Abstract
From 1 January to 31 December 2024, fifty-five institutions across Australia participated in the Australian Staphylococcus aureus Surveillance Outcome Program (ASSOP). The aim of ASSOP 2024 was to determine the proportion of Staphylococcus aureus bacteraemia (SAB) isolates in Australia that were antimicrobial resistant, with particular emphasis on methicillin resistance, and to characterise the molecular epidemiology of methicillin-resistant S. aureus (MRSA). A total of 3,358 SAB episodes were reported, of which 78.5% were community-onset. Overall, 14.9% of S. aureus were methicillin resistant. The 30-day all-cause mortality associated with methicillin-resistant SAB was 13.7%, which was not significantly different to the 14.1% 30-day all-cause mortality associated with methicillin-susceptible SAB (p = 0.9). With the exception of the β-lactams and erythromycin, antimicrobial resistance in methicillin-susceptible S. aureus (MSSA) was infrequent. However, in addition to the β-lactams, 34.8% of MRSA were resistant to erythromycin; 28.9% to ciprofloxacin; 13.1% to gentamicin; 11.0% to tetracycline; and 2.7% to cotrimoxazole. A daptomycin-resistant MRSA from New South Wales was identified. The isolate had a daptomycin minimum inhibitory concentration (MIC) of 6.0 mg/L, and was identified as ST5-V, with a S337L MprF mutation. When applying the European Committee on Antimicrobial Susceptibility Testing (EUCAST) breakpoints, teicoplanin resistance was detected in three MSSA isolates. Linezolid or vancomycin resistance was not detected. Resistance to the non-β-lactam antimicrobials was largely attributable to the predominant healthcare-associated MRSA (HA-MRSA) clone ST22-IV [2B] (EMRSA-15), and to the community-associated MRSA (CA-MRSA) clone ST45-V [5C2&5], which has acquired resistance to multiple antimicrobials including ciprofloxacin, clindamycin, erythromycin, gentamicin, and tetracycline. Overall, 89.6% of methicillin-resistant SAB episodes were caused by CA-MRSA clones. Although polyclonal, approximately 72% of CA-MRSA clones were characterised as ST93-IV [2B] (Queensland clone), ST5-IV [2B], ST45-V [5C2&5], ST8-IV [2B], ST30-IV [2B], ST1-IV [2B], ST6-IV [2B], ST97-IV [2B] and Panton-Valentine leucocidin positive ST22-IV [2B]. As CA-MRSA is well established in the Australian community, it is important to monitor antimicrobial resistance patterns in community- and healthcare-associated SAB, as this information will guide therapeutic practices in treating S. aureus bacteraemia.
