Survival Mechanisms of Lactic Acid Bacteria Immobilized in Silanol-Humate Gels with Organic Acids

Bacterial survival under unfavorable conditions is an important issue. Long-term preservation of bacterial viability, an applied aspect of this problem, is of special importance for the storage of lactic acid bacteria due to the high biotechnological importance of this microbial group and its high death rate during storage. The goal of the present work was to investigate long-term survival of members of different physiological groups of lactic acid bacteria (heterofermentative Enterococcus faecium M3185 and homofermentative Lactobacillus paracasei АК 508) in silanol-humate gels (SHG) containing various organic acids used as titrating agents in SHG production (malic, lactic, acetic, ascorbic, and citric acids). Cell immobilization in SHG with organic acids for long-term storage (up to 5 months) resulted in a resulted in viable cell titers considerably exceeding (up to 200-fold) those in the control, depending on the strain and storage duration. The factors providing for long-term bacterial survival in SHG were: (1) hypometabolic state of most of the cells (with the metabolic rate 1000 lower compared to growing cells) and consumption of organic acids, as was seen from a decrease in their concentration and (in the case of E. faecium) from CO2 release; (2) absence of large-scale cell autolysis, probably due to separation of the cells in the gel, preventing sufficient accumulation of autoregulators and autolytic enzymes; and (3) dormant state of some of the cells (as stress-resistant cyst-like cells). Moreover, an alternative (biofilm) phenotype with increased stress resistance is probably formed in the course of gel immobilization. These results confirm that immobilization of lactic acid bacteria in SHG with organic acids is a promising approach to their long-term storage.