From physiology to systems metabolic engineering for the production of biochemicals by lactic acid bacteria

Fortification of goat milk yogurts with date palm (Phoenix dactylifera L.) coproducts: Impact on their quality during cold storage

The aim of this study was to investigate the impact of different concentrations (3% and 6%) of two ingredients (paste and flour) obtained from the valorization of date fruit coproducts on the nutritional (proximate composition and mineral profile), technological (coagulation curve, pH, acidity, sugar and organic acid content and syneresis), physicochemical (color, water activity and texture), microbiological and sensory properties of goat's yogurt during 21 days of refrigerated storage. Both ingredients enhanced the growth and stability of the yogurt starter culture, thereby improving the probiotic potential of date-added yogurts. Physicochemically, the addition of date flour (at both concentrations) induces stronger modifications (texture, color and syneresis) in yogurts than the date paste. During storage, date paste reduced the syneresis and hence maintained yogurts' physical quality. Consumers preferred the yogurts with date paste (3% and 6%) rather than with date flour, because its addition led to a more brownish color and granular texture.

Genomic Modifications of Lactic Acid Bacteria and Their Applications in Dairy Fermentation

Lactic Acid Bacteria (LAB) have a long history of safe use in milk fermentation and are generally recognized as health-promoting microorganisms when present in fermented foods. LAB are also important components of the human intestinal microbiota and are widely used as probiotics. Considering their safe and health-beneficial properties, LAB are considered appropriate vehicles that can be genetically modified for food, industrial and pharmaceutical applications. Here, this review describes (1) the potential opportunities for application of genetically modified LAB strains in dairy fermentation and (2) the various genomic modification tools for LAB strains, such as random mutagenesis, adaptive laboratory evolution, conjugation, homologous recombination, recombineering, and CRISPR (clustered regularly interspaced short palindromic repeat)- Cas (CRISPR-associated protein) based genome engineering. Lastly, this review also discusses the potential future developments of these genomic modification technologies and their applications in dairy fermentations.

Microbial synthesis of anthocyanins and pyranoanthocyanins: current bottlenecks and potential solutions

Anthocyanins (ACNs) are secondary metabolites found in plants. Due to their impressive biological activities, ACNs have gained significant popularity and extensive application within the food, pharmaceutical, and nutraceutical industries. A derivative of ACNs: pyranoanthocyanins (PACNs) possesses more stable properties and interesting biological activities. However, conventional methods for the production of ACNs, including chemical synthesis and plant extraction, involve organic solvents. Microbial synthesis of ACNs from renewable biomass, such as amino acids or flavonoids, is considered a sustainable and environmentally friendly method for large-scale production of ACNs. Recently, the construction of microbial cell factories (MCFs) for the efficient biosynthesis of ACNs and PACNs has attracted much attention. In this review, we summarize the cases of microbial synthesis of ACNs, and analyze the bottlenecks in reconstructing the metabolic pathways for synthesizing PACNs in microorganisms. Consequently, there is an urgent need to investigate the mechanisms behind the development of MCFs for PACNs synthesis. Such research also holds significant promise for advancing the production of food pigments. Meanwhile, we propose potential solutions to the bottleneck problem based on metabolic engineering and enzyme engineering. Finally, the development prospects of natural food and biotechnology are discussed.

Sodium carboxymethyl celluloses as a cryoprotectant for survival improvement of lactic acid bacterial strains subjected to freeze-drying

This study investigated the possibility of sodium carboxymethyl celluloses (Na-CMC) in protecting the viability of lactic acid bacteria (LAB) against freeze-drying stress. 1 % concentration of Na-CMC with a 0.7 substitution degree and viscosity of 1500 to 3100 (MPa.s) was found to protect Lactobacillus delbrueckii subsp. bulgaricus CICC 6098 best, giving a high survival rate of 23.19 ± 0.88 %, high key enzymatic activities, and 28-day storage stability. Additionally, Na-CMC as cryoprotectant provided good protection for other 7 lactic acid bacterial strains subjected to freeze-drying. The highest survival rate was 48.79 ± 0.20 U/mg for β-GAL, 2.75 ± 0.15 U/mg for Na+-K+-ATPase, and 2.73 ± 0.41 U/mg for Ca2+-Mg2+-ATPase as 48.48 ± 0.46 % for freeze-dried Pediococcus pentosaceus CICC 22228. It was Interesting to note that the presence of Na-CMC reduced the freezable water content of the lyophilized powders containing the tested strains through its hydroxyl group, and supplied micro-holes and fibers for protecting the integrated structure of LAB cell membrane and wall against the freezing damage. It is clear that addition of Na-CMC should be promising as a new cryoprotective agent available for processing the lyophilized stater cultures of LAB strains.

