Use of pyruvate fermentation compared with tetrazolium reduction in the differentiation of group D streptococci

The reduction of 2, 3, 5 triphenyltetrazolium chloride in the original medium of Barnes (Journal of General Microbiology, 14, 57, 1956), and in a modified medium, was compared with the ability to ferment pyruvate as a means of separating Streptococcus faecalis from all other group D streptococci. The tetrazolium reduction test gave an occasional negative reaction with Str. faecalis strains. In addition, a number of strains gave a weakly positive reaction in the test, as did some strains of Str. faecium and Str. bovis. With some batches of tetrazolium, these weak reactions with Str. faecalis were so frequent that interpretation of the results was difficult. On the other hand, all Str. faecalis strains, and no other group D streptococci, gave a positive pyruvate-fermentation reaction in 48 hours.