The influence of temperature on the autolytic activity of thermophilic lactobacilli

In this work in vitro autolytic activity of 91 thermophilic lactobacilli strains of Lactobacillus helveticus, Lactobacillus delbrueckii subsp. bulgaricus and Lactobacillus delbrueckii subsp. lactis at different temperatures was studied. The autolytic activity was determined as OD 650 nm decrease of cells suspended in 0.2 M phosphate buffer, pH 7.0 incubated at 15°C, 42°C and 55°C for 24 h. Autolysis appeared to be a highly strain-dependent phenotypic characteristics, whose extent varied according to the temperature. Strains corresponding to Lb. helveticus and L. delbrueckii subsp. lactis species reached on average their highest autolysis values at 42°C, while L. delbrueckii subsp. bulgaricus strains reached the same values at 42°C and 55°C. For all the species considered autolysis appeared to be almost negligible at 15°C. On account of the species average values, Lb. helveticus was found to have the most autolytic activity, though it showed higher behaviour heterogeneity with respect to the Lb. delbrueckii species. Optimum time for autolysis was also determined for each incubation temperature.