Multicenter Evaluation of the In Vitro Susceptibility of Staphylococci to Vancomycin

Abstract

We conducted a prospective study to identify the frequency of glycopeptide-intermediate Staphylococcus aureus (GISA), heteroresistant GISA, and glycopeptide-intermediate and heteroresistant coagulase-negative staphylococci (CNS) in 4 participating centers and to determine strain relatedness among GISA and heteroresistant GISA isolates. Among 3117 isolates screened, 227 (63 S. aureus and 164 CNS) grew on brain-heart infusion agar plates supplemented with 4 mg/mL vancomycin. The population susceptibility profiles of these isolates were by 2 different methods. By the agar dilution MIC method, 3 S. aureus isolates and 14 CNS isolates had population susceptibility profiles of 4 mg/mL, and 6 CNS isolates had population susceptibility profiles of 8 mg/mL with MICs of 6 to 16 mg/mL by broth microdilution. The same 227 isolates were tested by a different method of performing E-test on Mueller-Hinton agar following an overnight growth in broth. Among the 227 isolates tested, 27 isolates (7 S. aureus and 20 CNS) showed growth in the clear zone of inhibition, 3 S. aureus isolates had subpopulations with MIC of 6 mg/mL, and 7 CNS isolates had subpopulations with MIC of 6 to 16 mg/mL. Using the vancomycin agar screen method, 2.5% of S. aureus and 28.4% of CNS had reduced susceptibility to vancomycin. No S. aureus and only 1.2% of CNS were confirmed as such by vancomycin dilution subpopulation analysis. By using the vancomycin E-test method, 0.12% of S. aureus and 1.2% of CNS isolates were confirmed to have reduced susceptibility to vancomycin. E-test analysis of an overnight growth of the isolate and evaluation of colonies in the clear zone of inhibition improved the accuracy of detection of staphylococcal subpopulations with reduced susceptibility to vancomycin. (Infect Dis Clin Pract 2006;14:89-92)