In vitro assessment of urinary isolates of ampicillin-resistant enterococci

OBJECTIVE: Susceptibility and minimum inhibitory concentration (MIC) studies of ampicillin-resistant enterococci (ARE) were performed with vancomycin, ciprofloxacin, and trovafloxacin. Ampicillin MICs were determined to make comparisons with achievable urinary concentrations of ampicillin. DESIGN: From July 1998 to April 1999, all enterococci isolated from urinary specimens were tested for susceptibility to ampicillin by disk diffusion. For all ARE, vancomycin, ciprofloxacin, and trovafloxacin susceptibilities were determined by use of either disk diffusion or the E-test. Ampicillin MICs were determined for these isolates by liquid agar microdilution testing. ARE were identified to the species level on the basis of biochemical reactions. SETTING: The study was performed at a university-affiliated tertiary care hospital. OUTCOME MEASURES: In vitro susceptibility studies and MIC determinations were performed in accordance with the National Committee for Clinical Laboratory Standards. RESULTS: A total of 310 urine samples were culture positive for enterococcus. Thirty (9.7%) unduplicated isolates were resistant to ampicillin. Of these, nine ARE isolates (30%) were also vancomycin resistant, whereas only 2 ampicillin-susceptible isolates were vancomycin resistant (p < 0.05). All ARE were resistant to ciprofloxacin, and 29 (96.7%) were resistant to trovafloxacin. Nine (30%), 18 (60%), and 3 (10%) isolates had an ampicillin MIC of 128, 256, and 512 μg/mL, respectively. Ampicillin MICs did not differ significantly between vancomycin-susceptible and -resistant isolates (p = 0.963). Twenty-seven isolates (90%) were identified as Enterococcus faecium; the other 3 were either Enterococcus avium or Enterococcus raffinosus. CONCLUSIONS: Ampicillin resistance is associated with resistance to vancomycin. Most ARE are resistant to fluoroquinolone antibiotics such as ciprofloxacin and trovafloxacin. Ampicillin MICs for ARE found in these urinary specimens were all within 1 dilution of 256 μg/mL, a concentration achievable in the urine with higher doses of oral amoxicillin or intravenous ampicillin. Additional studies are needed to assess the clinical implications of these data.