Community onset staphylococcus aureus infections in Melbourne

<b>Objective:</b> To investigate the relative importance of methicillin resistant Staphylococcus aureus (MRSA) in the community in Melbourne by describing circulating S. aureus strains and infection characteristics.<br /><br /><b>Methods:</b> Patients with any community-onset S. aureus infection were identified via clinical specimens submitted to a community-based pathology service in 2006. The referring doctors confirmed community onset and defined site and severity of each infection. Patient isolates were characterised by antibiotic resistance subtype and presence of the Panton-Valentine leukocidin gene (pvl).<br /><br /><b>Results:</b> Between April and September 2006, 2,094 S. aureus isolates were processed. Of these, 133 (6.4%) were multiresistant MRSA (mMRSA) and 110 (5.3%) were resistant to less than 3 non-betalactam antibiotics (non-multiresistant MRSA or nmMRSA). We followed-up all nmMRSA (34) and mMRSA (15) confirmed community-onset infections, and a random subset of eligible patients with MSSA infections (57), for whom clinical data were available from referring doctors (82% response).<br /><br />The majority of isolates were from skin infections (99/106), but drainage was performed in less than one third of cases (29/99). Antibiotics were prescribed for 89% (95%CI: 82, 94) of infections. The isolates were resistant to the prescribed antibiotic 100% of the time for mMRSA infections and 80% for nmMRSA. Those with infections caused by MRSA had on average one additional visit to their doctor compared with MSSA infections.<br /><br />Ten nmMRSA clones were identified, including one new pvl positive nmMRSA. Of the 29 nmMRSA isolates, 14 were pvl positive (48%; 95%CIs: 30%, 66%) compared with 16% of MSSA and 0% mMRSA.<br /><br />Patients with an infection caused by pvl positive strains (23) were younger ((mean age 23 years (95%CI: 16, 30) compared with the 55 years (95%CI: 50, 61)). Infection site also varied with presence of pvl; more pvl positive infections were found in the axilla (17.9% compared with 0%) and head and neck (35.7% compared with 8.2%), and less for the leg or foot (21.4% compared with 55.7%).<br /><br /><b>Conclusions: </b>We estimate that 3.5% of community-onset S. aureus infections in Melbourne in 2006 were caused by MRSA, and 70 to 90% of patients with MRSA infections were treated initially with antibiotics to which their isolate was resistant. pvl positive isolates of S. aureus were associated with younger age and axillary or head and neck infections.<br />