Cytotoxicity of extracellular Legionella pneumophila

Legionella pneumophila, the causative agent of Legionnaire's disease and Pontiac fever, is known to produce a cytopathic effect on macrophages. The capacity of extracellular L. pneumophila to mediate toxicity for guinea pig peritoneal macrophages and J774 mouse macrophages was assessed. Extracellular organisms were found to be capable of mediating toxicity; however, toxic activity appeared to require close proximity with the mononuclear cell surface. Serogroup 1 strains grown on supplemented Mueller-Hinton agar exhibited variable expression of toxic activity. One strain positive on supplemented Mueller-Hinton agar was cytotoxic and unable to replicate in J774 macrophages but remained virulent for guinea pigs at high doses.