Application of the Etest to antimicrobial susceptibility testing of Legionella spp

Development of susceptibility tests for Legionella spp. has been difficult because of the specific growth requirements of this organism. The most commonly used media, buffered charcoal-yeast extract (BCYE) agar contains charcoal, which is known to inactivate some antimicrobial agents. This study compared five antimicrobial (erythromycin, clindamycin, ofloxacin, doxycycline, and rifampin) minimum inhibitory concentration (MIC) values as determined by agar dilution using BCYE, agar dilution using this same media without the charcoal [buffered yeast extract (BYE) agar], and the Etest on BCYE media. The MIC50 and MIC90 for this group of Legionella spp. were greater with BCYE than with BYE. Erythromycin and ofloxacin (fourfold change) were the most affected by the charcoal in the medium. The Etest MIC and agar dilution MIC with BYE were comparable. The Etest rifampin results demonstrated that the Legionella spp. strains were very susceptible (< 0.016 micrograms/ml, producing very large zones) requiring use of one half of the Etest strip, a whole test strip on an individual 100-mm plate of BCYE, or use of a new low-MIC-range Etest strip. The Etest on BCYE provides a simple, readily available, and accurate method unaffected by medium components for susceptibility testing of Legionella spp.