Evaluation of culture media for selective enumeration of Bifidobacterium spp. in combination with different strains of Streptococcus thermophilus isolated from commercial yogurt starter cultures

Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus are involved in the manufacture of yogurt, and Bifidobacterium spp. can be supplemented if probiotic properties are expected. In such products, enumeration of each bacterial component is a hard task since they have similar nutritional requirements. For this reason, there is still no official and reliable methodology for this purpose. The objectives of the present study were to evaluate the most appropriate culture medium for the enumeration of four different Bifidobacterium species in the presence of S. thermophilus and L. delbrueckii subsp. bulgaricus isolated from commercial cultures and investigate the influence of different strains of S. thermophilus on the choice of medium. Among 11 isolates of S. thermophilus obtained from these cultures, and which were submitted to repetitive element sequence-based PCR (rep-PCR), nine were found to be distinct strains. For selective enumeration of the Bifidobacterium species, results were influenced by the S. thermophilus strain used, but De Man, Rogosa and Sharpe (MRS) agar containing bile salts showed the best results for all of the Bifidobacterium species.