Osmotic stress induced by salt increases cell yield, autolytic activity, and survival of lyophilization of Lactobacillus delbrueckii subsp. lactis

Growth and stress adaptation of an autolytic strain of Lactobacillus delbrueckii subsp. lactis FAM-10991 was studied during pH-controlled batch fermentations. After an initial growth to an optical density at 650 nm of 0.8 under controlled optimal growth conditions (pH 5.5, 37 °C, no salt), exponentially growing cells were exposed to salt at concentrations from 1 to 3.5%, and temperatures between 48 and 53.5 °C, without pH control or with pH controlled at 5.5 or 4.5. Autolysis was induced by salt concentrations of 2.5 or 3.5% and suppressed at 53.5 °C or pH 4.5. Salt at concentrations of 2.5 or 3.5% or a temperature of 53.5 °C, without pH control or with pH controlled at 5.5, significantly enhanced (p < 0.05) survival of lyophilization as compared with the survival of cells in control cultures or cultures with salt at concentration of 1 and 1.5%. The former conditions increased survival by 125- and 200-fold, respectively. However, no correlation was found between autolytic activity and survival of lyophilization. Cultures grown with salt at 2.5% gave high yields of viable cells in broths before and after lyophilization, with numbers being 27-fold higher than with control cultures, but with autolytic activity that was 2.5-fold higher than in cells from control cultures.