Evaluations of the Etest for antimicrobial susceptibility testing of Legionella pneumophila, including validation of the imipenem and sparfloxacin strips.

Development of antimicrobial testing for Legionella spp. has been technically compromised by the fastidious growth requirements of this organism and by the most commonly used medium, buffered charcoal yeast extract agar (BCYE) that contains substances known to inhibit some antimicrobial agents. This study validated the potency of two newer antimicrobials (sparfloxacin and imipenem) and their Etest strip. The comparisons of antimicrobial minimum inhibitory concentration (MIC) values as determined by Etest on BCYE agar (98 Legionella pneumophila) demonstrated that sparfloxacin was the most potent drug (MIC90, 0.19 microgram/ml) among the fluoroquinolones, macrolides, tetracyclines, and beta-lactams tested. Imipenem MICs (MIC90, < or = 0.38 microgram/ml) were also determined by a reference agar dilution method and validated Etest strips on buffered yeast extract agar, buffered charcoal yeast extract agar, and buffered charcoal yeast extract agar with defined supplements. The non-Legionella control strains used to demonstrate medium component influences on the imipenem MICs demonstrated the addition of supplements (particularly L-cysteine) markedly elevated the MICs. These data indicate that the Etest was a simple and accurate quantitative method for susceptibility tests with Legionella isolates. Sparfloxacin among the fluoroquinolones and imipenem among the beta-lactams require further clinical studies for legionellosis therapy.