Selective enumeration of Lactobacillus casei in yoghurt-type fermented milks based on a 15 degrees C incubation temperature

A procedure was developed to enumerate selectively Lactobacillus casei populations in yoghurt-type fermented milks that can also contain strains of Streptococcus thermophilus, Lactobacillus delbrueckii ssp. bulgaricus, Lactobacillus acidophilus and Bifidobacterium infantis. Commercial LBS agar was acidified to pH 5.4, and the plates were incubated at 15 degrees C for 14 days under anaerobic conditions. Acidification prevented the development of streptococci, and incubation at 15 degrees C limited the development of the lactobacilli and the bifidobacteria. L. casei formed colonies on HHD medium which were different from those obtained with L. bulgaricus. Counts of L. casei on HHD confirmed results obtained on LBS - pH 5.4 medium and incubated at 15 degrees C. L. casei did not form colonies on M17, nor did L. acidophilus or L. bulgaricus.