Streptococcus thermophilus Isolation Agar is recommended for determining the ratio of Streptococcus thermophilus and Lactobacillus bulgaricus in yoghurt. Streptococcus Thermophilus Isolation Agar, recommended by APHA. Symboitic activity and equal population of Streptococcus thermophilus and Lactobacillus bulgaricus is essential for optimum consistency, flavor and odor generation during yoghurt production. Increase in the numbers of either species can lead to defects in the yoghurt characteristics.