Effects of tetracyclines on experimental Legionella pneumophila infection in guinea-pigs.

The activities of tetracycline, doxycycline and minocycline against Legionella pneumophila strain Philadelphia-1 were compared in vitro, in peritoneal macrophages and in-vivo experiments in guinea-pigs. Minocycline was the most effective in in-vitro minimum inhibitory concentration assays. In the assay measuring inhibitory effects of drugs on intracellular bacterial multiplication, minocycline and doxycycline were equally effective and tetracycline was the least effective of the three. In-vivo experiments were carried out using guinea-pigs infected intraperitoneally. From the analysis of cumulative survival rates, only minocycline had statistically significant effects. Doxycycline, however, did significantly prolong the infected animals' survival days. These data lend some support to the case reports showing that tetracycline derivatives are effective in the treatment of Legionnaires' disease.