Development of disk diffusion susceptibility test methods for Aerococcus spp. and updates to Clinical and Laboratory Standards Institute MIC breakpoints
Abstract
UNLABELLED: Aerococcus spp. are uropathogenic gram-positive bacteria that occasionally cause invasive infections. Minimal inhibitory concentration (MIC) breakpoints published by the Clinical and Laboratory Standards Institute (CLSI) were evaluated, and disk diffusion criteria developed, by testing 174 isolates of Aerococcus spp., by the CLSI M45 fourth edition workgroup. Aerococcus urinae growth was found to be significantly impacted by the brand of reference Mueller-Hinton broth base, and 14.6% of 117 A. urinae tested did not grow in cation-adjusted Mueller-Hinton broth supplemented with 5% lysed horse blood after 44 h incubation at 35°C in 5% CO2 in any brand. Ampicillin and nitrofurantoin MIC breakpoints were established, and penicillin MIC breakpoints were adjusted, based largely on MIC distributions and pharmacokinetic data. Disk criteria were established for penicillin, ampicillin, vancomycin, nitrofurantoin, ciprofloxacin, levofloxacin, and tetracycline. Disk breakpoints could not be established for cefotaxime, ceftriaxone, or meropenem using standard CLSI disk masses due to large zones of growth inhibition that did not differentiate between MIC values and error rates that far exceeded CLSI standards. Trimethoprim-sulfamethoxazole breakpoints were also not possible to establish due to differences in thymidine content between MIC and disk diffusion testing media, which led to no correlation between MIC values and zones of growth inhibition for this antibiotic. The data…
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