Bernard Poutrel, Jaquemine Vialard, Karine Groud, Francoise Grain, Francois Soury-Lavergne, Jacqueline Bastien, Veronique Lambert and Christian Guidarini
Milk and dairy products are frequently implicated in food-borne infections caused by Staphylococcus aureus and infected animals may contaminate bulk milk. In addition, human handlers, milking equipment, the environment, the udder and the teat skin of dairy animals are possible sources of bulk milk contamination. The main objective of this study was to identify and prioritize the sources of S. aureus contamination of bulk milk and raw milk cheese, and secondly to investigate the diversity of strains involved in bovine mastitis, and test the controversial hypothesis of cross-infection between humans and animals. Four farms manufacturing cheese from raw milk with a total of 135 dairy cows were selected. Bacteriological analyses were performed on quarter milk samples, swabs of udders and teats, the milking machine, bulk milk, cheeses, swabs taken from staff members’ hands and nasal cavities. Typing of S. aureus isolates was carried out using the Multiple Locus Variable Tandem Repeat Analysis [MLVA] including five genes [clfA, clfB coa, fnb and SAV1078] combined with the investigation of the presence of staphylokinase gene [sak]. A total of 537 isolates were genotyped. The genotyping results confirmed that most intramammary infections in each farm were due to a prevalent genotype. The majority of genotypes present on the teat skin were also isolated from quarter milk samples. These isolates are the main sources of the contamination of bulk milk and cheese. The identity of certain genotypes characterized in both humans and animals was sometimes associated with the presence of the sak gene and suggests the existence of cross-contamination and also the occasional involvement of human handlers in the contamination of milk and cheeses.
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