Intramammary infections of two common non-aureus staphylococci
Staphylococcus (S.) chromogenes and S. simulans are commonly found in intramammary infections (IMI) associated with bovine subclinical mastitis, but little is known about genotypic variation within species, let alone genes coding for antimicrobial resistance (AMR) and virulence factors (VF). The aim of this study was therefore to investigate these aspects by whole genome sequencing of milk isolates from Swedish dairy cows with subclinical mastitis. In total, 105 and 118 isolates of S. chromogenes and S. simulans, respectively, were included. Genotypes were identified using 7-MLST and core genome analysis, which were confirmed by wgMLST analysis. Additionally, in-silico analysis of AMR and virulence genes were performed. Forty-seven sequence types (STs) and 7 clusters of S. chromogenes were identified. A 7-locus MLST scheme for S. simulans was not available, but 3 clusters and 6 subclusters were identified using core genome analysis. Only a few AMR genes were detected in one or both bacterial species, mostly in a small number of isolates. However, resistance against penicillin, identified by presence of blaZ and production of betalactamase, was found in 22% of the isolates of S. chromogenes, but not in any of the S. simulans isolates. The average total number of VFs detected per isolate was similar (28 and 33) in the two species studied, but some variation in total numbers and presence of specific VFs or functional groups of VFs, was found between STs or clusters within species. In conclusion, the results generated new knowledge on epidemiology of bovine S. chromogenes and S. simulans IMI with implications for prevention and antimicrobial treatment of such infections.