Virulence of two mastitis strains of Staphylococcus aureus in bovine skin: enhancement by growth in high carbohydrate-high salt medium or in raw milk.

Cows were inoculated intradermally with two strains of Staphylococcus aureus, and the severity of the resulting skin lesions was assessed on the basis of size, degree of necrosis, and in some cases by measuring the area of inflammation in histological preparations of excised skin. At least 10(6) colony forming units were required to produce a detectable skin lesion, indicating that the cow has a resistance to staphylococcal skin infection comparable to man and rabbit. The severity of the lesions produced by the two strains correlated with their known virulence in the bovine udder as mastitis pathogens. A comparison between lesions produced by cocci grown in a stock laboratory medium and those grown in the high carbohydrate-high salt modified 110 medium showed that the virulence of cocci grown in the latter medium was enhanced. The comparable effect of milk to the modified 110 medium is discussed.