Comparative studies on synthesis of water-soluble vitamins among human species of bifidobacteria.

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The ability of bifidobacteria to synthesize six water-soluble vitamins (thiamine, folic acid, nicotinic acid, pyridoxine, vitamin B12 and riboflavin) was systematically investigated with twenty-four strains of five species derived from human feces. The vitamins synthesized were determined as those accumulated in cultures grown in a semi-synthetic medium. For the vitamins other than vitamin B12 and riboflavin, extracellular liberation was also examined with the supernatant fluids obtained after removing cells from the cultures. Many strains of the bifidobacteria investigated could indeed synthesize five of the vitamins, the exception being riboflavin. A large portion of each of the vitamins synthesized was excreted into the medium. The concentrations of the vitamins, especially thiamine, nicotinic acid and folic acid, accumulated varied widely among different species or strains. On the basis of the results, these bifidobacteria could be divided into three general types according to the abilities to accumulate thiamine, nicotinic acid and folic acid. These three vitamins were accumulated in all the strains of B. bifidum and B. infantis as well as in many strains of B. breve and B. longum but the vitamin concentrations were significantly higher in the former species (higher-accumulators) than in the latter species (lower-accumulators). On the other hand, none of these three vitamins were detected in most strains of B. adolescentis or in some strains of B. breve and B. longum (non-accumulators). Non-accumulator strains of B. adolescentis required thiamine and nicotinic acid for maximal growth in a complete synthetic medium. Further studies on thiamine synthesis revealed that the addition of exogenous thiamine to the medium significantly reduced the level of thiamine accumulated in a culture of a lower-accumulator strain of B. longum but did not affect the vitamin level in a higher-accumulator strain of B. bifidum. These findings were discussed in relation to the vitamin biosynthesis and its regulation in bacteria.

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