Development of two methods for detecting Streptococcus mitis/oralis-related species with meropenem-non-susceptibility

Several studies have reported clinical isolates of Streptococcus mitis, Streptococcus oralis, and their related species (S. mitis/oralis group) with reduced carbapenem susceptibility. However, to date, no simple and accurate methods of detecting S. mitis/oralis group isolates with reduced carbapenem susceptibility have been reported. The meropenem ETEST misinterpreted 3 of 25 (12 %) meropenem-non-susceptible S. mitis/oralis group isolates as susceptible to meropenem. Therefore, we screened 38 disks containing 38 different β-lactams to detect S. mitis/oralis group strains with reduced carbapenem susceptibility using the disk-diffusion method. Additionally, using the agar dilution method to determine susceptibility, we identified the optimal antibiotic concentrations in agar medium to enable selective culture of meropenem-non-susceptible S. mitis/oralis group by culturing 25 meropenem-non-susceptible (meropenem minimum inhibitory concentration [MIC] >0.5 μg/mL) and 24 meropenem-susceptible (meropenem MIC ≤0.5 μg/mL) S. mitis/oralis group isolates on agar containing different concentrations of six β-lactams. Meropenem-susceptible and non-susceptible S. mitis/oralis group isolates were successfully differentiated using oxacillin- and cefoxitin-containing disks. Moreover, cefdinir-, flomoxef-, meropenem-, and doripenem-containing disks showed clear differences in the growth inhibitory zone diameter between the meropenem-susceptible and meropenem-non-susceptible S. mitis/oralis group isolates. Media containing 2 μg/mL oxacillin, 4-16 μg/mL cefoxitin, 2 μg/mL cefdinir, 2-4 μg/mL flomoxef, 0.06-0.25 μg/mL meropenem, and 0.06-0.12 μg/mL doripenem allowed selective culture of meropenem-non-susceptible S. mitis/oralis group strains. The disk-diffusion method using six β-lactam-containing disks and selective culture agar medium are the first-reported simple and accurate methods of detecting meropenem-non-susceptible S. mitis/oralis group suitable for use in clinical microbiology laboratories.