In vitro activity of clinically implemented β-lactams against Aerococcus urinae: presence of non-susceptible isolates in Switzerland.

We analyzed the in vitro susceptibility to several ?-lactams and vancomycin of 80 Aerococcus urinae isolates collected during 2011-2012 in Switzerland. MICs were determined by Etest (bioMérieux) on Müller-Hinton agar with 5% sheep blood and interpreted according to the CLSI and EUCAST criteria set for viridans streptococci. MIC50/90 for penicillin, amoxicillin, ceftriaxone and vancomycin were 0.016/0.064 mg/l, 0.032/0.064 mg/l, 0.125/0.5 mg/l and 0.38/0.5 mg/l, respectively. Three (3.8%) isolates were resistant to ceftriaxone regardless of the criteria used (MICs ?2 mg/l); one of them was also non-susceptible to penicillin (MIC of 0.25 mg/l) according to CLSI. β-lactam resistance in A. urinae is a concern and suggests that more studies are needed to determine the molecular mechanisms of such resistance.