Susceptibility of Legionella pneumophila to ofloxacin in vitro and in experimental Legionella pneumonia in guinea pigs.

The antimicrobial activity of ofloxacin was tested against 15 standard strains and 37 clinical and environmental strains of Legionella pneumophila by agar dilution susceptibility studies with a new growth medium. The ofloxacin MICs were inoculum dependent and ranged from 0.03 to 0.125 microgram/ml. The antibacterial activities of other agents tested relative to ofloxacin were rifampin greater than ofloxacin greater than josamycin greater than pipemidic acid. Ofloxacin, at concentrations equal to or greater than 0.05 microgram/ml, inhibited the growth of L. pneumophila grown in human monocytes. The therapeutic efficacy of ofloxacin in experimental guinea pig L. pneumophila pneumonia was greater than that observed with erythromycin or josamycin therapy; it was less effective than was rifampin. Ofloxacin was very active against intracellular L. pneumophila in these experiments and should be studied in the therapy of human Legionnaires disease.