Australian Group on Antimicrobial Resistance (AGAR) Australian Staphylococcus aureus Surveillance Outcome Program...

From 1 January to 31 December 2023, fifty-seven institutions across Australia participated in the Australian Staphylococcus aureus Surveillance Outcome Program (ASSOP). The aim of ASSOP 2023 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 methicillin-resistant S. aureus (MRSA) molecular epidemiology. A total of 3,422 SAB episodes were reported, of which 77.0% were community-onset. Overall, 16.1% of S, aureus were methicillin resistant. The 30-day all-cause mortality associated with methicillin-resistant SAB was 14.8%, which was not significantly different to the 16.5% all-cause mortality associated with methicillin-susceptible SAB (p = 0.44). 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, approximately 33% of MRSA were resistant to ciprofloxacin; 30% to erythromycin; 13% to tetracycline; 13% to gentamicin; and 3% to co-trimoxazole. Two New South Wales daptomycin-resistant MRSA, with minimum inhibitory concentrations (MICs) of 3.0 and 4.0 mg/L, were identified as ST22-IV, with a V351E mprF mutation, and ST45-V with a T345I mprF mutation respectively. Three daptomycin-resistant MSSA were identified. One from Tasmania, with a daptomycin MIC of 1.5 mg/L, identified as ST9295 with a L341I mprF mutation; one from New South Wales, with a daptomycin MIC of 3.0 mg/L, identified as ST97 with a L776S mprF mutation; and one from Western Australia, with a daptomycin MIC of 2.0 mg/L, identified as ST5. No previously reported mutations in known loci were detected in the Western Australian isolate. When applying the European Committee on Antimicrobial Susceptibility Testing breakpoints, teicoplanin resistance was detected in three MSSA isolates and one MRSA isolate. Vancomycin or linezolid resistance was not detected. Resistance to non-β-lactam antimicrobials was largely attributable to the healthcare-associated MRSA (HA-MRSA) clone ST22-IV [2B] (EMRSA-15), and 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. ST22-IV [2B] (EMRSA-15) was the predominant HA-MRSA clone in Australia. Overall, 85% of methicillin-resistant SAB were caused by community-associated MRSA (CA-MRSA) clones. Although polyclonal, approximately 70.3% of CA-MRSA clones were characterised as ST93-IV [2B] (Queensland clone); ST5-IV [2B]; ST1-IV [2B]; ST45-V [5C2&5]; ST30-IV [2B]; ST8-IV [2B]; ST6-IV [2B]; ST97-IV [2B]; and ST953-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.