Abstract
BackgroundPrevious studies regarding the prognosis of patients infected with MRSA isolates characterized by a high minimum inhibitory concentration (MIC) for vancomycin have generally used a commercial Etest. Little research has been conducted on determining the vancomycin susceptibility of MRSA using a reference microdilution. Additionally, there is discordance between the MIC result from an Etest and the value determined using the reference microdilution method.MethodsUsing a reference microdilution method, we determined the MIC of vancomycin for isolates from 123 consecutive patients with nosocomial MRSA bacteremia. The clinical features and outcome for these patients were recorded and the MRSA isolates were genotyped.ResultsAmong the 123 non-duplicated isolates, 21.1% had a MIC = 2 mg/L, 76.4% had a MIC = 1 mg/L and 2.4% had MIC = 0.5 mg/L. Patients with MRSA bacteremia in the ICU or those who had been hospitalized for a long time were more likely to be infected with strains of high vancomycin MIC MRSA (MIC = 2 mg/L; p < 0.05). Cox regression analysis demonstrated that the high MIC group had a significantly higher 30-day mortality than the low MIC group (HR: 2.39; 95% CI: 1.20-4.79; p = 0.014). Multivariate analyses indicated that the presence of high MIC isolates, pneumonia, post-cardiothoracic surgery and a high Charlson comorbidity index were all independent predictors of a 30-day mortality. Genotyping of these high vancomycin MIC isolates demonstrated that SCCmec III, spa type037, was the predominant strain (> 80%). The rates of resistance to trimethoprim/sulfamethoxazole, gentamicin, levofloxacin, rifampin and tetracycline were also higher in the high MIC group than in the isolates belonging to low MIC group (p < 0.05).ConclusionsIn a high vancomycin MIC group in Taiwan, SCCmec III, spa type t037, was the predominant strain of MRSA identified. Patients with MRSA bacteremia in the ICU or who had prolonged hospitalization were more likely to be infected with S. aureus strains with high vancomycin MICs. The mortality rate was higher among patients infected with these strains compared to patients infected with low MIC strains.
Highlights
Previous studies regarding the prognosis of patients infected with MRSA isolates characterized by a high minimum inhibitory concentration (MIC) for vancomycin have generally used a commercial Etest
Univariate analysis indicated that patients with MRSA bacteremia who were in the ICU (p = 0.028) or who had been hospitalized for a long time, i.e., > 60 days, (p = 0.022) were more likely to be infected with MRSA strains with a high vancomycin MIC (MIC = 2 mg/L)
Patients infected with high MIC isolates had a greater 30day mortality rate than patients infected with low MIC isolates, as indicated by univariate analysis (hazard ratio (HR) = 2.20; 95% confidence interval (CI): 1.13-4.27; p =
Summary
Previous studies regarding the prognosis of patients infected with MRSA isolates characterized by a high minimum inhibitory concentration (MIC) for vancomycin have generally used a commercial Etest. A previous genotype study in the U.S showed that SCCmec II was the genotype that was most predictive of high vancomycin MIC isolates [10]. We do not know if the high vancomycin MIC isolates from patients in Taiwan have a similar genotype as in those seen in patients in the U.S. we collected bacteria from consecutive patients with nosocomial MRSA bacteremia and, using the Clinical and Laboratory Standards Institute (CLSI) reference broth microdilution, determined vancomycin MICs. We examined the association of infection with high MIC strains (MIC = 2 mg/L) and mortality and genotyped the isolates
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