Isolation and Characterization of Fast Acid-Producing Antibiotic-Resistant Mutants of Lactic Streptococci

Single-step streptomycin-resistant mutants of nine strains of lactic streptococci used in commercial Cheddar cheese manufacture were isolated. In each case a 24-h broth culture was concentrated by centrifugation and pour plated in medium containing 600 /ag/ml of streptomycin. Mutant colonies were selected and subcultured in broth and then streaked onto antibiotic-containing fast-slow differential agar, which allowed isolation of fast acidproducing organisms. A fast rifampinresistant mutant also was obtained by this technique.