Journal of Infection (2015) 71, 395e412
LETTERS TO THE EDITOR In vitro susceptibility of Aerococcus urinae isolates to antibiotics used for uncomplicated urinary tract infection
KEYWORDS Aerococcus; Antimicrobial susceptibility testing; Urinary tract infections
To the Editor Rasmussen and colleagues, in this Journal, recently highlighted the emerging problem of Aerococcal infections.1 Aerococcus urinae is a Gram-positive, a-hemolytic, and catalase-negative coccus ranking sixth among uropathogens.1,2 However, data regarding its susceptibility to antibiotics classified as first-line agents for the empirical treatment of uncomplicated urinary tract infections (UTIs) by the Infectious Disease Society of America (IDSA) and the European Society of Clinical Microbiology and Infectious Diseases (ESCMID) are scarce.3 By broth microdilution, Humphries et al. reported susceptibility rates of 84% for levofloxacin (LEV) and 97% for trimethoprim-sulfamethoxazole (SXT) using the Clinical Laboratory Standards Institute (CLSI) interpretive criteria.4 As stated in a recent review by Rasmussen et al., the fosfomycin (FOS) susceptibility rate for A. urinae was 90%1; however, these data were only based on twenty isolates. In an older study from 1997, which may not represent our current situation, Schuur et al. reported that their collection of A. urinae was uniformly susceptible to nitrofurantoin (NIT).5 To establish the in vitro susceptibility of contemporary A. urinae to NIT, SXT, LEV and FOS, we tested 50 nonduplicate isolates detected in urine samples collected between January 2013 and December 2014 at the Laboratory of Microbiology of the Institute for Infectious Diseases, University of Bern (Switzerland). Species identification was obtained by using matrixassisted laser desorption ionization time-of-flight mass spectrometry (Bruker Daltonics, Bremen, Germany).
Minimum inhibitory concentrations (MIC) were determined rieux, Marcy l’Etoile, by Etest (bioMe France) on cationadjusted Mueller-Hinton (CAMH) agar plates supplemented rieux) for NIT, FOS and LEV or with 5% sheep blood (bioMe rieux). On with 5% lysed horse blood (LHB) for SXT (bioMe account of the conflicting SXT susceptibility rates reported in studies using different methods,4,6,7 we also determined the SXT MIC values by microdilution using Todd-Hewitt broth (THB) supplemented with 2.5% LHB. A. urinae ATCC 700306 was used as control. Due to the lack of specific breakpoints for A. urinae, we interpreted the results according to the European Committee on Antimicrobial Susceptibility Testing (EUCAST) and to the CLSI criteria set for Enterobacteriaceae.8,9 As shown in Fig. 1, all isolates were susceptible to NIT irrespective of the criteria used (MIC90 of 4 mg/ml). For FOS, all isolates were susceptible using the CLSI criteria, whereas 90% were susceptible using the EUCAST cutoff (MIC90 of 32 mg/ml). Most isolates were susceptible to LEV regardless of the criteria used (88%; MIC90 of 4 mg/ml). Using the Etest method only 16% of A. urinae isolates remained susceptible to SXT (MIC90 64 mg/ml). This was in strong contrast to the results obtained using broth microdilution where 60% of the isolates were susceptible to SXT (MIC90 8 mg/ml). Detailed information on the discrepancies between Etest and broth microdilution results for each tested strain is provided in the Supplementary Table. Our results indicate that NIT and FOS, the two antibiotic agents recommended by the IDSA and ESCMID as empirical first-line treatment for uncomplicated UTIs, demonstrate good in vitro activity against A. urinae.3 In contrast, we found in our local strains of A. urinae increased levels of resistance to quinolones compromising their role in empiric treatment. We recently demonstrated that such resistant isolates possess amino acid substitutions in GyrA and/or ParC.2 A. urinae has been classically reported as resistant to SXT in vitro, an antibiotic otherwise considered as first empirical choice for UTIs, as long as the local resistance rate for uropathogens is