The influence of different glucose concentrations on the synthesis of lactic acid by Enterococcus bacteria
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In the present work, weinvestigated the effect of glucose concentration in the nutrient medium on the growth of bacterial isolates Enterococcus faecium and Enterococcus sp. from wheat bran, degraded by the action of native microbiota for various periods of time, and lactic acid biosynthesis. The cultures were identified by sequencing the 16S rRNA gene. Cultivation was carried out in a shaker incubator for 50 hours. The kinetics of bacterial growth was evaluated by measuring the optical density of the culture fluid at a wavelength of 600 nm; the lactic acid content was determined by photometric method using iron (III) chloride. Glucose and yeast extract, as well as beet molasses, were used as components of the culture medium for cultivation. It is shown that the maximum optical density characterizing the accumulation of biomass and the lactic acid content for both strains were achieved with a minimum concentration of glucose in the nutrient medium (50 g/l). These observations indicate a low resistance of the studied bacteria to elevated concentrations of glucose in the medium. Glucose concentrations of 100 g/l and above inhibited biomass growth and lactic acid synthesis. The revealed effect allowed us to make an assumption about the belonging of bacterial isolates to oligocarbophiles. When culturing a bacterial culture of E. sp., the highest yield (11.65 g/l) of lactic acid was achieved in comparison with E. faecium. It has been shown that when the glucose content in the culture medium is 50 g/l, the conversion rate of the substrate to lactate reaches 23.3%. When culturing the studied bacterial isolates on a medium containing beet molasses as a carbon source, the lactate yield was 23 g/l for the E. faecium strain and 22 g/l for E. sp. (the degree of carbohydrate conversion to MK is 46% and 44%, respectively). The high content of simple carbohydrates in molasses contributes to an increase in the productivity of biomass for the biosynthesis of lactic acid.
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