Laboratory features of invasive community-onset methicillin-resistant Staphylococcus aureus - correlation with clinical features in a prospective case-control study

  • Michael Wehrhahn, Australia
  • Ms Julie Pearson, Gram Positive Bacteria Typing and Research Unit, School of Biomedical Sciences, Curtin University of Technology, Australia
  • Dr Frances O'Brien, Gram Positive Bacteria Typing and Research Unit, School of Biomedical Sciences, Curtin University of Technology, Australia
  • Ms Hui Tan, Gram Positive Bacteria Typing and Research Unit, School of Biomedical Sciences, Curtin University of Technology, Australia
  • Dr James Robinson, Department of Microbiology and Infectious Diseases, Royal Perth Hospital and PathWest Laboratory Services WA, Perth, Australia
  • Ms Rebecca Lee, Department of Microbiology and Infectious Diseases, Royal Perth Hospital and PathWest Laboratory Services WA, Perth, Australia
  • Geoff Coombs, Department of Microbiology and Infectious Diseases, Royal Perth Hospital and PathWest Laboratory Services WA, Perth, Australia
  • Dr Ronan Murray, Department of Microbiology and Infectious Diseases, Royal Perth Hospital and PathWest Laboratory Services WA, Perth, Australia

    • Objective: To compare the laboratory features of isolates from invasive community-onset methicillin-resistant Staphylococcus aureus (cMRSA) infections with those caused by methicillin-susceptible S. aureus (cMSSA) and relate these findings to clinical data.

    Methods: Patients with invasive cMRSA infection were identified prospectively at two tertiary hospitals in Western Australia from April 2006 to April 2008. Two contemporaneous cMSSA cases acted as controls. Phenotypic and genotypic typing including Panton-Valentine leukocidin (PVL) PCR was performed on all isolates, and results correlated with clinical data.

    Results: 150 episodes of invasive S. aureus infection were studied (50 cMRSA and 100 cMSSA). Susceptibility testing revealed 44% of cMRSA isolates were resistant to clindamycin whilst 19% of cMSSA isolates retained susceptibility to penicillin. The PVL genes were present in 27% of all isolates (30% of cMRSA and 25% of cMSSA isolates respectively). PVL-positive S. aureus infection was associated with indigenous ethnicity (58% vs 20%, p <0.001), younger age (median age 35y vs 55y, p <0.001) and was more likely in isolates from skin and soft tissue infections (52% vs 16%, p<0.001). Markers of illness severity (SAPS II scores and length of stay) were significantly lower in patients with PVL-positive S. aureus infection (8 vs 21, p=0.001; 5.5d vs 16d, p<0.001 respectively), however all-cause 30-day mortality was not significantly different (5% vs 9%, p=0.4). Molecular typing demonstrated that most cMRSA isolates were the three most common cMRSA clones in our region: ST1-MRSA-IV (30%), ST78-MRSA-IV (20%) and ST93-MRSA-IV (20%). The remaining 30% comprised ST5-MRSA-IV, ST22-MRSA-IV, ST30-MRSA-IV, and ST45-MRSA-IV. cMSSA isolates were more heterogenous, with the commonest clones being ST93-MSSA (20%), ST47-MSSA (10%) and ST15-MSSA (10%). The majority of PVL-positive strains were ST93-MRSA-IV, ST30-MRSA-IV and ST93-MSSA.

    Conclusions: PVL was present in the minority of cMRSA isolates (30%), but in a surprisingly large proportion (25%) of cMSSA isolates. Infection with PVL-positive S.aureus was associated with younger age, indigenous ethnicity, skin and soft tissue infections, less morbidity and similar mortality compared to those with invasive PVL-negative S. aureus infection. A small number of clones are responsible for the majority of invasive cMRSA infection in our region, in contrast to the greater variety of clones causing invasive cMSSA infection.