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Introduction: To date, approximately 3,000 different multi-locus sequence types have been identified for the important opportunistic pathogen Staphylococcus aureus (www.mlst.net). However, only a limited number of these sequence types (e.g. ST8) is frequently reported for isolates of different geographic and host origin, including methicillin-resistant and -susceptible variants. Here we report on the prevalence, genetic composition and background of S. aureus isolated from the nose of free-living small rodents trapped at different sites in Mecklenburg-Western Pomerania and Thuringia, including one methicillin resistant strain (MRSA) harboring mecC. In a pilot study, 100 rodents were trapped by the network “Rodent-Borne Pathogens” in 2011-2013 at different sites in Germany. All animals were frozen at -20 °C after trapping. Nose tissue was homogenized and subsequently cultured in an S. aureus enrichment medium for 48 hours and plated on mannitol salt agar dishes in serial dilutions. Then, all morphotypes that could be distinguished visually were subcultured on blood agar plates. S. aureus identity was confirmed by gyrase gene-specific PCR.
All isolates were initially subjected to spatyping, and next-generation sequencing (Illumina, MiSeq®) was performed for all 29 S. aureus isolates representing diverse origins
and trapping sites. Further data analysis was carried out by use of Ridom SeqSphere plus®.
Results: S. aureus was isolated from 29 of 100 rodents belonging to five different species. The majority of the S. aureus isolates (24/29) belonged to sequence type ST8-t211. Analysis of altogether 2332 targets based on MLSTplus (n=1811 loci) together with the accessory gene content (n=521 loci) revealed a very close relationship among these isolates from different federal states, trapping sites and host species. Two isolates belonged to sequence type ST130-t843, including one mecC-positive MRSA, harboring a complete blaZ-mecCmecR1-mecI structure (class E mec complex) as well as the recombinase genes crrA1 and ccrB3 and associated loci (e.g. arsenic resistance operon) described for S. aureus LGA251. One further S. aureus isolate was assigned to ST88-t2311, harboring prophage L54a and genes encoding the LukD/E leukotoxin. Two further isolates belonged to ST890-t1736 and -t1773.
Conclusion: S. aureus ST8 strains, regardless if methicillin resistant or not, seem to be well established in wildlife and companion animals, as well as in hospitals and the human community. The success of certain lineages - denominated as extended host spectrum genotypes (EHSG) - in occupying multiple ecological niches and host species needs to be further evaluated. Furthermore, wild rodents seem to be a potential reservoir for mecC-MRSA.