Forms and mechanisms for Escherichia coli long-term survival in silanol-humate gels

Immobilization of bacteria with various types of metabolism (aerobic hydrocarbon oxidizers and fermentative lactic acid bacteria) into silanol-humate gels (SHG) resulted in increase in the number of the cells surviving long-term storage (for months or years) under room conditions by 1-3 orders of magnitude regarding non-stabilized cultures. The goal of the present work was to investigate the effect of SHG immobilization and storage on survival of Escherichia coli К-12 (MG1655), capable of both respiratory and fermentative metabolism. SHG immobilization of E. coli was shown to result in an adaptive response, including acquisition of a hypometabolic state, slow growth of surviving cells, and transition of some of the population to dormant forms. Survival efficiency in SHG depended on the presence and properties of organic acids used as titrants during gelation. The highest viable cell titer after 5 months of incubation was achieved in SHG obtained using ascorbic, malic, citric, or lactic acids (50-100 times higher than in the control, cells in liquid medium). Utilization of acids as energy sources for the maintenance of cell viability was monitored by their consumption and CO₂ emission. After long-term storage in SHG, the E. coli populations exhibited: (i) a hypometabolic state (the rate of metabolism was four orders of magnitude lower than in growing cultures); (ii) low heat resistance of the cells; (iii) high abundance of persister cells; and (iv) high level of antibiotic-tolerant cells. These results have high potential for application in biotechnologies requiring long-term stabilization of microbial biocatalysts.