Evolution of Staphylococcus aureus and MRSA during outbreaks

- During transmission in outbreaks, genomes vary in SNPs, small genome rearrangements and in MGEs encoding virulence and resistance genes.
- Enough variation is detectable by whole genome sequencing to enable MRSA tracking.
- Benchtop sequencers have the potential to enable real-time investigation of outbreaks.
- Individuals may carry multiple variants of isolates.
- Selection of variants during hospital transmission is not seen, but in the long term antibiotic resistance is selected in successful hospital clones.

Investigation of Staphylococcus aureus outbreaks, and particularly those due to methicillin-resistant S. aureus (MRSA) in hospitals, can identify infection reservoirs and prevent further colonization and infection. During outbreaks, S. aureus genomes develop single nucleotide polymorphisms (SNPs), small genetic rearrangements, and/or acquire and lose mobile genetic elements (MGE) encoding resistance and virulence genes. Whole genome sequencing (WGS) is the most powerful method for discriminating between related isolates and deciding which are involved in an outbreak. Isolates with only minor variations are detectable and can identify MRSA transmission routes and identify reservoirs. Some patients may carry 'clouds' of related isolates, and this has consequences for how we interpret the data from outbreak investigations. Different clones of MRSA are evolving at different rates, influencing their typability. S. aureus genome variation reveals the importance of antibiotic resistance in the long term evolution of successful hospital clones, contributing to strategies to prevent the spread of successful MRSA clones.