Abstract
From 1 January to 31 December 2021, forty-eight institutions around Australia participated in the Australian Staphylococcus aureus Surveillance Outcome Programme (ASSOP). The aim of ASSOP 2021 was to determine the proportion of Staphylococcus aureus bacteraemia (SAB) isolates in Australia that were antimicrobial resistant, with particular emphasis on susceptibility to methicillin and on characterisation of the molecular epidemiology of the methicillin-resistant isolates. A total of 2,928 SAB episodes were reported, of which 78.4% were community-onset. Overall, 16.9% of S. aureus isolates were methicillin resistant. The 30-day all-cause mortality associated with methicillin-resistant SAB was 15.0%, which was not significantly different from the 14.4% all-cause mortality associated with methicillin-susceptible SAB (p = 0.7). With the exception of the β-lactams and erythromycin, antimicrobial resistance in methicillin-susceptible S. aureus was rare. However, in addition to the β-lactams, approximately 36% of methicillin-resistant S. aureus (MRSA) were resistant to ciprofloxacin; 30% to erythromycin; 15% to tetracycline; 16% to gentamicin; and 3% to co-trimoxazole. When applying the European Committee on Antimicrobial Susceptibility Testing (EUCAST) breakpoints, teicoplanin resistance was detected in three S. aureus isolates. Resistance to vancomycin or linezolid 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 multiple antimicrobial resistance determinants including ciprofloxacin, erythromycin, clindamycin, gentamicin and tetracycline. The ST22-IV [2B] (EMRSA-15) clone is the predominant HA-MRSA clone in Australia. Nonetheless, 85% of methicillin-resistant SAB episodes were due to CA-MRSA clones. Although polyclonal, approximately 68% of CA-MRSA clones were characterised as ST93-IV [2B] (Queensland clone); ST45-V [5C2&5]; ST5-IV [2B]; ST1-IV [2B]; ST30-IV [2B]; and ST97-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.
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