Comparison of Slow and Fast Acid-Producing Streptococcus Lactis

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Abstract
A fast strain which produced sufficient acid to coagulate sterile nonfat milk in 18 hr. at 21 C. isolated from a slow mutant of S. lactis C2 required incubation for at least 48 hr. to effect coagulation. Nonfat milk cultures of these organisms were compared to determine the reason for the apparent reduced acid production by the slow strain. The organisms had the same generation time (1.4 hr.) in the log phase of growth and produced the same amount of acid per cell. The average viable population of the fast culture after 18 hr. at 21 C was about 4 times as great as the slow. The fast culture was 4 times more proteolytic in nonfat milk than the slow. A direct relationship between available N and total growth in milk was reflected by the final amount of acid in the cultures.