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Factors Affecting Growth of Streptococcus agalactiae in Milk

National Animal Disease Laboratory, Animal Disease and Parasite Research Division, USDA, Ames, Iowa

ABSTRACT

Factors that might affect the growth of Streptococcus agalactiae in milk were studied. Raw or pasteurized milk was serially diluted in steamed milk (100 C, 30 min) and acid production used to measure the growth of S. agalactiae 660 by determining the pH of the samples after incubation at 37 C for 24 hr. The inhibitory titer, represented by the reciprocal of the highest dilution of milk showing maximum inhibition of acid production, was lowered when the milk used for diluent was heated for 30 min at 80 or 90 C, for 10 min at 100 C, or 15 min at 121 C, and when the incubation period of the test was increased to 48 hr. The inhibitory titer was progressively increased when 5 to 30 µg of ammonium thiocyanate per ml of milk was added to pasteurized milk and the diluent. A stimulatory substance that counteracted the inhibitory activity of milk for S. agalactiae could be removed from whey by dialysis and ultrafiltration. Based on a comparison of the inhibitory activity curves, 20 colostrum samples tested had more stimulatory substance but less lactoperoxidase than milk. The inactivation of the inhibitory activity of milk by cysteine, glutathione, catalase, and horse-radish peroxidase confirmed the work of others who used organisms other than S. agalactiae.