Molecular characterization of Staphylococcus aureus isolated from bovine mastitis and close human contacts in South African dairy herds : genetic diversity and inter-species host transmission

摘要

Staphylococcus aureus is one of the most common etiological agents of contagiousbovine mastitis worldwide. The purpose of this study was to genetically characterizea collection of S. aureus isolates (bovine = 146, human = 12) recovered from casesof bovine mastitis and nasal swabs of close human contacts in the dairy environment.Isolates were screened for a combination of clinically significant antimicrobial andvirulence gene markers whilst the molecular epidemiology of these isolates and possibleinter-species host transmission was investigated using a combination of genotypingtechniques. None of the isolates under evaluation tested positive for methicillin orvancomycin resistance encoding genes. Twenty seven percent of the bovine S. aureusisolates tested positive for one or more of the pyrogenic toxin superantigen (PTSAg)genes with the sec and sell genes predominating. Comparatively, 83% of the humanS. aureus isolates tested positive for one or more PTSAg genes with a greater varietyof genes being detected. Genomic DNA macrorestriction followed by pulsed-field gelelectrophoresis (PFGE) of the bovine isolates generated 58 electrophoretic patterns including: ST8 (CC8), ST97 (CC97), ST351 (CC705), ST352 (CC97), ST508 (CC45),ST2992 (CC97) and a novel sequence type, ST3538 (CC97). Based on PFGE analysis,greater genetic diversity was observed among the human S. aureus isolates. Bovineand human isolates from three sampling sites clustered together and were genotypicallyindistinguishable. Two of the isolates, ST97 and ST352 belong to the common bovinelineage CC97, and their isolation from close human contacts suggests zoonotic transfer.In the context of this study, the third isolate, ST8 (CC8), is believed to be a human clone which has transferred to a dairy cow and has subsequently caused mastitis. Thedetection of indistinguishable S. aureus isolates from bovine and human hosts at threeof the sampling sites is suggestive of bacterial transmission and supports the need forvigilant monitoring of staphylococcal populations at the human-animal interface.which grouped into 10 pulsotypes at an 80% similarity level. The majority of the bovineisolates, 93.2% (136/146), clustered into four major pulsotypes. Seven sequence types(ST) were identified among the representative bovine S. aureus isolates genotyped,