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Journal of Dairy Science Vol. 68 No. 7 1608-1612
© 1985 by American Dairy Science Association ®
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Effects of Antibiotics on Proteinase-Positive and Proteinase-Negative Variants of Streptococcus cremoris1

Ragaa S. Hafez, Rodney J. Brown and Gary H. Richardson

Department of Nutrition and Food Sciences, Utah State University, Logan 84322

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 5 h.

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.

FOOTNOTES

1 Contribution number 2721 of the Utah Agricultural Experiment Station. Approved by the Director.






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Copyright © 1985 by the American Dairy Science Association ®.