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.