Exploring cellular fatty acid composition and intracellular metabolites of osmotic-tolerant mutant Lactobacillus paracasei NCBIO-M2 for highly efficient lactic acid production with high initial glucose concentration

Strain engineering of Bacillus coagulans with high osmotic pressure tolerance for effective L-lactic acid production from sweet sorghum juice under unsterile conditions

Open unsterile fermentation of the low-cost non-food crop, sweet sorghum, is an economically feasible lactic acid biosynthesis process. However, hyperosmotic stress inhibits microbial metabolism and lactic acid biosynthesis, and engineering strains with high osmotic tolerance is challenging. Herein, heavy ion mutagenesis combined with osmotic pressure enrichment was used to engineer a hyperosmotic-tolerant Bacillus coagulans for L-lactic acid production. The engineered strain had higher osmotic pressure tolerance, when compared with the parental strain, primarily owing to its improved properties such as cell viability, cellular antioxidant capacity, and NADH supply. In a pilot-scale open unsterile fermentation using sweet sorghum juice as a feedstock, the engineered strain produced 94 g/L L-lactic acid with a yield of 91 % and productivity of 6.7 g/L/h, and optical purity of L-lactic acid at the end of fermentation was 99.8 %. In short, this study provided effective and low-cost approach to produce polymer-grade L-lactic acid.

What are the main obstacles to turning foods healthier through probiotics incorporation? a review of functionalization of foods by probiotics and bioactive metabolites

Functional foods are gaining significant attention from people all over the world. When added to foods, probiotic bacteria can turn them healthier and confer beneficial health effects, such as improving the immune system and preventing cancer, diabetes, and cardiovascular disease. However, adding probiotics to foods is a challenging task. The processing steps often involve high temperatures, and intrinsic food factors, such as pH, water activity, dissolved oxygen, post-acidification, packaging, and cold storage temperatures, can stress the probiotic strain and impact its viability. Moreover, it is crucial to consider these factors during food product development to ensure the effectiveness of the probiotic strain. Among others, techniques such as microencapsulation and lyophilization, have been highlighted as industrial food functionalization strategies. In this review, we present and discuss alternatives that may be used to functionalize foods by incorporating probiotics and/or delivering bioactive compounds produced by probiotics. We also emphasize the main challenges in different food products and the technological characteristics influencing them. The knowledge available here may contribute to overcoming the practical obstacles to food functionalization with probiotics.

Enhancement effect of l-cysteine on lactic acid fermentation production

BACKGROUND

Environmental stress resistance is still a bottleneck for economical process for l-lactic acid fermentation. Chronological lifespan (CLS) extension has represented a promising strategy for improving stress resistance of microbial cell factories.

MAIN METHODS AND MAJOR RESULTS

In this study, addition of anti-aging drug cysteine, a kind of extending CLS of microbial cell factories, was systematically evaluated on cell viability and l-lactic acid production in Bacillus coagulans CICC 23843. The results revealed that 16 mm l-cysteine supplement significantly improved l-lactic acid titer in B. coagulans. The enhanced total antioxidant capacity (T-AOC) and key enzymes activities involving in glycolytic pathway as well as differentially expressed genes involved in cysteine synthesize and cysteine precursor synthesize pathways, and fatty acid degradation pathway may help to further understand the relative mechanism of l-cysteine effect on improving l-lactic acid accumulation. Finally, based on 16 mm l-cysteine supplement, a final l-lactic acid titer of 130.5 g L-1 with l-lactic acid productivity of 4.07 g L-1  h-1 and the conversion rate of 0.94 g g-1 total sugar was achieved in a 5 L bioreactor.

CONCLUSIONS AND IMPLICATIONS

This study provided a valuable option for engineering lactic acid bacteria lifespan for enhancement of lactic acid yield.

Microbial Fermentation Processes of Lactic Acid: Challenges, Solutions, and Future Prospects

The demand for lactic acid and lactic acid-derived products in the food, pharmaceutical, and cosmetic industries is increasing year by year. In recent decades, the synthesis of lactic acid by microbials has gained much attention from scientists due to the superior optical purity of the product, its low production costs, and its higher production efficiency compared to chemical synthesis. Microbial fermentation involves the selection of feedstock, strains, and fermentation modes. Each step can potentially affect the yield and purity of the final product. Therefore, there are still many critical challenges in lactic acid production. The costs of feedstocks and energy; the inhibition of substrates and end-product; the sensitivity to the inhibitory compounds released during pretreatment; and the lower optical purity are the main obstacles hindering the fermentation of lactic acid. This review highlights the limitations and challenges of applying microbial fermentation in lactic acid production. In addition, corresponding solutions to these difficulties are summarized in order to provide some guidance for the industrial production of lactic acid.

