Susceptibility testing by E-test and agar dilution of 30 strains of Legionella spp. isolated in Portugal

Abstract

Objective: To evaluate the activity of erythromycin, roxithromycin, ciprofloxacin, doxycycline, sulfamethoxazole trimethoprim and rifampin against 32 strains of Legionella spp. under different testing conditions. Methods: Minimum inhibitory concentrations (MICs) were determined by the E-test (Ab Biodisk, Solna, Sweden) and agar dilution reference technique (National Committee for Clinical Laboratory Standards (NCCLS), 1990) on two different media, buffered charcoal yeast extract agar (BCYE-α) and buffered yeast extract agar (BYE-α), under 48- and 72-h incubation, without CO2. Results: All the antimicrobial agents were inhibited by BCYE-α agar. The MIC90 values on BYE-α were lower than those on BCYE-α but the variation factor was not the same: ciprofloxacin and rifampin, followed by erythromycin, suffered the greatest inhibition by the charcoal in the culture medium. Except for ciprofloxacin and rifampin, the 72-h MIC90 readings were always higher than the 48-h results whenever the agar dilution method was used. The E-test results showed slight variations with some, but not all, antibiotics. The most active agents against the 32 Legionella strains tested were rifampin and ciprofloxacin. Conclusions: BCYE-α is not suitable for susceptibility testing of Legionella spp. The E-test method on BYE-α agar with 48-h incubation is recommended.