Susceptibility of Legionella species to five antibiotics and development of resistance by exposure to erythromycin, ciprofloxacin, and rifampicin

The minimal inhibitory concentration (MIC) values of erythromycin, ciprofloxacin, ofloxacin, rifampicin, and clindamycin were determined for 56 strains of Legionella pneumophila (38 patient, 3 environmental, and 15 reference strains) and 37 strains of other Legionella species (7 patient, 2 environmental, and 28 reference strains) using the ϵ-test system on BCYEα agar plates. High-level resistance (MIC ≥ 4 μg/mL) was found only for clindamycin (57%), with MIC values ranging from 0.25–32 μg/mL. Low-level resistance was found for erythromycin (18%) (0.5 < MIC < 8), ciprofloxacin (1%) (1< MIC < 4), and clindamycin (40%) (0.5 < MIC < 4), but not for ofloxacin and rifampicin. MIC 50 for the 45 Danish clinical Legionella strains were 0.25 μg/mL (erythromycin), 0.25 μg/mL (ciprofloxacin), 0.19 μg/mL (ofloxacin), below 0.016 μg/mL (rifampicin), and 4 μg/mL (clindamycin). Of the clinical isolates, 64% were resistant to clindamycin. There were no significant differences between the MIC 50 values obtained for clinical and nonclinical Legionella strains. Selected susceptible strains were exposed to increasing concentrations of either erythromycin, ciprofloxacin, or rifampicin to select for resistance. Isolates resistant to erythromycin (MIC 0.75–32 μg/mL) or ciprofloxacin (MIC 2–3 μg/mL) could be selected by a two-step procedure. One single strain recovered from media containing 50 μg/mL of erythromycin had an MIC value higher than 256 μg/mL to erythromycin. In contrast, high-level resistance toward rifampicin with MIC ≥ 256 μg/mL developed as a one-step phenomenon in several strains.