[Study of intraspecific variations of the bacterium Lactococcus lactis in adaptation to high acidity of the medium]

This paper reports on the study of acid tolerance of lactic acid bacteria as a property of cells, determining their ability to divide efficiently and retain viability under conditions of increased nutrient medium acidity during bacterial growth. The bacteria of the strain TV2, isolated from a self-soured curd, similar to the bacteria of the strain STE05 (Russian National Collection of Industrial Microorganisms), were assigned to the species Lactococcus lactis according to their G+C composition (36.7-36.5 mol %) and the high level of DNA-DNA hybridization (93%). However, these strains were essentially different in the number and size of the plasmids and the chromosomal DNA restriction fragments, as well as in the sensitivity to phages of lactic acid bacteria. It was found that bacteria of the strain TV2 were stable (i.e., they divided efficiently at a pH as low as 5.3) and tolerant to the lactic acid that they produced while growing (i.e., they retained viability at pH 4.4). Bacteria of the strain STE05 lacked acid tolerance (at pH below 6.5, growth was retarded, and pH 5.0 was the lowest value at which the cells remained viable). The acid tolerance and phage resistance of TV2 bacteria are likely to characterize their higher adaptive capacity in comparison with STE05 bacteria. Acid tolerance is inherited in a stable manner and retained by the segregants of TV2 strain obtained in the course of long-term storage of the bacteria. Specifically, the strains TV29, TV13, and TV 229, which displayed this property, had altered physiological and biochemical characteristics (accumulation of biomass and fermentation of lactose) in spite of their genetic identity to the original strain (pulsed-field gel electrophoresis of chromosomal DNA restriction fragments).