Enhanced acid reduction in lactic acid bacteria: Breeding through irradiation-induced mutation and functional...

High concentrations of citric acid (CA), malic acid (MA), and tartaric acids (TA) are the primary contributors to the sour taste of fruit and fruit products. However, lactic acid bacteria that are capable of efficiently degrading these organic acids are scarce. Here, three brands of sauerkraut (Xinxi, X; Yuyuan, Y; and Zou Youcai, Z) with various doses of ⁶⁰Co γ-irradiation could be treated to induce mutations in their associated lactic acid bacteria and then the abilities of the resulting microbial communities to degrade CA, MA, and TA were evaluated. Sauerkraut X treated with 0.4 kGy irradiation demonstrated the greatest ability of acid reduction. Metagenomic analyses of irradiated (0.4 kGy) and non-irradiated bacterial communities from sauerkraut X revealed a slight decrease in microbial diversity due to irradiation, with a substantial decline in the relative abundance of Lactiplantibacillus xiangfangensis. Concurrently, the relative abundance of dominant acid-reducing lactic acid bacteria such as Levilactobacillus brevis, Pediococcus ethanolidurans, and Lentilactobacillus parafarraginis increased. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis revealed an increase in metabolism-related genes after irradiation, indicating that fatty acid synthesis and aspartate metabolism might be key pathways involved in the enhanced degradation of CA, MA, and TA. Analysis using the Carbohydrate-Active enzymes Database (CAZy) database revealed that glycoside hydrolase (GH) and glycosyltransferase (GT) genes were the most abundant carbohydrate-associated enzyme genes in the bacterial community of sauerkraut X. This finding proved that the oligosaccharides and monosaccharides produced by GH and GT might indirectly affect rates of organic acid degradation. Three highly effective acid-reducing lactic acid bacteria from the microbial community of irradiated sauerkraut X a were isolated and identified via 16S rRNA sequencing as Pediococcus ethanolidurans, Levilactobacillus brevis, and Loigolactobacillus coryniformis. The individual strains showed degradation rates as high as 92.02 % for citric acid (Pediococcus ethanolidurans), 83.04 % for malic acid (Levilactobacillus brevis), and 90.33 % for TA (Loigolactobacillus coryniformis). This study provides a theoretical basis and technical support for the development of enhanced microbial strains that can reduce the acid content of fruit materials.