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

Abstract

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 600μg/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 acid-producing organisms. A fast rifampin-resistant mutant also was obtained by this technique. Characterization involved comparing parent and mutant strains for growth, acid production, proteolytic abilities, optimum temperatures, and phage sensitivities. The use of this technique to obtain isolates resistant to other antibiotics and their possible use in commercial cheese manufacture to overcome antibiotics in the milk supply is discussed.