Effects of Antibiotics on Proteinase-Positive and Proteinase-Negative Variants of Streptococcus cremoris

Abstract

The growth and acid-producing ability of proteinase-positive and proteinase-negative starter cultures were studied in the presence of several antibiotics commonly used in treating bovine mastitis. Proteinase-positive and proteinase-negative variants of two different strains of Streptococcus cremoris were inoculated separately into heat-sterilized, reconstituted nonfat milk containing a range of concentrations of either dihydrostreptomycin, erythromycin, or penicillin G. Generation times and pH changes were determined for each sample following incubation at 38°C for 5h.At 2% inoculation, proteinase-positive cultures produced more pH change than their proteinase-negative counterparts at all concentrations of all three antibiotics. Proteinase-negative cultures at 8% inoculation produced more acid than proteinase-positives did when inoculated at 2%. Generation times for proteinase positives and proteinase negatives were the same when both cultures were inoculated at 2%, but 8% inoculation of proteinase negatives produced significantly shorter generation times. Increases in antibiotic concentrations affected proteinase-positives more than proteinase-negatives in both pH change and generation time. Penicillin affected both proteinase positives and negatives more than the other two antibiotics did but affected positives more than negatives.