RAFFINOSE-BIFIDOBACTERIUM (RB) AGAR, A NEW SELECTIVE MEDIUM FOR BIFIDOBACTERIA

A new selective medium for the detection of bifidobacteria was develop ed. The medium owes its selectivity to the presence of propionate (15 g/l) and lithium chloride (3 g/l) as inhibitory agents, and raffinose (7.5 g/l) as a selective carbon source. In addition, casein (5 g/l) is used as a protein source, which results in a zone of precipitation ar ound the colonies of bifidobacteria. The medium, Raffinose-Bifidobacte rium (RB) agar, is free of antibiotics and easy to prepare. Bifidobact eria growing on RB agar show a yellow colony with a yellow halo and a precipitation zone around the colony. All human and dairy bifidobacter ia grew well on RB agar, except for some Bifidobacterium bifidum strai ns. Some uncommon or animal species, Bifidobacterium gallicum, Bifidob acterium asteroides, Bifidobacterium animalis, Bifidobacterium pulloru m, and some Bifidobacterium bifidum either did not grow or did not sho w the characteristic reactions. No non-bifidobacterial strains used in dairy products grow with the distinctive characteristics. Some faecal strains showed good growth and acid production, resulting in characte ristic reactions. These strains were Actinomyces species, a Clostridiu m perfringens strain and some lactobacilli of animal origin. Compariso n with other media indicated that RB agar is more selective than other media used.