Leuconostoc mesenteroides and Liquorilactobacillus mali strains, isolated from Algerian food products, are producers of the postbiotic compounds dextran, oligosaccharides and mannitol

Six lactic acid bacteria (LAB) isolated from Algerian sheep's milk, traditional butter, date palm sap and barley, which produce dextran, mannitol, oligosaccharides and vitamin B2 have been characterized. They were identified as Leuconostoc mesenteroides (A4X, Z36P, B12 and O9) and Liquorilactobacillus mali (BR201 and FR123). Their exopolysaccharides synthesized from sucrose by dextransucrase (Dsr) were characterized as dextrans with (1,6)-D-glucopyranose units in the main backbone and branched at positions O-4, O-2 and/or O-3, with D-glucopyranose units in the side chain. A4X was the best dextran producer (4.5 g/L), while the other strains synthesized 2.1-2.7 g/L. Zymograms revealed that L. mali strains have a single Dsr with a molecular weight (Mw) of ~ 145 kDa, while the Lc. mesenteroides possess one or two enzymes with 170-211 kDa Mw. As far as we know, this is the first detection of L. mali Dsr. Analysis of metabolic fluxes from sucrose revealed that the six LAB produced mannitol (~ 12 g/L). The co-addition of maltose-sucrose resulted in the production of panose (up to 37.53 mM), an oligosaccharide known for its prebiotic effect. A4X, Z36P and B12 showed dextranase hydrolytic enzymatic activity and were able to produce another trisaccharide, maltotriose, which is the first instance of a dextranase activity encoded by Lc. mesenteroides strains. Furthermore, B12 and O9 grew in the absence of riboflavin (vitamin B2) and synthesized this vitamin, in a defined medium at the level of ~ 220 μg/L. Therefore, these LAB, especially Lc. mesenteroides B12, are good candidates for the development of new fermented food biofortified with functional compounds.

Lactococcus lactis, a bacterium with probiotic functions and pathogenicity

Lactococcus lactis (L. lactis) is the primary organism for lactic acid bacteria (LAB) and is a globally recognized safe microorganism for the regulation of the intestinal micro-ecological balance of animals and improving the immune performance of the host. L. lactis is known to play a commercially important role in feed fortification, milk fermentation, and vaccine production, but pathogenic L. lactis has been isolated from many clinical cases in recent years, such as the brain of silver carp with Lactococcosis, the liver and spleen of diseased waterfowl, milk samples and padding materials with cow mastitis, and blood and urine from human patients with endocarditis. In dairy farming, where L. lactis has been used as a probiotic in the past, however, some studies have found that L. lactis can cause mastitis in cows, but the lack of understanding of the pathogenesis of mastitis in cows caused by L. lactis has become a new problem. The main objective of this review is to analyze the increasingly serious clinical mastitis caused by L. lactis and combined with the wide application of L. lactis as probiotics, to comprehensively discuss the characteristics and diversity of L. lactis.

Lactic acid bacteria in the functional food industry: biotechnological properties and potential applications

With the growing demand for functional foods having better nutraceutical properties, lactic acid bacteria (LAB) has become an important industrial microorganism. LAB play a significant role in the functional food industry by exhibiting probiotic properties and has the ability to produce various biologically active metabolites such as γ-aminobutyric acid (GABA), exopolysaccharides (EPSs), conjugated linoleic acid (CLA), bacteriocins, reuterin and reutericyclin, which provides enhanced nutraceutical properties to the final food products. LAB are also known to produce several specific enzymes essential for producing substrate-derived bioactive compounds, such as polyphenols, bioactive peptides, inulin-type fructans and β-glucans, fatty acids, and polyols. These compounds exhibit many health benefits, including better mineral absorption, oxidative stress protection, blood glucose and cholesterol-lowering properties, prevention of gastrointestinal tract infections and improved cardiovascular function. Further, metabolically engineered LAB have been widely used for the nutritive enhancement of different food products and the application of CRISPR-Cas9 holds tremendous potential for the engineering of food cultures. This review provides an overview of the use of LAB as probiotics, its application in producing fermented foods and nutraceutical products, and its health benefits on the host.

