The article presents the results of a study of the biological properties of the Lactobacillus brevis 2k.Gv strain (morphological, cultural, biochemical, physiological and probiotic) in order to assess the biotechnological potential and create alternative forms of probiotics for highly productive fish farming. Under the conditions of an aquarium experiment, a positive effect of the strain on the physiological parameters of the rainbow trout Parasalmo mykiss Walbaum was found. L. brevis 2k. Gv was orally administered to fish for 30 days at a concentration of 2×108 cells/mg feed as a supplement to the main diet. It was found that the studied strain has a multifactorial effect on the fish organism. Throughout the entire period of feeding juvenile trout with extruded feed enriched with lactobacilli, no signs of infectious and somatic diseases were recorded in the experimental group of trout; hepatosomatic and splenosomatic indices of organs, as well as hematological parameters of blood remained normal. The results obtained prove the presence of probiotic properties in the Lactobacillus brevis 2k. Gv strain and the possibility of its further use as part of feed additives for therapeutic and prophylactic purposes for aquaculture objects.

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The article presents the results of quantitative indicators of the growth of rainbow trout when growing feed with a partial replacement of animal proteins with plant proteins. The results obtained show that the growth rate of fish and the feed coefficient when replacing protein by 10% with plant proteins do not change, provided that the animal, as well as the macro- and microelement balance of feed, are observed.

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D-lactic acid stereoisomer (D-lactate) is produced by the intestinal microflora and can enter the bloodstream and cause in some cases a condition of acute D-lactic acidemia known as short gut syndrome. The level of D-lactate in blood and in the contents of the intestine is considered as a promising marker of the development of inflammation associated with microflora disorders, as well as with the development of a bacterial infection, while the mechanism of its entry into the blood of vertebrates from the intestine has not been studied in detail.The aim of the study. To investigate the relationship between the level of D-lactate in blood and in the intestine, taking into account the permeability of the intestinal epithelium.Materials and methods. As a model object of the study, we used juvenile rainbow trout O. mykiss. For 54 days, they were high-carbohydrate or high-protein fed. Since different types of bacteria prefer different substrates, it was expected that at the end of the experiment, the composition of the intestinal microflora would be significantly different in fish fed with different diets. The content of D-lactate in blood plasma in vitro was assessed by the Larsen method with modifications; intestinal permeability was assessed by the intensity of fluorescence of the FITС-Dextran stain in the blood of fish. The analysis of the metagenome of samples of the contents and epithelium of the fore and hind intestine was carried out. The hematological profile was partially characterized using blood smears taken immediately after fish blood sampling. By the means of a different diet, it was possible to obtain two groups of fish that differ significantly in the permeability of the intestinal epithelium and in the content of D-lactate in the intestine. At the same time, despite the differences between the experimental groups in the content of D-lactate in the intestine and in intestinal permeability, no significant differences in D-lactate level in blood were found between them. Analysis of the composition of the intestinal microbiome by metabarcoding for the 16S rRNA gene revealed the absence of lactobacilli in the production of D-lactate in fish.Results. It was shown that the mechanism of accumulation of D-lactate in the blood plasma in fish is less associated with increased intestinal permeability or hyperproduction of this metabolite by the intestinal microflora and is more associated with the utilization of D-lactate in the body. In the experiment, it was not possible to achieve a significant change in the species composition of the intestinal microflora of trout under the influence of a highcarbohydrate diet for 54 days compared to fish that received high-protein diet. Some tendencies towards changes in the composition of the microflora were found in the contents of the hindgut, and perhaps with a longer exposure, these changes could reach a statistically significant level.