Comparative genomics and transcriptome analysis of Lactobacillus rhamnosus ATCC 11443 and the mutant strain SCT-10-10-60 with enhanced L-lactic acid production capacity

Multi-omics analysis reveals the mechanism for galactose metabolism in mutant Streptococcus thermophilus IMAU20551Y

Streptococcus thermophilus (S. thermophilus) is a species widely used in the dairy industry to accelerate the acidification rate and improve the texture and flavour characteristics of dairy products. However, most S. thermophilus have galactose-negative (Gal-) phenotypes, which can lead to accumulation of free galactose in fermented dairy products. In a previous study, a mutant of S. thermophilus IMAU20551Y was obtained by N-methyl-N'-nitro-N-nitrosoguanidine (NTG) mutagenesis in which key enzymes related to galactose metabolism were significantly changed compared with the wild type. β-galactosidase and galactokinase activity were higher in the mutant while glucokinase and pyruvate kinase activities were significantly decreased compared with the wild type. In this study, the ability of the mutant to metabolize galactose was verified by high performance liquid chromatography (HPLC), and the mechanism for enhanced galactose metabolism elucidated by multi-omics analysis. HPLC analysis showed that accumulation of galactose in milk fermented by mutant S. thermophilus IMAU20551Y was reduced by 41.4%, compared with the wild type. Although no mutations in gene sequences associated with galactose metabolism were detected by genome sequencing, transcriptomic data showed up-regulation in expression of galM, galK, galT, galE (associated with the Leloir pathway) and LacI family transcriptional regulator GalR, resulting in enhanced galactose metabolism in the mutant. This study provides a reference for genetic engineering modification of galactose-positive (Gal+) S. thermophilus, which is expected to be used as a starter for the production of low galactose fermented dairy products.

Improvement in L-ornithine production from mannitol via transcriptome-guided genetic engineering in Corynebacterium glutamicum

BACKGROUND

L-Ornithine is an important medicinal intermediate that is mainly produced by microbial fermentation using glucose as the substrate. To avoid competition with human food resources, there is an urgent need to explore alternative carbon sources for L-ornithine production. In a previous study, we constructed an engineered strain, Corynebacterium glutamicum MTL13, which produces 54.56 g/L of L-ornithine from mannitol. However, compared with the titers produced using glucose as a substrate, the results are insufficient, and further improvement is required.

RESULTS

In this study, comparative transcriptome profiling of MTL01 cultivated with glucose or mannitol was performed to identify novel targets for engineering L-ornithine-producing strains. Guided by the transcriptome profiling results, we modulated the expression of qsuR (encoding a LysR-type regulator QsuR), prpC (encoding 2-methylcitrate synthase PrpC), pdxR (encoding a MocR-type regulator PdxR), acnR (encoding a TetR-type transcriptional regulator AcnR), CGS9114_RS08985 (encoding a hypothetical protein), and CGS9114_RS09730 (encoding a TetR/AcrR family transcriptional regulator), thereby generating the engineered strain MTL25 that can produce L-ornithine at a titer of 93.6 g/L, representing a 71.6% increase as compared with the parent strain MTL13 and the highest L-ornithine titer reported so far for C. glutamicum.

CONCLUSIONS

This study provides novel indirect genetic targets for enhancing L-ornithine accumulation on mannitol and lays a solid foundation for the biosynthesis of L-ornithine from marine macroalgae, which is farmed globally as a promising alternative feedstock.

Comparative transcriptome analysis for the biosynthesis of antioxidant exopolysaccharide in Streptococcus thermophilus CS6

BACKGROUND

Food grade Streptococcus thermophilus produces biological exopolysaccharides (EPSs) with great potential with respect to catering for higher health-promoting demands; however, how S. thermophilus regulates the biosynthesis of EPS is not completely understood, decelerating the application of these polymers. In our previous study, maltose, soy peptone and initial pH were three key factors of enhancing EPS yield in S. thermophilus CS6. Therefore, we aimed to investigate the regulating mechanisms of EPS biosynthesis in S. thermophilus CS6 via the method of comparative transcriptome and differential carbohydrate metabolism.

RESULTS

Soy peptone addition (58.6 g L^-1 ) and a moderate pH (6.5) contributed to a high bacterial biomass and a high EPS yield (407 mg L^-1 ). Maltose, soy peptone and initial pH greatly influenced lactose utilization in CS6. Soy peptone addition induced a high accumulation of mannose and arabinose in intracellular CS6, differential monosaccharide composition (mannose, glucose and arabinose) in EPS and high radical [2,2-diphenyl-1-picrylhydrazyl, superoxide and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)] scavenging activities. Carbohydrate transportation, sugar activation and eps cluster-associated genes were differentially expressed to regulate EPS biosynthesis. Correlation analysis indicated high production of EPSs depended on high expression of lacS, galPMKUTE, pgm, gt2-5&4-1 and epsLM.

CONCLUSION

The production of antioxidant EPS in S. thermophilus CS6 depended on the regulation of galactose metabolism cluster and eps cluster. The present study recommends a new approach for enhancing EPS production by transcriptomic regulation for further food and health application of EPS. © 2022 Society of Chemical Industry.