Effect of Peptide-Rich Extracts on Acid Production by Oxytetracycline and Penicillin-Resistant Starters

The effect of peptide-rich extracts, including pancreas extract, yeast extract, liver fraction L, tryptone, tryptose, neopeptone, casein hydrolysate, milk protein hydrolysate, and peptonized milk, on the acid production of oxytetracycline-sensitive and -resistant counterparts of Streptococcus lactis (10D) in skimmilk was determined. Additions of 0.5% of each stimulant enabled the resistant culture to produce acid in 8 hr., in concentrations equal to or in excess of that of the sensitive culture. The additives did not enable the sensitive culture to produce acid in the presence of the antibiotic.