Comparison of the Etest with a conventional agar dilution method in evaluating the in vitro activity of moxifloxacin

Abstract

The Etest was introduced in 1988 as an alternative method of determining MICs.Although it is used principally to confirm resistance, rather than as a primary susceptibility testing method, the pharmaceutical industry is increasingly adopting it as a means of evaluating new antimicrobial agents, these data being used to confirm or suggest tentative MIC breakpoints.In the present study we have compared the MICs of moxifloxacin determined by two methodologies: according to recommendations of the British Society for Antimicrobial Chemotherapy (BSAC), 1 and with the Etest adapted for use in the UK. 2 M o x i floxacin was provided by Bayer AG (Wuppertal, G e r m a n y ) .The 149 clinical isolates studied comprised 11 strains of Bacteroides fragilis, 17 of anaerobic streptococci, 12 of Haemophilus influenzae, 11 of Streptococcus pneumoniae, 12 of Moraxella catarrhalis, 52 of Enterobacteriaceae, 12 of Pseudomonas aeruginosa and 22 of staphylococci; 11 NCTC/ATCC strains were included as controls.The media used were those recommended by the BSAC, i.e.Iso-Sensitest agar (ISTA) (Oxoid, Basingstoke, UK) for facultative anaerobes and Wilkins & Chalgren agar (WCA; Oxoid) for anaerobes.Where appropriate, ISTA was supplemented with 5% defibrinated horse blood (DFB) and/or -nicotinamide adenine dinucleotide (Sigma Diagnostics, Poole, UK) in a concentration of 20 mg/L and WCA was supplemented with 5% DFB.The inoculum for the agar dilution method was 10 4 cfu/spot, and that for the Etest was a suspension with a density equivalent to that of a 0.5 McFarland standard.Incubation was at 35-37ºC for 18-20 h in air, except in the case of fastidious organisms, for which the atmosphere was enriched with 4-6% CO 2 , and anaerobes, which were incubated in an anaerobic cabinet.Susceptibility categories were assigned according to the following tentative MIC breakpoints: susceptible 1 mg/L

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Affiliated Institutions

• Birmingham City Hospital GB

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