Rifampin resistance of Legionella pneumophila is not increased during therapy for experimental Legionnaires disease: study of rifampin resistance using a guinea pig model of Legionnaires disease.

Isolates of Legionella pneumophila serogroup 1, obtained from guinea pigs with experimentally induced Legionnaires disease, were tested for rifampin resistance. Thirteen isolates were from animals treated with rifampin alone, four isolates were from animals treated with saline, and three isolates each were from animals treated with erythromycin or erythromycin plus rifampin; all of these isolates were derived from the same parent strain, F889. Most of the isolates were obtained from rifampin-treated animals that survived infection but had persistence of bacteria in their lungs at necropsy. No differences in rifampin agar dilution MICs were detected for the 23 isolates and parent strain that were tested. None of the 13 isolates from animals treated with rifampin alone had a high number of resistant organisms detected by using a rifampin gradient plate assay. Thirteen isolates plus the parent strain were tested by using a quantitative method of determining resistance frequency. Considerable heterogeneity among isolates was observed, but there was no evidence of increased resistance for any treatment group. The range of rifampin resistance frequencies was 10(-7) to 10(-8). No evidence for rifampin-induced resistance of L. pneumophila was found in this study.