Differential growth of Legionella pneumophila in guinea pig versus mouse macrophage cultures.

Legionella pneumophila is a facultative intracellular bacterium which replicated well in inbred guinea pig strain 2 peritoneal macrophages at a low infectivity ratio. In contrast, the growth of this organism was markedly restricted in mouse (BDF1) peritoneal macrophages, even at a relatively high infectivity ratio. The initial uptake of L. pneumophila organisms by macrophages was similar in both animal species, and both groups of macrophages supported the growth of Listeria monocytogenes. Treatment of L. pneumophila with immune guinea pig serum did not result in restriction of bacterial growth in macrophages, but guinea pig macrophages were readily induced to suppress the growth of L. pneumophila by preincubation with supernatants obtained from mitogen-activated normal guinea pig splenocyte cultures. Thus, lymphokines generated from mitogen-stimulated guinea pig lymphocytes induced a restriction of growth of these organisms similar to that observed naturally with macrophages from mice, which are considered highly resistant to these bacteria. Although guinea pigs are considered highly susceptible to L. pneumophila infections and mice are considered relatively resistant, the mechanism of this difference is not clear. The results of the present study suggest that the restriction of L. pneumophila growth by macrophages relates to host susceptibility to infection and that cell populations permissive for L. pneumophila can be transformed to nonpermissive by products from stimulated lymphocytes but not by opsonization with immune serum.