Frequency and immunologic cross-reactivity of encapsulated Staphylococcus aureus in bovine milk in New York

A total of 213 milk samples, which were positive by bacteriologic cultural examination for Staphylococcus aureus, were obtained from lactating cows on 55 farms throughout the dairying areas of New York. The S aureus isolates were evaluated for encapsulation directly from milk by the serum-soft agar technique. Results indicated that 93% of S aureus field isolates showed evidence of encapsulation on direct evaluation from milk. Suppression of capsule formation in vitro was seen in 72.7% of isolates when grown in the presence of 1% of anticapsular sera. One of the 3 anticapsular sera used exhibited a significantly higher level of cross-reactivity with the field isolates than did the 2 other sera. Increasing the concentrations of the antisera to 2% and 4% in the serum-soft agar had no effect on the expression of capsule in those isolates which had shown no evidence of capsule suppression with the specific antisera at the 1% level. Limited subculture of randomly selected encapsulated isolates on artificial media resulted in a decrease in the percentage of encapsulation from 100 to 33.3.