Streptococcus thermophilus Isolation HiVeg™ Agar

Principle and Interpretation

Streptococcus Thermophilus Isolation HiVeg Agar is prepared by using HiVeg hydrolysate which is free from BSE/TSE risks. Streptococcus Thermophilus Isolation HiVeg Agar is the modification of Streptococcus Thermophilus Isolation Agar which is based on the formulation originally developed by Lee at al (1) and recommended by APHA (1) for isolation and determination of ratio of Streptococcus thermophilus and Lactobacillus bulgaricus in yoghurt. Both these species are essential in fermentation of milk - yoghurt and are active in a symbiotic relationship. Dominance of either species can cause defects in the yoghurt affecting its consistency, flavour and odour etc. Equal numbers of both the species produce desirable yoghurt.

The medium contains sucrose, which is not fermented by majority of the Lactobacillus bulgaricus strains but is readily utilized by Streptococcus thermophilus. At pH 6.8, restricted growh of Lactobacillus bulgaricus is seen on this medium (3) where as Streptococcus thermophilus grows well. The former grow as tiny white cottony colonies as against the discrete colonies of the latter. Bifidobacterium if present in yogurt does not grown on this medium if incubated aerobically. For easy differentiation, often Bromocresol purple may be added to the medium. However if lactose is incorporated in this medium it is utilized by both the species. Yet another media was devised by Lee (1) with sucrose, lactose and added Bromocresol purple to differentiate between these species. With a suitable combination of sucrose and lactose, the rate of acid production by Streptococcus thermophilus is enhanced while that of Lactobacillus is restricted. However later on Driessen et al (4) reported two separate media to enumerate cocci and rods respectively from mixed cultures.

HiVeg hydrolysate and yeast extract provide nitrogenous nutrients, vitamin B complex and trace elements for the growth of Streptococcus thermophilus. Dipotassium phosphate prevents pH imbalance in the medium.