Evaluation of media for selective enumeration of Lactobacillus acidophilus and Bifidobacterium species

Abstract

Seven different media were evaluated to determine their suitability for selective enumeration of Lactobacillus acidophilus and Bifidobacterium spp using six strains of Streptococcus salivarius ssp thermophilus, five strains of Lactobacillus delbrueckii ssp bulgaricus, six strains of Lactobacillus acidophilus and nine strains of Bifidobacterium spp. The media which were evaluated were NNLP agar, Bile agar, MRS-galactose agar, MRS-maltose agar, MRS-dextrose agar, MRS-L-arabinose agar and RCA agar. NNLP agar was found to be suitable for selective enumeration of B. bifidum, B. longum, B. pseudolongum and B. thermophilum as this medium suppressed the growth of S. salivarius thermophilus, L. delbrueckii ssp bulgaricus and of L. acidophilus. Bile agar can be used for selective enumeration of L. acidophilus from a product containing L. acidophilus and B. bifidum, B. adolescentis, B. breve or B. longum 20097. Other strains of bifidobacteria grew in Bile agar. MRS-maltose agar can be used for selective enumeration of L. acidophilus from yogurt organisms in a product which does not contain Bifidobacterium spp. MRS-L arabinose agar suppressed the growth of S. salivarius ssp thermophilus, L. delbrueckii ssp bulgaricus, L. acidophilus and bifidobacteria except that of B. longum and B. pseudolongum. Thus, this medium can be used for selective enumeration of B. longum and B. pseudolongum only. In a separate experiment, MRS-L-arabinose agar and NNLP agar were used to selectively enumerate B. longum 1941 from yogurt prepared with S. salivarius ssp thermophilus, L. delbrueckii ssp bulgaricus, L. acidophilus and B. longum 1941 as starter cultures. B. longum colonies were easily differentiated from that ofL. acidophilus, which formed tiny colonies, while S. salivarius ssp thermophilus and L. delbrueckii ssp bulgaricus did not form any colonies.