The results of variations in acid production in 1 per cent dextrose fermented and unfermented veal broth modified by the addition of 4 per cent of horse serum or 2 per cent of peptone have been recorded. Human streptococci and a group of low acid-producing streptococci from milk produce less acid in the simpler broths (fermented and unfermented). 2 per cent peptone fails to increase the amount of acid produced by these two groups.
The bovine streptococci act much the same as those of human origin. The equine streptococci apparently do not require serum in addition to carbohydrate since they tend to produce less acid in serum media.
The following figures indicate the average minimum and maximum acid production of the various streptococci under the conditions set forth in the experiment.
Human: pH 4.97 to 5.66; titratable acidity 4.51 to 3.66
Bovine: pH 4.56 to 4.77; titratable acidity 7.0 to 5.74
Equine: pH 4.86 to 5.42; titratable acidity 5.38 to 4.24
Low acid-producing streptococci from milk: pH 6.28 to 5.14; titratable acidity 2.56 to 4.28