Molecular characterization of Staph aureus associated with the cow-human interface.

Discipline: mastitis; 

The virulence of Staph aureus combined with the ability to develop, or acquire, resistance to different classes of antimicrobials makes it a formidable pathogen. Knowledge of the S. aureus strains circulating within herds may aid in the formulation and implementation of strain-specific control strategies to curb the spread of the pathogen between dairies.  Presently, very little data is available regarding the genetic diversity of S. aureus strains responsible for causing mastitis in South African dairy herds.  Therefore, the objectives of the study referenced below were (i) to screen isolates for clinically significant antimicrobial and virulence gene markers; (ii) investigate the diversity of S. aureus strains causing mastitis within KZN and (iii) investigate the transmission of S. aureus between cows with mastitis and close human contacts in the dairy environment.

Thirteen dairy herds were visited between October 2013 and March 2014.  At each of the sampling sites, milk samples were collected from all cows in milk and the staff was requested to provide a self-collected nasal swab for analysis.  Following bacteriological culture, isolates resembling S. aureus were identified phenotypically by a M-PCR assay.  All S. aureus isolates were characterised further using a series of M-PCR assays to screen for antibiotic resistance genes and selected virulence genes including the super antigen (SAg) toxin genes. The diversity and genetic relatedness of the S. aureus isolates were also studied. 

A total of 146 bovine and 12 human S. aureus isolates were recovered from the milk and nasal swabs analysed.  None of the isolates tested positive for the methicillin or vancomycin resistance genes.  The β-lactam encoding resistance gene was the most common resistance determinant detected and was identified in 29% of the bovine and 75% of the human S. aureus isolates respectively.  Twenty seven percent of the bovine S. aureus isolates tested positive for one or more SAg genes.  Comparatively, 83% of the human S. aureus isolates tested positive for one or more SAg genes with a greater variety of genes being detected.  Greater genetic diversity was observed among the human S. aureus.  Human S. aureus isolates from three sampling sites clustered together with bovine isolates recovered from milk samples at the sites. The three human isolates were genotypically indistinguishable from bovine S. aureus isolates present at the respective sites. 

Conclusions:  The results showed a high degree of clonality among the bovine S. aureus isolates.  The detection of genotypically indistinguishable S. aureus isolates from bovine and human hosts at three of the sampling sites is disturbing because it suggests bacterial transmission. This indicates the need for vigilant monitoring of staphylococcal populations at the human-animal interface.

Reference:

Schmidt T, Kock MMand Ehlers MM, 2016. Molecular characterisation of Staphylococcus aureus isolated from cases of bovine mastitis and close human contacts in KwaZulu-Natal, South Africa. Proc. IDF Mastitis Conf. Sept. 2016, Nantes.