468. Evolution of in vitro susceptibility of Staphylococcus aureus (SA) blood isolates at a university hospital
Session: Poster Session: Hospital-acquired and Transplant Infections
Friday, October 30, 2009: 12:00 AM
Room: Poster Hall A
Background: In recent years, increasing resistance to vancomycin (V) among SA isolates has emerged, including strains that are completely resistant and others that are intermediately susceptible or show heteroresistance (hGISA). Some investigators have reported upward MIC drift for SA versus V . We evaluated the comparative in vitro activity of V, daptomycin (D), and linezolid (L) against SA isolated from blood of patients at RWJ University Hospital over 3 decades to examine whether MICs of SA changed during this time period.
Methods: Since 1980, all blood isolates causing bacteremia at RWJUH have been saved and stored at -70oC. 12 MSSA and 12 MRSA from 1980-85, 1986-89, 1990, and 1995 as well as 24 MSSA and 24 MRSA for each year from 2000-2008 were tested by CLSI reference broth microdilution (BMD) and Etest for V, D, and L MIC values. MBCs were measured by the CLSI reference method, and the presence of glycopeptide-intermediate (GISA) or heteroresistant (hGISA) strains was assessed using Etest GRD VA/TP strips.
Results: To date, 529 strains have been tested. Although no upward MIC drift was noted for V, D, or L by BMD, an upward trend of geometric mean (GM) MICs for V between 2000 and 2008 was evident for MRSA strains using the Etest method [GM=1.23 (2000) rose to 1.86 (2008); r=0.76]. GM MICs did not change for D or L. Etest GM MRSA MICs for V in 1990 and 1995 were elevated (1.8 and 1.76, respectively), possibly suggesting the presence of a dominant clone with more resistance to V during these years. Given these high MICs, analysis for all years did not show an upward MIC trend (r=0.07). For the period 2000-2008, 2 GISA and 35 hGISA (of 201 MRSA tested, 17.5%) were detected; only 1 of 26 MRSA tested from the years prior to 2000 was hGISA.
Conclusion: Upward MIC drift of V MICs for MRSA was noted for the years 2000-2008 but was only detected by the Etest method. However, this trend was not detectable during the full study period due to high V MICs in 1990 and 1995. The possibility of a resistant clone during these years is being investigated. hGISA emerged in 2000 and was present in 17.5% of MRSA strains.
Alexandra Sonyey, MD, Robert Wood Johnson Medical School, New Brunswick, NJ, Melvin Weinstein, MD and  A. Sonyey, None. 
M. P. Weinstein,
Cubist Pharmaceuticals Role(s): Grant Investigator, Received: Research Grant.
Becton Dickinson Role(s): Other, Honorarium for lectures, Received: Speaker Honorarium.
Siemens Role(s): Grant Investigator, Received: Research Grant.