Spontaneous phage-resistant mutants of Streptococcus thermophilus: Isolation and technological characteristics

Spontaneous phage-resistant mutants were isolated from 12 commercial Streptococcus thermophilus strains by secondary-culture method. They were characterized by cell and colony morphology, carbohydrate fermentation patterns, phage-resistance stability, efficiency of plaquing (EOP), acidifying and proteolytic powers and milk acidification kinetics. Only 22.6% ( n = 100 ) of isolates proved to be true phage-resistant mutants. Phage resistance stability was a variable parameter among the mutants isolated, while EOP values were mostly very low ( 0.05 ) between phage-sensitive strains and their respective phage-resistant derivatives. Groups of mutants were found showing a milk acidifying activity which was slower or faster than that observed for the parent strain. Some mutants exhibiting high phage resistance and good technological capacities could be used as improved strains for industrial processes.