Sensitivity and resistance of Legionella pneumophila to some antibiotics and combinations of antibiotics

For the treatment of Legionella pneumophila infections erythromycin and rifampicin are the antibiotics of choice. In view of reported therapy failures other antibiotics, e.g. the quinolones, are currently under investigation. The sensitivity of L. pneumophila to four antibiotics and to combinations of antibiotics was investigated and the rate of mutations was calculated. For 20 L. pneumophila strains we determined the MIC of rifampicin (0.002–0.004 mg/l), erythromycin (0.063–0.125 mg/l), norfloxacin (0.125 mg/l) and ciprofloxacin (0.016–0.032 mg/l). Mutation rates ranged from 1 × 10−8 for ciprofloxacin to >1 × 10−7 for erythromycin, resulting in high-level resistance to rifampicin in most strains and erythromycin resistance in one strain, but not in resistance to the quinolones. The combination of erythromycin and rifampicin was synergistic (FIC index >0.5) against four of the L. pneumophila strains and showed indifference (FIC index 0.5–2.0)for the remainder (mean FIC index 0.79). Combinations of ciprofloxacin and erythromycin and of rifampicin and ciprofloxacin showed only indifference (mean FIC index respectively 1.05 and 1.21). Combining rifampicin with ciprofloxacin was not effective in reducing the number of mutants for either of these antibiotics, whereas the other combinations did prevent this.