A spontaneous mutant of Streptococcus lactis C2 which formed abnormally large colonies on agar medium was isolated. The mutant grew as rapidly in broth or milk as the parent culture but was slower in acid production. When grown in complex broth at 32 C, the parent culture lowered the pH to 4.8 in 8 h whereas the mutant required 15 h to produce pH 5.0. Mutant cells consumed 6 times as much oxygen as S. lactis C2 and produced carbon dioxide derived primarily from carbons 3 and 4 of glucose. The mutant was defective in lactic dehydrogenase possessing only .1 units as compared to 138 units for S. lactis C2. This enzymatic defect caused the mutant to produce about 1,000 /,g/ ml of Westerfeld positive material when grown in broth. S. lactis C2 produced only trace amounts. Alth,'mgh acetoin was a major metabolic end-product in the mutant, diacetyl was also detected. The results indicate a type of mutation which could be responsible for the spontaneous occurrence of aroma and carbon dioxide producing strains of S. lactis in dairy starter cultures.
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McKay et al. (1974) studied this question.