Susceptibility of Legionella pneumophila to twenty antimicrobial agents

Thirty-three isolates of Legionella pneumophila, all except one of which were clinical isolates, were tested against 20 antimicrobial agents by using an agar dilution technique. Erythromycin, rifamp]in, and rosaramycin were the most active agents tested. Aminoglycosides, chloramphenicol, and cefoxitin also inhibited the organisms at low concentrations. Other agents, including moxalactam, cefoperazone, and cephalosporins, exhibited moderate to little activity. Tetracycline, doxycycline and minocyeline were apparently inactivated by charcoal-yeast extract medium. There was slight inoculum dependence noted with most of the antimicrobials tested, particularly the beta-lactam agents. There was no consistent difference in susceptibility between Center for Disease Control-supplied stock strains and recent clinical isolates, but there were marked differences with some agents. Susceptibility testing needs to be standardized in view of the influence of inoculum size, strain variation, and the medium used.