Improved media for differentiation of rods and cocci in yogurt

Selected yogurt starters and commercial samples grew on Elliker's lactic agar supplemented with .1% Tween 80 and 50 micrograms/ml of 2,3,5-triphenyltetrazolium chloride to produce small, red Streptococcus thermophilus colonies and larger, white Lactobacillus bulgaricus colonies. The distinction was somewhat strain dependent but was satisfactory in most cases. Addition of 7% skim milk (11% solids) to lactic agar in place of 2,3,5-triphenyltetrazolium chloride allowed good rod-coccus differentiation, regardless of strain or yogurt brand. On this medium, called yogurt lactic agar, L. bulgaricus appeared as large white colonies surrounded by a cloudy zone and S. thermophilus as smaller white colonies devoid of a surrounding halo. Casein precipitation was responsible for the halo effect around the more acidogenic L. bulgaricus colonies. Yogurt lactic agar compared favorably with S. thermophilus and Lactobacillus agar media for the recovery of S. thermophilus and L. bulgaricus in single and mixed cultures.