Analysis of differential metabolites of Brevibacterium casei G20 in response to saline-alkali stress

Abstract:

[Objective] To explore the metabolite composition and metabolite synthesis potential of Brevibacterium casei G20 in response to saline-alkali stress, and to provide a reference for the mining of potential functional molecules and the rapidly and stably responsive genetic logic gate line induced by saline-alkali. [Methods] We used LC-MS to detect the metabolites of G20 at four growth phases in saline-alkali and normal environments, and focused on the analysis of the lag and logarithmic phases rich in metabolites with high differential fold changes. [Results] B. casei G20 grew well in the environment with pH 10.0 and 9% NaCl, and the pH of the environment gradually decreased with the growth. According to the detection result in positive ion mode and negative ion mode, the numbers of differential metabolites of B. casei G20 during growth in the saline-alkali environment were 0.69, 0.75, and 0.81 times that in the normal environment, respectively. The metabolites that differed between the lag and logarithmic phases under saline-alkali stress were mainly benzenoids, organic acids and their derivatives, and organic heterocyclic compounds. Among them, the MS signal intensity of the organic acids such as indole-3-acetic acid, kynurenic acid, and gluconic acid that were up-regulated in the logarithmic phase was lower than that in the lag phase. The possible osmoprotectants in the strain were l-citrulline, l-proline, N-acetylornithine, l-carnitine, etc. The differential compounds with larger fold changes or higher MS signal intensity in the lag phase were pilocarpine, phytosphingosine, and citrate, and in the logarithmic phase were histamine, l-proline, and thiamine. The differential metabolic pathways of the strain were mainly the amino acid metabolism and carbohydrate metabolism. There were also structural analogs of betaine and trigonelline in the metabolites of the strain. In addition, for metabolites with large changes induced by saline-alkali stress, such as histamine, l-proline, and choline, we could infer their synthetic and metabolic pathways through the coupling of metabolomic and genomic data. [Conclusion] The strain secreted organic acids such as indole-3-acetic acid, kynurenic acid, and Gluconic acid to reduce the pH value of the environment, and accumulated osmoprotectants such as l-citrulline and l-proline to maintain osmotic balance. In addition, the strain may also have the ability to synthesize structural analogs of betaine, trigonelline, and pilocarpine, allowing the construction of a new synthetic pathway based on prokaryotic bacteria. Moreover, the enzyme-coding genes and upstream and downstream element sequences in the synthetic and metabolic pathways of compounds such as histamine, l-proline, and choline in the strain can serve as a reference for sorting out the logic gate gene circuits induced by saline-alkali stress.