Bacterial factors influencing the mortality for methicillin-resistant Staphylococcus aureus (MRSA) bacteremia

In the present journal, an association of high vancomycin minimal inhibitory concentration (MIC) with mortality in patients with methicillin-susceptible Staphylococcus aureus bacteremia was recently reported, regardless of antibiotic treatment used. [1] The impact of vancomycin minimal inhbition concentration (MIC) on the outcome of methicillin-resistant Staphylococcus aureus (MRSA) bacteremia was reported in several studies. [2] In this study, we aimed to investigate the role of other bacterial factors as clonal identity, Panton Valentine Leukocidin (PVL) and toxin (enterotoxin A-J, staphylococcal toxic shock toxin-TSST) genes, the accessory gene regulator (agr) locus types on the outcome of MRSA bacteremia. A total of 131 non-duplicate MRSA strains isolated from hospital-acquired and/or healthcare-associated bloodstream infections of adult patients between January 2004 and January 2012 at Hacettepe Adult Hospital were included in the study. The isolates were identified as MRSA by Phoenix automated system (BD Diagnostic Instrument Systems, Franklin Lakes, NJ) and stored at 80 C. Then the isolates were re-inoculated onto blood agar and phenotypical identification was repeated by Gram staining, catalase and coagulase tests. Methicillin-resistance was confirmed by oxacillin Etest (BioMerieux, Marcy-l’ Etoile, France), carried out according to the Clinical Laboratory Standards Institute (CLSI) criteria,[3] and for presence of mecA gene by PCR. [3] SCCmec, PVL, enterotoxin, TSST genes and agr types were investigated by PCR. [4–8] Vancomycin MIC was determined by Etest (BioMerieux, Marcy-l’ Etoile, France) and evaluated according to the CLSI criteria. [3] MRSA isolates were typed by Pulse-field gel electrophoresis (PFGE) using the procedures as described previously. [9] Cluster analysis was performed by the unweighted pair-group method on the basis of arithmetic averages (UPGMA). The resulting dendogram was also verified by inspection of the banding patterns by eye. Hospital acquisiton was defined on the basis of Center for Disease Control (CDC) definition criteria. [10] Healthcare-associated MRSA bacteremia was defined according to the definition by Friedman and collagues. [11] The patients were divided into two groups according to the survival status 30 days after the first positive blood culture was obtained. All statistical analyses were performed with SPSS software for Windows (version 15.0; SPSS, Chicago, IL). Group comparisons were done by using the v tests. All tests for statistical significance were two-tailed and p-values<0.05 were considered to be statistically significant. A logistic regression analysis was performed to identify the risk factors that independently influence mortality due to MRSA bacteremia. The results of these logistic regression analyses were reported as adjusted relative risks with 95% confidence intervals (CI). A total of 131 mecA-positive MRSA strains were typed and grouped. Fourteen out of 131 cases of MRSA bacteremia were defined as healthcare-associated, while the remaining 117 were hospital-acquired. One hundred and twenty-four strains belonged to SCCmec III. The rest of the strains belonged to SCCmec type-V (n1⁄4 3), type-VI (n1⁄4 3), and type-I (n1⁄4 1). None of the isolates carried PVL or TSST genes. Ninety-seven isolates (74%) were positive for a combination of enterotoxin genes as follows: seaþ seeþ sei (n1⁄4 48), seaþ sei (n1⁄4 35), seaþ secþ seeþ sei (n1⁄4 5) and other combinations (n1⁄4 9). Eighteen isolates (14%) were carrying sea gene alone and 16 (12%) was not carrying any of the enterotoxin genes. Agr typing revealed that 57 (44%) were type I, and one was type II. The remaining 66 isolates (50%) was carrying two or three agr groups. In seven isolates, agr groups could not be identified. According to PFGE patterns, there were 23 different clones and 76 of the strains could be placed in four main