Background
In a study of whole genome sequences of Staphylococcus aureus sequence type 93 (ST93) isolates from across the globe, a cluster of MRSA isolates collected from the Northern Territory (NT), Australia were observed as an early diverging group of MRSA isolates, predating the acquisition of the SCCmecIVa that forms the most widespread ST93 MRSA clones.
Aim
Define the resistome profile and the structure of the SCCmec element in a cluster of ST93 isolates.
Methods
Whole genome sequencing of 12 S. aureus ST93 isolates collected between 2004 and 2010 in the NT, Australia. Short read data was used to interrogate a S. aureus resistome database using SRST2. Genome assemblies for the 12 were constructed, and long range PCR was used to bridge contig gaps in the SCCmec element. The reconstructed SCCmec element was compared with previously described SCCmec elements.
Results
Resistome analysis of the isolates showed a unique presence of the aadD aminoglycoside resistance gene compared with the larger ST93 dataset, in addition to the presence of mecA, sdrM, blaZ, cadD and ermC genes. The assemblies showed co-localisation of the mecA and aadD genes in the SCCmec and consistent contig breaks involving IS431 elements. Reconstruction using long range PCR confirmed the structure to be novel, composed of a SCCmecIVa backbone in which the pUB110 plasmid containing the aadD gene, has been inserted, flanked by IS431 elements. The sequence of the novel SCCmec element has been submitted to the SCC International Working Group under the proposed name SCCmecIVn.
Conclusions
The presence of the aadD gene in the novel SCCmec element extends the resistance profile to include high level tolerance to tobramycin and kanamycin. The SCCmec structure remains relatively small and is unlikely to be a fitness-burden on the bacterium, as suggested by its maintenance in the population over at least a 6-year time period.
Disclosure
The authors declare no conflicts of interest