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Comparison of Slow and Fast Acid-Producing Streptococcus Lactis¹

Department of Microbiology, Oregon State University, Corvallis

ABSTRACT

From Streptococcus lactis C2, a fast strain which produced sufficient acid to coagulate sterile nonfat milk in 18 hr at 21 C, a slow mutant was isolated which required incubation for at least 48 hr to effect coagulation. Nonfat milk cultures of these two organisms were compared to find causes for the apparent reduced acid production by the slow strain. The two 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, however, was about four times as great as the slow. Also, the fast culture was four times more proteolytic in nonfat milk than the slow. There was, therefore, a direct relationship between available nitrogen and total growth in milk, reflected by the final amount of acid produced in the cultures.

¹ Technical paper no. 1742. Oregon Agricultural Experiment Station. Supported by N.I.H. grant No. EF-69.