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Examination of Strain Interaction in a Multiple-Strain Lactic Starter Culture by Streptomycin-Resistant Mutants of Lactic Streptococci¹

Department of Microbiology, Oregon State University, Corvallis 97331–3804

ABSTRACT

Streptomycin-resistant mutants of lactic streptococci were used to determine strain dominance and suppression in a multiple-strain Cheddar cheese starter culture composed of two Streptococcus lactis and four Streptococcus cremoris strains. This was accomplished by growing the six strains together in six separate batches of nonfat milk and substituting a different streptomycin-resistant mutant in each experiment. Samples were taken periodically and plated onto media with and without added streptomycin. In this manner, the resistant strain in each batch could be identified and counted during growth in mixed culture. Two-strain combinations were studied similarly. Both Streptococcus lactis strains significantly inhibited two of the four Streptococcus cremoris strains, and one instance of Streptococcus cremoris-Streptococcus cremoris inhibition was found. Speculation regarded a dominance pattern in this starter.

¹ Technical Paper No. 6578, Oregon Agricultural Experiment Station.

² Genentech, Inc., South San Francisco, CA 94080.