Streptococcus thermophilus Isolation Agar

Principle And Interpretation

Yoghurt is a fermented milk product in which Streptococcus thermophilus and Lactobacillus bulgaricus are the essential microbial species and are active in a symbiotic relationship. To obtain optimum consistency, flavour and odour, the two species should be present in about equal numbers in the culture. Dominance by either species can cause defects. Equal numbers of both the species produce desirable yoghurt.

Streptococcus Thermophilus Isolation Agar, recommended by APHA (1), is used for determining the ratio of S. thermophilus and L. bulgaricus in yoghurt.

Streptococcus Thermophilus Isolation Agar is based on the formulation originally developed by Lee et al (2). However later on Driessen et al (3) reported two separate media to enumerate cocci and rods respectively from mixed cultures where S. thermophilus is grown on Streptococcus Thermophilus Isolation Agar (M948) and L. bulgaricus is cultivated on Lactococcus Bulgaricus Agar (LB Agar, M927).

The medium contains sucrose, which is not fermented by majority of the L. bulgaricus strains but is readily utilized by S. thermophilus. However if lactose is incorporated in this medium it is utilized by both the species. With a suitable combination of sucrose and lactose, the rate of acid production by S. thermophilus is enhanced while that of L. bulgaricus is restricted. Casein enzymic hydrolysate and yeast extract provide nitrogenous nutrients, vitamin B complex and trace elements for the growth of S. thermophilus. Dipotassium phosphate prevents pH imbalance in the medium.