Nosocomial methicillin-resistant Staphylococcus aureus (MRSA) bacteremia in Taiwan: mortality analyses and the impact of vancomycin, MIC = 2 mg/L, by the broth microdilution method.
BACKGROUND
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. 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.


METHODS
Using 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.


RESULTS
Among 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).


CONCLUSIONS
In 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.
Department of Internal Medicine, E-Da Hospital/I-Shou University, Kaohsiung, Taiwan.
Jiun-Ling
JL
Wang
Jann-Tay
JT
Wang
Wang-Huei
WH
Sheng
Yee-Chun
YC
Chen
Shan-Chwen
SC
Chang
D000368:Aged; D000369:Aged, 80 and over; D016470:Bacteremia / Q000382:microbiology / Q000401:mortality*; D015373:Bacterial Typing Techniques; D003428:Cross Infection / Q000382:microbiology / Q000401:mortality*; D005260:Female; D005838:Genotype; D006801:Humans; D008297:Male; D055624:Methicillin-Resistant Staphylococcus aureus / Q000145:classification / Q000187:drug effects* / Q000235:genetics / Q000302:isolation & purification*; D008826:Microbial Sensitivity Tests; D013203:Staphylococcal Infections / Q000382:microbiology / Q000401:mortality*; D013624:Taiwan; D020713:Vancomycin Resistance