuclear DAPI counterstaining. Induction of apoptosis in M M6 cells by a given species of the genus Legionella correlates with their h uman prevalence rather than with their ability to multiply within this huma n monocytic cell line. Furthermore, we found that initiation of apoptosis o f Mono Mac 6 cells was dependent on direct adherence of the pathogenic bact eria to the host cell and was triggered by extracellular bacteria.