Characterization of methicillin-resistant Staphylococcus aureus CC398 obtained from humans and animals on dairy farms

In this study MRSA isolates from dairy farms were investigated for their genetic relationships and antimicrobial susceptibility. In total, 125 MRSA isolates from 26 dairy farms were studied, including isolates from milk samples (n = 46), dairy cattle (n = 24), calves (n = 6), dust samples from pig (n = 16) and veal calf sheds (n = 1), dogs (n = 2), a horse, a sheep and humans (n = 28). CC398-specific PCRs, spa typing, SCCmec typing and ApaI macrorestriction analysis were conducted. Susceptibility testing was performed by broth microdilution. All 125 isolates belonged to CC398. Eight spa types (t011, t108, t034, t567, t1184, t1451, t2287, t3934) were detected. SCCmec elements of types IV (n = 48) and V (n = 67) were identified with 10 isolates being non-typeable. Six main macrorestriction patterns - with up to 23 sub-patterns - and twelve resistance patterns were identified. Sixty-eight isolates showed a multiresistance phenotype. Farm-by-farm analysis revealed different scenarios: in some farms, the MRSA CC398 isolates from dairy cattle, humans, pig sheds and/or sheep were indistinguishable suggesting an interspecies exchange of the same MRSA CC398 subtype. In other farms, several MRSA CC398 subtypes were detected in different host species/sources with occasionally even more than one MRSA CC398 subtype from the same host species/source. These latter results may suggest that either different MRSA subtypes associated with humans or animals have been imported into the respective farm or that one MRSA CC398 strain has undergone diversification, reflected by more or less expanded changes in PFGE patterns, spa type or resistance pattern, during colonization of different hosts on the same farm.