HYA AGAR

Description

Yoghurt is a fermentable milk product in which Streptococcus thermophilus and Lactobacillus bulgaricus are the essential microbial species and are active in a symbiotic relationship. The large number of media proposed for lactic acid bacteria, particularly for Streptococci and lactobacilli is indicative of the difficulties encountered in growing some strains of these organisms. The choice of medium is governed to some extent by the particular strains under study and therefore by products or habitat. In general, lactic acid bacteria are tolerant to low pH, they can be very sensitive to other adverse conditions. Samples to be examined for numbers of viable lactic acid bacteria should not be frozen prior to analysis. Porubcan and Sellars described this medium on which L. bulgaricus grow as diffuse, low mass colonies (2-10 mm in diameter) and S. thermophilus as discrete high mass colonies (1-3 mm in diameter). To obtain optimum consistency, flavour and odour, many investigators claim that the two species should be present in about equal numbers in the culture. Dominance by either species can cause defects. Because of the emphasis on maintaining balance between coccus and rods, techniques are needed to determine the relative proportion of S. thermophilus and L.bulgaricus when grown together in milk culture. Differentiation of two species on HYA Agar is based on colony morphology. Also this media is recommended by APHA.