Molecular characterisation and epidemiology of transmission of intraoperative Staphylococcus aureus isolates stratified by vancomycin minimum inhibitory concentration (MIC)

BACKGROUND: Reduced vancomycin susceptibility in Staphylococcus aureus (S. aureus) is considered a more pathogenic strain characteristic and is associated with treatment failure. We aimed to characterise the epidemiology of intraoperative transmission of S. aureus isolates with reduced vancomycin susceptibility. METHODS: S. aureus isolates (N=173) collected from 274 randomly selected operating room environments at three major academic medical centres in 2009–2010 were characterised by vancomycin minimum inhibitory concentration (MIC). We aimed to characterise the transmission dynamics for VISA and isolates with relatively reduced vancomycin (MIC= 2μg/mL) susceptibility at the range of therapeutic differentiation. RESULTS: Intraoperative S. aureus MIC was 1.38 ± 0.34 μg/mL. No VISA isolates were identified (95% upper confidence limit 2.1%) and those with an MIC of 2 μg/mL accounted for 12.72% (22/173) of all isolates. MIC=2 μg/mL isolates were more frequently cultured from the hands of healthcare providers [19.3% (16/83)] versus otherwise [6.7% (6/90)], with unadjusted risk ratio 2.89, P=0.021, and from patients with >2 major comorbidities [25.0% (8/32)] versus otherwise [9.9% (14/141)], with unadjusted risk ratio 2.52, P=0.035. Both were significant when tested simultaneously. The adjusted relative risk for provider hands was 2.77 (95% CI 1.15 to 6.69, P=0.024). The adjusted relative risk for patients with >2 major comorbidities was 2.37 (95% CI 1.11 to 5.05, P=0.026). MIC=2μg/mL was not associated with greater risk of clonal transmission (unadjusted P=0.34, adjusted P=0.18). CONCLUSION: Intraoperative VISA is a rare event. S. aureus isolates MIC=2μg/mL isolates were not associated with increased risk of intraoperative transmission. The epidemiology of detected intraoperative transmission is consistent with Centers for Disease Control guidelines.