Poster Presentation Australian Society for Microbiology Annual Scientific Meeting 2016

A trimethoprim-sulphamethoxazole-resistant, methicillin-resistant Staphylococcus aureus clone isolated from skin sores of children in remote communities of the Northern Territory, Australia (#321)

Tegan M Harris 1 , Asha C Bowen 1 2 3 , Deborah C Holt 1 , Derek S Sarovich 1 , Philip M Giffard 1 , Jonathan R Carapetis 3 , Steven YC Tong 1
  1. Menzies School of Health Research, Casuarina, NT, Australia
  2. Princess Margaret Hospital for Children, Subiaco, WA, Australia
  3. Telethon Kids Institute, Subiaco, WA, Australia

Trimpethoprim-sulphamethoxazole (SXT) is efficacious for the treatment of skin and soft tissue infections including impetigo, and provides an oral antimicrobial treatment option where methicillin-resistant Staphylococcus aureus (MRSA) is prevalent. Increasing rates of SXT-resistance have recently been reported in community-associated MRSA from Western Australia (WA) and the Northern Territory (NT). We describe the genomic epidemiology of SXT-resistant S. aureus strains cultured from children with impetigo living in remote NT communities, and identify the mechanism of SXT-resistance.

Children in an antibiotic treatment trial of impetigo had ≥1 skin sore and anterior nares swabbed for microbiological culture pre- and post-antibiotic treatment. Antimicrobial susceptibility testing of S. aureus strains was performed using Vitek2. Whole genome sequences of SXT-resistant strains were obtained. De novo genome assemblies were screened for antibiotic resistance determinants and virulence markers. A maximum likelihood tree was built to examine SXT-resistant S. aureus sequence type (ST) 5 phylogeny.

Twenty-two SXT-resistant S. aureus isolates were recovered from 8/508 (1.6%) trial participants from five geographically distinct NT communities. SXT-resistant S. aureus was detected pre- (7/8, 87.5%) and post-antibiotic treatment (6/8, 75%) from either skin sores or anterior nares. Nineteen of the 22 (86.4%) SXT-resistant isolates were ST5. Features of all the SXT-resistant ST5-MRSA included: 1) the presence of dfrG, a known SXT-resistance determinant, associated with an insertion sequence within SCCmecIVc; 2) a single chromosomal copy of dfrA, a known trimethoprim-resistance determinant, with a single nucleotide polymorphism in the dfrA promoter region; and 3) carriage of lukSF-PV, encoding for Panton-Valentine leucocidin (PVL).

SXT-resistant S. aureus, predominantly ST5, is becoming more prevalent in the NT. The co-location of dfrG with SCCmec in this ST5 clone provides a mechanism for co-selection of SXT and methicillin resistance. Ongoing genomic surveillance will be crucial to assess the impact on the overall S. aureus population of recent regional and national recommendations for the use of SXT for skin and soft tissue infections.