Evolution of ST228 SCCmec-I MRSA 5 years after outbreak

Phylogenetic studies showed that infections caused by methicillin-resistant Staphylococcus aureus (MRSA) often occur in epidemic waves. The reasons why some clones replace others are still unclear. This study aimed at investigating isolates of MRSA clone ST228-I in order to identify genetic differences that might explain the fall of this clone in our region. The first 20 consecutive isolates of ST228 recovered five years after a large outbreak were selected and sequenced on an Illumina platform. In addition, 265 genomes representative of the diversity of the ST228 clone observed in Swiss isolates between 1999 to 2012 and in Europe were selected from a previous study. SNPs, acquired resistance and virulence loci, mutational resistance, phages and pathogenicity islands were detected using Bionumerics V8.0. All 20 isolates recovered after the outbreak clustered within the clade of isolates recovered between 2006 and 2012 in our region. None of these isolates have lost or gained a specific characteristic, such as acquired and mutational resistance determinant, virulence factors or phage, when compared to ancestral isolates in the same clade. The stability of this clone suggests that intrinsic bacterial characteristics were not directly related to its decline in our region and may not play an important role in the dynamic of MRSA clones. Extrinsic factors, such as infection control, have likely played a more important role. Some mystery remains around the rise and fall of MRSA clones and further studies are needed understand the mechanisms underlying the evolution of MRSA clones and to better control this pathogen.