The effects of single and combined antibiotics on the growth of Legionella pneumophila using time-kill studies.

The combinations of erythromycin with rifampicin, with ciprofloxacin or with amoxycillin, and ciprofloxacin plus rifampicin were tested by the time-kill curve method to assess their bactericidal activity against Legionella pneumophila. Rifampicin-resistant strains were found in broth cultures of the micro-organism even before exposure to the drug. In the presence of MBCs of rifampicin, the sensitive organisms were killed, allowing the resistant mutants to multiply. In broths containing both erythromycin and rifampicin, the rifampicin-resistant mutants were killed more rapidly than by erythromycin alone. In addition, erythromycin was effective in preventing the growth of amoxycillin- and ciprofloxacin-resistant organisms. Fortunately resistance to erythromycin was not detected by these time-kill studies. Thus for patients with severe forms of Legionnaires' disease, erythromycin should be combined with other more inhibitory drugs, such as rifampicin or ciprofloxacin, to enhance bactericidal activity.