SUSCEPTIBILITY OF LEGIONELLA SPECIES TO ANTIMICROBIAL AGENTS

Thirty-five clinical isolates of Legionella species were tested agains t 7 antimicrobial agents using an agar dilution technique. Results obt ained on charcoal-supplemented (BCYE) and charcoal-free agar (BSYE) we re compared. On BCYE, the most active agent was rifampicin; the minima l inhibitory concentration inhibiting 90% of the strains (MIC90) was 0 .008 mg/L. Imipenem was the next most active in vitro (MIC90 0.06 mg/L ). The macrolide antibiotics and ciprofloxacin also inhibited the orga nisms at low concentrations (MIC90 less-than-or-equal-to 2 mg/L). In g eneral, MIC's obtained on BCYE agar were at least twofold higher than on BSYE agar except for that of imipenem. BSYE agar is a suitable alte rnative medium for susceptibility testing of most Legionella species. Erythromycin and rifampicin continue to demonstrate good in vitro acti vity against legionellae in Australia. On the basis of in vitro suscep tibility tests, the other macrolides and ciprofloxacin are likely to b e suitable alternatives for the treatment of legionellosis.