Evolution of Staphylococcus aureus and MRSA during outbreaks.

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.