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Appl Environ Microbiol. 1972 December; 24(6): 947-952
Copyright © 1973 American Society for Microbiology. All Rights Reserved.

Agar Medium for Differential Enumeration of Lactic Streptococci1

M. S. Reddy, E. R. Vedamuthu2, C. J. Washam3 and G. W. Reinbold

a Department of Food Technology, Iowa State University, Ames, Iowa 50010

ABSTRACT

An agar medium containing arginine and calcium citrate as specific substrates, diffusible (K2HPO4) and undiffusible (CaCO3) buffer systems, and bromocresol purple as the pH indicator was developed to differentiate among lactic streptococci in pure and mixed cultures. Milk was added as the sole source of carbohydrate (lactose) and to provide growth-stimulating factors. Production of acid from lactose caused developing bacterial colonies to seem yellow. Subsequent arginine utilization by Streptococcus lactis and S. diacetilactis liberated ammonia, resulting in a localized pH shift back toward neutrality and a return of the original purple indicator hue. The effects of production of acid from lactose and ammonia were fixed around individual colonies by the buffering capacity of CaCO3. After 36 hr at 32 C in a candle oats jar, colonies of S. cremoris were yellow, whereas colonies of S. lactis and S. diacetilactis were white. S. diacetilactis, on further incubation, utilized suspended calcium citrate, and, after 6 days, the citrate-degrading colonies exhibited clear zoning against a turbid background, making them easily distinguishable from the colonies of the other two species. The medium proved suitable for quantitative differential enumeration when compared with another widely used general agar medium for lactic streptococci.

FOOTNOTES

2 Present address: Department of Microbiology, Oregon State University, Corvallis, Ore. 97331.

3 Present address: Department of Biology, Southwestern Union College, Keene, Tex. 76059.

1 Journal Paper J-7273 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa. Project 1838.

Appl Environ Microbiol. 1972 December; 24(6): 947-952
Copyright © 1973 American Society for Microbiology. All Rights Reserved.





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Copyright © 1972 by the American Society for Microbiology. All rights reserved.