Development of disk diffusion susceptibility test methods for Aerococcus spp. and updates to Clinical and Laboratory Standards Institute MIC breakpoints.

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 summarized herein are the basis for proposed changes to the M45 guideline for Aerococcus spp., which will be published in the fourth edition.

Importance: Aerococcus species are an increasingly recognized cause of urinary tract infections and may cause serious infections, including endocarditis. Antimicrobial susceptibility testing (AST) is required for the management of these infections, as species within the genus display resistance to various antimicrobial agents. This study describes the data that informed updates to the Clinical and Laboratory Standards Institute M45 guideline for AST of fastidious or infrequently isolated bacteria. We describe the introduction of new ampicillin and nitrofurantoin breakpoints, updated MIC breakpoints for penicillin, and disk diffusion correlates for penicillin, ampicillin, ciprofloxacin, levofloxacin, nitrofurantoin, and tetracycline.