Controversies and confusion regarding antimicrobial susceptibility testing of enterococci

Antimicrobial resistance in staphylococci and enterococci in 10 Portuguese hospitals in 1996 and 1997. POSGAR. Portuguese Study Group of Antimicrobial Resistance

During a 2-year period, 10 Portuguese hospitals located throughout the country studied antimicrobial susceptibilities of clinically relevant staphylococci and enterococci. Of more than 12,000 Staphylococcus aureus isolates tested, two main patterns were found, methicillin-sensitive organisms most of them resistant only to penicillin but a few to other antimicrobials and methicillin-resistant S. aureus (MRSA) strains (prevalence 48.2%) resistant to most of the antimicrobials tested and uniformly susceptible to vancomycin. Among coagulase-negative staphylococci (CNS), 71% of S. epidermidis (approximately 5,000 isolates tested) and 84% S. haemolyticus (approximately 1,000 isolates tested) were also resistant to methicillin as well as most other antimicrobials except vancomycin. Most of the 5,000 Enterococcus faecalis isolates tested were susceptible to ampicillin and vancomycin, in contrast to 650 E. faecium isolates, 70% of which were resistant to ampicillin and 20% to vancomycin and all other antibiotics. A high prevalence of aminoglycoside resistance occurred in both Enterococcus species. This survey showed that resistance profiles of staphylococci and enterococci hospital isolates have not changed in the last 5 years in Portugal, with the exception of the rise in vancomycin resistance in E. faecium. The high prevalence of methicillin resistance in S. aureus and in the CNS remains an issue of medical concern.

Emerging multiply resistant enterococci among clinical isolates. II. Validation of the etest to recognize glycopeptide-resistant strains

Accurate, quantitative susceptibility testing of the enterococci for glycopeptide (vancomycin and teicoplanin) activity has become very important to guide chemotherapy as a result of emerging resistance. Reference dilution tests using broth and agar are generally difficult to perform, or in some commercial forms have demonstrated interpretive error. An alternative method, the Etest has promise as a system with qualitative and quantitative accuracy. Nearly 2000 enterococci from 97 laboratories in the United States were tested using broth microdilution, disk diffusion, and Etest methods. The Etest quantitative accuracy (+/- 1 log2 dilution) compared to the broth microdilution minimum inhibitory concentration was 90.1% (teicoplanin) to 94.1% (vancomycin). The qualitative interpretive accuracy of the Etest ranged from 98.7% for vancomycin to 99.9% for teicoplanin (no false-susceptible errors). All three tests were in remarkable agreement, with < or = 0.8% maximal discord by interpretive category. The Etest appears to be an excellent alternative to reference and standardized methods for producing quantitative activity measurements of glycopeptide susceptibility or resistance.

Emerging multiply resistant enterococci among clinical isolates. I. Prevalence data from 97 medical center surveillance study in the United States. Enterococcus Study Group

To assess the evolving problem of therapeutic drug resistances among enterococci, we organized a comprehensive national (United States) surveillance trial using 99 recruited microbiology laboratories in 48 of the 49 contiguous states or districts. All but two sites completed the protocol that generated information from nearly 2000 enterococci, usually isolated from blood cultures. All strains were speciated by the same method (API 20S) and were susceptibility tested by three methods (broth microdilution, disk diffusion, and Etest) against ampicillin, penicillin, vancomycin, teicoplanin, gentamicin, and streptomycin. Strains resistant to a glycopeptide or penicillin, or possessing high-level aminoglycoside resistance were referred to the monitor's laboratory for validation and additional susceptibility testing against other alternative antimicrobial agents. The most common species were Enterococcus faecalis and Enterococcus faecium. However, antimicrobial resistance occurred most often among the E. faecium isolates. Twenty-three percent of participant centers (22 sites) reported 87 vancomycin-resistant isolates, which accounts for 4.4% of the isolates evaluated. A recent audit (March 1994) of the laboratories not reporting vancomycin resistance during the study interval (October-December 1992) revealed that 61% of sites have now recognized these strains, a threefold increase in 12-15 months. Teicoplanin remained active against 28% (Van B phenotype) of vancomycin-resistant enterococci (10 E. faecalis, 13 E. faecium, and one Enterococcus spp.). Ampicillin-resistant beta-lactamase-positive strains were found only at one medical center (two strains, 0.2% of referred or validated strains); however, ampicillin-resistant strains represented 12% of all enterococcal, but nearly 60% of E. faecium strains.(ABSTRACT TRUNCATED AT 250 WORDS)