Antimicrobial susceptibility of Legionella pneumophila Serogroups I--IV.

Strains of Legionella pneumophila from 10 geographic areas were evaluated for their in vitro susceptibility to 14 antimicrobial agents. Included in this study were clinical and environmental isolates as well as strains from all 4 known serogroups. The minimal inhibitory concentration was established by agar dilution with a Steers replicator. The inhibitory index was then calculated considering the mean peak serum level for the associated antibiotic. Rifampicin, cefoxitin, chloramphenicol, ticarcillin, and parenteral erythromycin had the highest inhibitory indices. The only difference among serogroups was the increased susceptibility among serogroup II isolates to the penicillins and the increased susceptibility of serogroups III and IV to sulfamethoxazole-trimethoprim. Though there have been recent reports of the inadequacy of oral erythromycin in clinical cases of Legionnaires' disease, there was no erythromycin resistance noted among the 14 isolates tested.