Thermoinducible Lysis of Temperature-Sensitive Streptococcus cremoris Strains

Temperature-sensitive Streptococcus cremoris SK11 appeared to harbor a temperate phage that was induced at the maximum cooking temperatures used in Cheddar cheese manufacture, i.e., 38 to 40°C. When incubated at 30°C in M17-lactose broth, followed by a shift to 40°C, lysis of the culture occurred within 2 h. Results were similar when S. cremoris SK11 was propagated in M17-lactose broth through a simultated Pearce activity test. In 11% solids reconstituted NDM, this strain also exhibited decreased cell numbers approximately 2 h after the temperature had reached 40°C in the Pearce activity test. To support the hypothesis that lysis of the cells was due to induction of phage, electron microscopy of the temperature-induced lysate revealed the presence of particles resembling phage heads and tails but only a few intact phage-like particles. Examination of other temperature-sensitive or temperature-insensitive strains of S. cremoris also suggested that lysis of temperature-sensitive strains occurred after cells were incubated at 40°C, but no lysis was observed for temperature-insensitive strains.