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.