The Effect of Proline on the Freeze-Drying Survival Rate of Bifidobacterium longum CCFM 1029 and Its Inherent Mechanism

Amino acids, which are important compatible solutes, play a significant role in probiotic lyophilization. However, studies on the functions of Bifidobacterium during freeze-drying are limited. Therefore, in this study, we compared the freeze-drying survival rate of Bifidobacterium longum CCFM 1029 cultivated in different media containing different kinds of compatible solutes. We found that the addition of 21 g/L proline to the culture media substantially improved the freeze-drying survival rate of B. longum CCFM 1029 from 18.61 ± 0.42% to 38.74 ± 1.58%. Interestingly, this change has only been observed when the osmotic pressure of the external culture environment is increased. Under these conditions, we found that proline accumulation in this strain increased significantly. This change also helped the strain to maintain its membrane integrity and the activity of some key enzymes during freeze-drying. Overall, these results show that the addition of proline can help the strain resist a tough environment during lyophilization. The findings of this study provide preliminary data for producers of probiotics who wish to achieve higher freeze-drying survival rates during industrial production.

Characterization, High-Density Fermentation, and the Production of a Directed Vat Set Starter of Lactobacilli Used in the Food Industry: A Review

Lactobacilli have been widely concerned for decades. Bacteria of the genus Lactobacillus have been commonly employed in fermented food to improve the appearance, smell, and taste of food or prolong its shelf-life. They comprise 261 species (by March 2020) that are highly diverse at the phenotypic, ecological, and genotypic levels. Some Lactobacilli strains have been documented to be essential probiotics, which are defined as a group of living microorganisms that are beneficial to the health of the host when ingested in sufficiency. However, the characterization, high-density fermentation, and the production of a directed vat set (DVS) starter of Lactobacilli strains used in the food industry have not been systematically reported. This paper mainly focuses on reviewing Lactobacilli as functional starter cultures in the food industry, including different molecular techniques for identification at the species and strain levels, methods for evaluating Lactobacilli properties, enhancing their performance and improving the cell density of Lactobacilli, and the production techniques of DVS starter of Lactobacilli strains. Moreover, this review further discussed the existing problems and future development prospects of Lactobacilli in the food industry. The viability and stability of Lactobacilli in the food industry and gastrointestinal environment are critical challenges at the industrial scale. The new production equipment and technology of DVS starter of Lactobacilli strains will have the potential for large-scale application, for example, developing low-temperature spray drying, freezing granulation drying, and spray freeze-drying.

Production of GABA-enriched tomato juice by Lactiplantibacillus plantarum KB1253

To produce tomato juice with health-promoting functions, lactic acid bacteria (LAB) capable of converting l-glutamic acid in tomatoes into γ-aminobutyric acid (GABA) was screened from LAB stocks isolated from Japanese pickles. Lactiplantibacillus plantarum KB1253 was selected as the highest GABA producer among 74 strains of LAB stocks. gad gene expression and glutamic acid decarboxylation activity increased at low pH (3.0-3.5), whereas the growth decreased. Under optimal reaction conditions using resting cells as catalysts, this strain produced 245.8 ± 3.4 mM GABA. Furthermore, this strain produced 41.0 ± 1.1 mM GABA from l-glutamic acid in tomato juice under optimal fermentation conditions (pH 4.0, 20°Bx). This study may provide the basis for developing health-promoting functional foods rich in GABA from tomatoes and other agricultural products.

Protective effects of whey protein hydrolysate on Bifidobacterium animalis ssp. lactis Probio-M8 during freeze-drying and storage

We evaluated the potential of whey protein hydrolysate as a lyoprotectant for maintaining the cell viability of Bifidobacterium animalis ssp. lactis Probio-M8 during freeze-drying and subsequent storage. The moisture content and water activity of the lyophilized samples treated by different concentrations of whey protein hydrolysate were ≤5.23 ± 0.33 g/100 g and ≤0.102 ± 0.003, respectively. During storage at 25°C and 30°C, whey protein hydrolysate had a stronger protective effect on B. lactis Probio-M8 than the same concentration of whey protein. Using the Excel tool GinaFit, we estimated the microbial inactivation kinetics during storage. Whey protein hydrolysate reduced cell damage caused by an increase in temperature. Whey protein hydrolysate could protect cells by increasing the osmotic pressure as a compatible solute. Whey protein hydrolysate improved cell membrane integrity and reduced the amounts of reactive oxygen species and malondialdehyde produced. The findings indicated that whey protein hydrolysate was a novel antioxidant lyoprotectant that could protect probiotics during freeze-drying and storage.

