Antimicrobial susceptibility of Staphylococcus aureus from blood culture isolated from Srinagarind Hospital in 1997 and 2011

Abstract

Bacteremia is a serious illness and a leading cause of morbidity and mortality. Staphylococcus aureus is one of the most common cause of bloodstream infection and also a major cause of nosocomial infection. The increase of antimicrobial-resistant S. aureus especially methicillin-resistant isolate (MRSA) is currently problematic. The present study was conducted to examine antimicrobial susceptibility of S. aureus isolates from blood of patients at Srinagarind Hospital compared in two time periods: 1997 and 2011. A total of 166 S. aureus isolates including 85 isolates collected in 1997 (52 methicillin-susceptible S. aureus (MSSA) and 33 MRSA) and 81 isolates collected in 2011 (52 MSSA and 29 MRSA) were studied. Minimum inhibitory concentrations (MICs) of oxacillin, cefoxitin, gentamicin, tetracycline, erythromycin, cefazolin, ofloxacin and vancomycin were determined using an agar dilution method. Genes encoding for special virulence factors such as Panton-Valentine leukocidin (lukS-PV and lukF-PV) and toxic shock syndrome toxin (tst) were detected by polymerase chain reaction. The MSSA in 1997 showed comparable susceptibility rates to those of isolates in 2011 (59-100 and 53-100% respectively). The susceptibility rates to antimicrobial tested of MRSA in both periods were also similar (3-100%) except that of gentamicin. Thirteen antimicrobial susceptibility patterns were found in each MSSA group. In contrast, susceptibility patterns of MRSA were less variety than those of MSSA as only 3 and 5 patterns were found among MRSA in 1997 and 2011 respectively. The lukS-PV and lukF-PV genes were detected only in MSSA isolates (19 % and 11 % in 1997 and 2011 respectively) whereas tst was not found in any isolate in 1997 but was discovered from 38 % of MRSA and 22% of  MSSA in 2011. In the present study, no isolate had vancomycin MIC of > 2 µg/ml.  In addition MICs of all antimicrobial tested for both MRSA and MSSA among 1997 and 2011 were comparable except that gentamycin MICs for the MRSA isolates in 1997 and 2011 were significantly different (P < 0.05). These antibiograms may be useful for physicians to choose appropriate antimicrobial therapy, leading to reduction of mortality rate due to S. aureus infection.