Evaluation of the etest for antimicrobial spectrum and potency determinations of anaerobes associated with bacterial vaginosis and peritonitis

One hundred ninety-seven anaerobic organisms (24 Gardnerella vaginalis, 16 Mobiluncus spp., 19 Peptostreptococcus spp., 20 Lactobacillus spp., 20 Prevotella bivia/disiens, 81 Bacteroides fragilis group, 12 Clostridium spp., and five Fusobacterium spp.) were processed by the Etest (AB Biodisk, Solna, Sweden) and a reference ( Brucella blood agar) method against 10 antimicrobial agents. For the bacterial vaginosis-associated pathogens, the Etest was more reproducible and correlated acceptably with the reference agar test: within ±1 log 2 dilution for 74.4% of Mobiluncus spp. to 96.0% for Peptostreptococcus spp. (all organisms, 83.4%). The quantitative correlation ±2 log 2 dilution steps between test results was 94.3%. Results with B. fragilis group strains demonstrated 97.3% correlation (±2 log 2 dilution) with a trend toward slightly lower Etest minimum inhibitory concentrations for ampicillin-sulbactam, cefotaxime, imipenem, and clindamycin. The absolute qualitative interpretive agreement between Etest and the reference agar dilution method results was 94.4%, with only a 0.4% false-susceptible error rate. The Etest appears to be a very practical, quantitatively accurate, alternative procedure for clinical microbiology laboratories routinely testing the susceptibilities of anaerobes and, by these presented data, organisms associated with female tract infections.