Recovery of Salts from Synthetic Erythritol Culture Broth via Electrodialysis: An Alternative Strategy from the Bin to the Loop

Sustainability and circularity are currently two relevant drivers in the development and optimisation of industrial processes. This study assessed the use of electrodialysis (ED) to purify synthetic erythritol culture broth and for the recovery of the salts in solution, for minimising the generation of waste by representing an efficient alternative to remove ions, ensuring their recovery process contributing to reaching cleaner standards in erythritol production. Removal and recovery of ions was evaluated for synthetic erythritol culture broth at three different levels of complexity using a stepwise voltage in the experimental settings. ED was demonstrated to be a potential technology removing between 91.7–99.0% of ions from the synthetic culture broth, with 49–54% current efficiency. Besides this, further recovery of ions into the concentrated fraction was accomplished. The anions and cations were recovered in a second fraction reaching concentration factors between 1.5 to 2.5 times while observing low level of erythritol losses (<2%), with an energy consumption of 4.10 kWh/m3.

Ribosome Profiling and RNA Sequencing Reveal Genome-Wide Cellular Translation and Transcription Regulation Under Osmotic Stress in Lactobacillus rhamnosus ATCC 53103

To determine whether osmotic pressure affects the translation efficiency of Lactobacillus rhamnosus, the ribosome profiling assay was performed to analyze the changes in translation efficiency in L. rhamnosus ATCC 53103. Under osmotic stress, differentially expressed genes (DEGs) involved in fatty acid biosynthesis and metabolism, ribosome, and purine metabolism pathways were co-regulated with consistent expression direction at translation and transcription levels. DEGs involved in the biosynthesis of phenylalanine, tyrosine, and tryptophan, and the phosphotransferase system pathways also were co-regulated at translation and transcription levels, while they showed opposite expression direction at two levels. Moreover, DEGs involved in the two-component system, amino acid metabolism, and pyruvate metabolism pathways were only regulated at the transcription level. And DEGs involved in fructose and mannose metabolism were only regulated at the translation level. The translation efficiency of DEGs involved in the biosynthesis of amino acids was downregulated while in quorum sensing and PTS pathways was upregulated. In addition, the ribosome footprints accumulated in open reading frame regions resulted in impaired translation initiation and elongation under osmotic stress. In summary, L. rhamnosus ATCC 53103 could respond to osmotic stress by translation regulation and control the balance between survival and growth of cells by transcription and translation.

Lactobacillus, Bifidobacterium and Lactococcus response to environmental stress: Mechanisms and application of cross-protection to improve resistance against freeze-drying

The review deals with lactic acid bacteria in characterizing the stress adaptation with cross-protection effects, mainly associated with Lactobacillus, Bifidobacterium and Lactococcus. It focuses on adaptation and cross-protection in Lactobacillus, Bifidobacterium and Lactococcus, including heat shocking, cold stress, acid stress, osmotic stress, starvation effect, etc. Web of Science, Google Scholar, Science Direct, and PubMed databases were used for the systematic search of literature up to the year 2020. The literature suggests that a lower survival rate during freeze-drying is linked to environmental stress. Protective pretreatment under various mild stresses can be applied to lactic acid bacteria which may enhance resistance in a strain-dependent manner. We investigate the mechanism of damage and adaptation under various stresses including heat, cold, acidic, osmotic, starvation, oxidative and bile stress. Adaptive mechanisms include synthesis of stress-induced proteins, adjusting the composition of cell membrane fatty acids, accumulating compatible substances, etc. Next, we reveal the cross-protective effect of specific stress on the other environmental stresses. Freeze-drying is discussed from three perspectives including the regulation of membrane, accumulation of compatible solutes and the production of chaperones and stress-responsive proteases. The resistance of lactic acid bacteria against technological stress can be enhanced via cross-protection, which improves industrial efficiency concerning the survival of probiotics. However, the adaptive responses and cross-protection are strain-dependent and should be optimized case by case.

