Genetic diversity and within-farm transmission of Staphylococcus aureus from ewes in Norway

Abstract Background Staphylococcus aureus is the most commonly isolated bacterial species from ovine mastitis. Previous studies have mainly focused on dairy sheep, whereas knowledge about S. aureus in sheep kept for mutton and wool production remains limited. Furthermore, data on the occurrence of antimicrobial resistance genes and molecular epidemiology of S. aureus are scarce. In Norway, most sheep are kept for meat or wool, and mastitis caused by S. aureus results in substantial economic losses. This study aimed to determine the persistence, genetic relatedness and diversity, and antimicrobial resistance (AMR) of S. aureus in ewes at four farms in Norway using whole genome sequencing. The potential effect of vaccination on the occurrence of S. aureus with capsular polysaccharide type 8 (cap 8) was also determined. Results A total of 101 isolates from 70 ewes across four farms were characterized. Persistent colonization was observed in 24% of ewes. Among the isolates, 65% carried the capsular polysaccharide type 8 (cap 8) gene and belonged to sequence types (STs) 8, 9, 49, and 8875. The remaining isolates were cap5-positive and belonged to STs 30, 130, 133, and 1640. No statistically significant differences were seen between isolates from vaccinated and unvaccinated ewes regarding the cap gene. Single-nucleotide polymorphism (SNP) analysis showed no evidence of inter-farm transmission but supported within-farm transmission. AMR genes were rare; the only clinically relevant gene identified was blaZ, present solely in ST8 isolates. Virulence gene profiling showed that the toxin genes tst-1, sel, and sec were common in ST133 and ST49 isolates and that no specific human-associated lineages could be detected. Conclusions Despite the limited number of isolates and farms studied, the findings revealed high genetic diversity between farms, while one or two genotypes tended to dominate within a farm. AMR genes were uncommon, consistent with Norway’s low antibiotic usage and low prevalence of AMR. Most genotypes detected in this study have previously been described in ovine populations, indicating host-adapted strains. Vaccination appeared to have limited effect on the occurrence or distribution of S. aureus with cap 8.