Arginine degradation and citrate utilization, the major differentiating characteristics among lactic streptococci, formed the basis of a differential broth for separating Streptococcus cremoris, Streptococcus lactis, and Streptococcus diacetilactis strains in pure cultures.
The medium contains milk as the sole source of carbohydrate (lactose), arginine and sodium citrate as specific substrates, and a suitable pH indicator (bromcresol purple), in addition to other ingredients. The pH of the medium is adjusted to 6.2 ± 0.05 (which becomes 6.15 ± 0.05 after sterilization) to increase citrate utilization and the broth is dispensed into test tubes containing Durham fermentation tubes. Streptococcus cremoris produces a yellow reaction (acid) in the broth. Streptococcus lactis initially turns the broth yellow, but on liberation of NH3 reverses the color back to the original violet hue. Streptococcus diacetilactis produces a violet reaction, and CO2 accumulates in the Durham fermentation tubes from the fermentation of sodium citrate.
1Journal Paper No. J-6727 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa. Project No. 1839.