Metabolic engineering coupled with adaptive evolution strategies for the efficient production of high-quality L-lactic acid by Lactobacillus paracasei

The main indicators for industrial production of high-quality lactic acid at elevated temperatures are high titer, productivity, yield, and optical purity. However, no such strains have been reported to meet all these requirements simultaneously. In this study, a high optical purity L-lactic acid producing strain is developed through the CRISPR-Cas9 gene editing platform. Further, adaptive evolution was used to breed and select a high-performance strain (NCBIO01-M2-ldhL1-HT) that could efficiently produce L-lactic acid at a high temperature of 45℃. This strain produced 221.0 g/L of L-lactic acid in open fermentation with high initial glucose concentration. Also, L-lactic acid productivity and yield was above 7.5 g/L/h and 0.96 g/g respectively, as well as the optical purity of L-lactic acid in the fermentation broth exceeded 99.1%. In short, this breeding strain possess high potential to be considered for the commercial production of polymer-grade L-lactic acid.

Combined proteomics and transcriptomics analysis of Lactococcus lactis under different culture conditions

During industrial handling, Lactococcus lactis needs to adapt to different culture conditions by regulating its metabolic pathways. Modifying culture conditions may be an important way to control the biomass and functional metabolites of lactic acid bacteria. In this study, we identified the differentially expressed genes and proteins of L. lactis under different culture conditions by integrating transcriptomics and proteomics. We also analyzed the data using a bioinformatic approach to reveal the regulatory mechanisms affected by culture conditions. The transcriptome and proteome studies indicated that different culture conditions (fructose, calcium ion, palmitic acid, low pH) affected gene and protein expressions. The levels of differentially expressed proteins did not significantly correlate with the expression levels of their corresponding genes. Our results highlight the importance of comparative transcriptomics and proteomics analyses. In this study, fructose and pH significantly affected sugar metabolism of L. lactis. When lactose was replaced by fructose, fructokinase expression was promoted, and fructose metabolism was accelerated, whereas starch and sucrose metabolism and galactose metabolism system were inhibited. Low pH may be beneficial to homofermentation of L. lactis, which may also metabolize galactose through the tagatose pathway and the Leloir pathway. Fatty acid metabolism and fatty acid biosynthesis were significantly downregulated under calcium ion and palmitic acid. The purine metabolism was upregulated under fructose treatment and downregulated under palmitic acid treatment.

Lactic acid production - producing microorganisms and substrates sources-state of art

Lactic acid is an organic compound produced via fermentation by different microorganisms that are able to use different carbohydrate sources. Lactic acid bacteria are the main bacteria used to produce lactic acid and among these, Lactobacillus spp. have been showing interesting fermentation capacities. The use of Bacillus spp. revealed good possibilities to reduce the fermentative costs. Interestingly, lactic acid high productivity was achieved by Corynebacterium glutamicum and E. coli, mainly after engineering genetic modification. Fungi, like Rhizopus spp. can metabolize different renewable carbon resources, with advantageously amylolytic properties to produce lactic acid. Additionally, yeasts can tolerate environmental restrictions (for example acidic conditions), being the wild-type low lactic acid producers that have been improved by genetic manipulation. Microalgae and cyanobacteria, as photosynthetic microorganisms can be an alternative lactic acid producer without carbohydrate feed costs. For lactic acid production, it is necessary to have substrates in the fermentation medium. Different carbohydrate sources can be used, from plant waste as molasses, starchy, lignocellulosic materials as agricultural and forestry residues. Dairy waste also can be used by the addition of supplementary components with a nitrogen source.

Fermentation for tailoring the technological and health related functionality of food products

Fermented foods are experiencing a resurgence due to the consumers' growing interest in foods that are natural and health promoting. Microbial fermentation is a biotechnological process which transforms food raw materials into palatable, nutritious and healthy food products. Fermentation imparts unique aroma, flavor and texture to food, improves digestibility, degrades anti-nutritional factors, toxins and allergens, converts phytochemicals such as polyphenols into more bioactive and bioavailable forms, and enriches the nutritional quality of food. Fermentation also modifies the physical functional properties of food materials, rendering them differentiated ingredients for use in formulated foods. The science of fermentation and the technological and health functionality of fermented foods is reviewed considering the growing interest worldwide in fermented foods and beverages and the huge potential of the technology for reducing food loss and improving nutritional food security.