An update on Methicillin-resistant Staphylococcus aureus (MRSA) / Vancomycin-resistant Enterococci (VRE)

Professor Geoffrey Coombs1,2

1Chair of Public Health | School of Veterinary and Life Sciences, Murdoch University
2Senior Clinical Scientist | Microbiology Department, PathWest Laboratory Medicine – WA, Fiona Stanley Hospital

Funded by the Australian Commission on Safety and Quality, the Australian Group on Antimicrobial Resistance (AGAR) is a longstanding network of laboratories located across Australia that tests and gathers information on the level of antimicrobial resistance in bacteria that cause important and life-threatening infections. Since 2013 AGAR has focused on bloodstream infections and has conducted three ongoing programs including the Australian Staphylococcal (ASSOP) and the Australian Enterococcal (AESOP) Sepsis Outcome Programs.

The aim of ASSOP is to determine the proportion of Staphylococcus aureus bacteraemia (SAB) isolates in Australia that are antimicrobial resistant with particular emphasis on susceptibility to methicillin, and to characterise the molecular epidemiology of the methicillin-resistant isolates. In 2018 2,673 SAB episodes from 36 institutions were reported of which 79% were community on-set and almost one in five (17.4%) were methicillin-resistant (MRSA). Overall the thirty-day-all-cause mortality was 14.2%. The 30 day mortality for MRSA (17.1%) was not significantly higher than the mortality associated with methicillin-susceptible S. aureus (MSSA). Similarly, there was no significant difference in hospital mean length of stay (LOS) between MRSA and MSSA episodes. Almost one in four MRSA were characterised as healthcare-associated MRSA of which ST22-MRSA-IV (EMRSA-15) dominated. ST93-MRSA-IV (Queensland MRSA clone) was the dominant community-associated MRSA (CA-MRSA) across Australia.

The aim of AESOP is to determine the proportion of enterococcal bacteraemia isolates in Australia that are antimicrobial resistant, and to characterise the molecular epidemiology of the E. faecium isolates. In 2018 1,248 enterococcal episodes were reported of which 93.5% were caused by either E. faecalis (54.2%) or E. faecium (39.3%). Overall 49.3% of E. faecium harboured the vanA or vanB genes or both, with 52.7% of vancomycin-resistant E. faecium harbouring vanA. Although there was a significant difference in the 30 day all-cause mortality between E. faecium (27.2%) and E. faecalis (14.7%), there was no significant difference in mortality and hospital mean LOS between vancomycin non-susceptible and vancomycin susceptible E. faecium. The most predominant clone of E. faecium was ST17, identified in most states and territories.

As CA-MRSA is now well established in the Australian community and enterococcal bacteraemia is frequently caused by polyclonal ampicillin-resistant high-level gentamicin-resistant vanA or vanB E. faecium it is important antimicrobial resistance patterns in SAB and enterococcal bacteraemia continues to be monitored and this information is used to guide therapeutic practices in treating sepsis in Australia.

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