A vancomycin-resistant Staphylococcus aureus (VRSA) isolate was obtained from a patient in Pennsylvania in September 2002. Species identification was confirmed by standard biochemical tests and analysis of 16S ribosomal DNA, gyrA, and gyrB sequences; all of the results were consistent with the S. aureus identification. The MICs of a variety of antimicrobial agents were determined by broth microdilution and macrodilution methods following National Committee for Clinical Laboratory Standards (NCCLS) guidelines. The isolate was resistant to vancomycin (MIC ‫؍‬ 32 g/ml), aminoglycosides, ␤-lactams, fluoroquinolones, macrolides, and tetracycline, but it was susceptible to linezolid, minocycline, quinupristin-dalfopristin, rifampin, teicoplanin, and trimethoprim-sulfamethoxazole. The isolate, which was originally detected by using disk diffusion and a vancomycin agar screen plate, was vancomycin susceptible by automated susceptibility testing methods. Pulsed-field gel electrophoresis (PFGE) of SmaI-digested genomic DNA indicated that the isolate belonged to the USA100 lineage (also known as the New York/Japan clone), the most common staphylococcal PFGE type found in hospitals in the United States. The VRSA isolate contained two plasmids of 120 and 4 kb and was positive for mecA and vanA by PCR amplification. The vanA sequence was identical to the vanA sequence present in Tn1546. A DNA probe for vanA hybridized to the 120-kb plasmid. This is the second VRSA isolate reported in the United States.

Disk diffusion and broth microdilution (BMD) were used to perform clindamycin (CLI) induction testing on؉ phenotype by disk diffusion were also detected by BMD using a variety of CLI and ERY concentrations; however, isolates with the D phenotype were more difficult to detect by BMD and will likely require optimization of ERY and CLI concentrations in multilaboratory studies to ensure adequate sensitivity. Thus, at present, disk diffusion is the preferred method for testing S. aureus isolates for inducible CLI resistance.Erythromycin (ERY) (a macrolide) and clindamycin (CLI) (a lincosamide) represent two distinct classes of antimicrobial agents that inhibit protein synthesis by binding to the 50S ribosomal subunits of bacterial cells. In staphylococci, resistance to both of these antimicrobial agents can occur through methylation of their ribosomal target site (25). Such resistance is typically mediated by erm genes. Resistance to macrolides also can occur by efflux, typically mediated by the msrA gene (16). Another resistance mechanism, inactivation of lincosamides by chemical modification (such as mediated by the inuA gene), appears to be rare (1,8,15). The target site modification mechanism, also called macrolide-lincosamide-streptogramin B (MLS B ) resistance, results in resistance to ERY, CLI, and streptogramin B. This mechanism can be constitutive, where the rRNA methylase is always produced, or can be inducible, where methylase is produced only in the presence of an inducing agent. ERY is an effective inducer, but CLI is a weak inducer. In vitro, Staphylococcus aureus isolates with constitutive resistance are resistant to ERY and CLI, and isolates with inducible resistance are resistant to ERY but appear susceptible to CLI. In vivo, therapy with CLI may select for constitutive erm mutants (7), which may lead to clinical failure (2,20,24). Isolates with msrA-mediated efflux also appear ERY resistant and CLI susceptible by in vitro tests; however, such isolates do not typically become CLI resistant during therapy.An in vitro induction test can distinguish staphylococci that have inducible erm-mediated resistance from those with msrAmediated resistance. The test is performed by disk diffusion, placing a 15-g ERY disk in proximity to a 2-g CLI disk on an agar plate that has been inoculated with a staphylococcal isolate; the plate is then incubated overnight (5,9,22,23). A flattening of the zone of inhibition around the CLI disk proximal to the ERY disk (producing a zone of inhibition shaped like the letter D) is considered a positive result and indicates that the ERY has induced CLI resistance (a positive "D-zone test").

No abstract

No abstract

We assessed the in vitro activities of daptomycin, linezolid, and quinupristin-dalfopristin (QD) against a contemporary challenge panel of 88 staphylococcal and 90 enterococcal isolates. The staphylococci selected included vancomycin-intermediate Staphylococcus aureus (VISA), methicillin-resistant S. aureus, and coagulasenegative staphylococci. Enterococcal isolates included vancomycin-resistant Enterococcus faecium (VREF) containing either vanA, vanB1, or vanD. The MICs of daptomycin, linezolid, and QD were determined using commercial broth microdilution panels. All three VISA isolates were susceptible to daptomycin, linezolid, and QD. QD was the most active agent against staphylococcal isolates (MIC50 < or = 0.5 microg/ml and MIC90 = 1 microg/ml), including those with decreased susceptibility to vancomycin. QD was also the most active agent against VREF (MIC90 < or = 0.5 microg/ml). No differences were seen for susceptibility of vanA, vanB1, and vanD VREF strains for daptomycin, linezolid, or QD. Daptomycin was the most effective against E. faecalis. On the basis of manufacturer-suggested interpretive criteria, 92% of isolates were susceptible (MIC90 = 4 microg/ml). All isolates tested were susceptible to at least one antimicrobial agent for which interpretive criteria have been defined. Population analysis of three S. aureus isolates for which the daptomycin MICs were 8 microg/ml showed a pattern of homogeneous resistance.