Understanding the Probiotic Bacterial Responses Against Various Stresses in Food Matrix and Gastrointestinal Tract: A Review

Probiotic bacteria are known to have ability to tolerate inhospitable conditions experienced during food preparation, food storage, and gastrointestinal tract of consumer. As probiotics are living cells, they are adversely affected by the harsh environment of the carrier matrix as well as low pH, bile salts, oxidative stress, osmotic pressure, and commensal microflora of the host. To overcome the unfavorable environments, many probiotics switch on the cell-mediated protection mechanisms, which helps them to survive, acclimatize and remain operational in the harsh circumstances. In this review, we provide comprehensive understanding on the different stresses experienced by the probiotic when added in carrier food as well as during human gastrointestinal tract transit. Under such situation how these health beneficial bacteria protect themselves by activation of several defense systems and get adapted to the lethal environments.

The lactate dehydrogenase gene is involved in the growth and metabolism of Lacticaseibacillus paracasei and the production of fermented milk flavor substances

Objective

Lactate dehydrogenase (ldh) in lactic acid bacteria is an important enzyme that is involved in the process of milk fermentation. This study aimed to explore the changes and effects of fermented milk metabolites in mutant strains after knocking out the ldh gene of Lacticaseibacillus paracasei.

Methods

The ldh mutant ΔAF91_07315 was obtained from L. paracasei using clustered regularly interspaced short palindromic repeats technology, and we determined fermented milk pH, titratable acidity, viable count, and differential metabolites in the different stages of milk fermentation that were identified using metabolomic analysis.

Results

The results showed that the growth rate and acidification ability of the mutant strain were lower than those of the wild-type strain before the end of fermentation, and analysis of the differential metabolites showed that lactate, L-cysteine, proline, and intermediate metabolites of phenylalanine, tryptophan, and methionine were downregulated (P < 0.05), which affected the growth initiation rate and acidification ability of the strain. At the end of fermentation (pH 4.5), the fermentation time of the mutant strain was prolonged and all differential metabolites were upregulated (P < 0.05), including amino acids and precursors, acetyl coenzyme A, and other metabolites involved in amino acid and fatty acid synthesis, which are associated with the regulation of fermented milk flavors. In addition, riboflavin was upregulated to promote the growth of the strain and compensate for the growth defects caused by the mutation.

Conclusion

Our data established a link between the AF91_07315 gene and strain growth and metabolism and provided a target for the regulation of fermented milk flavor substances.

Impact of Spontaneous Fermentation and Inoculum with Natural Whey Starter on Peptidomic Profile and Biological Activities of Cheese Whey: A Comparative Study

Fermentation is a promising solution to valorize cheese whey, the main by-product of the dairy industry. In Parmigiano Reggiano cheese production, natural whey starter (NWS), an undefined community of thermophilic lactic acid bacteria, is obtained from the previous day residual whey through incubation at gradually decreasing temperature after curd cooking. The aim of this study was to investigate the effect of fermentation regime (spontaneous (S) and NWS-inoculated (I-NWS)) on biofunctionalities and release of bioactive peptides during whey fermentation. In S and I-NWS trials proteolysis reached a peak after 24 h, which corresponded to the drop out in pH and the maximum increase in lactic acid. Biological activities increased as a function of fermentation time. NWS inoculum positively affected antioxidant activity, whilst S overcame I-NWS in angiotensin-converting enzyme (ACE) and DPP-IV (dipeptidyl peptidase IV) inhibitory activities. Peptidomics revealed more than 400 peptides, mainly derived from β-casein, κ-casein, and α-lactalbumin. Among them, 49 were bioactive and 21 were ACE-inhibitors. Semi-quantitative analysis strongly correlated ACE-inhibitory activity with the sum of the peptide abundance of ACE-inhibitory peptides. In both samples, lactotripeptide isoleucine-proline-proline (IPP) was higher than valine-proline-proline (VPP), with the highest content in S after 24 h of fermentation. In conclusion, we demonstrated the ability of whey endogenous microbiota and NWS to extensively hydrolyze whey proteins, promoting the release of bioactive peptides and improving protein digestibility.

Bioactive Compounds Extracted from Saudi Dates Using Green Methods and Utilization of These Extracts in Functional Yogurt

The bioactive compounds of four Saudi date flesh extracts (Ambara (AF), Majdool (MF), Sagai (SF), and Sukkari (SKF)) prepared using different extraction methods-namely, supercritical fluid extraction (SFE), subcritical CO₂ extraction (SCE), and Soxhlet extraction (SXE)-were evaluated. A total of 19 bioactive compounds were detected in extracts prepared using SFE and SCE methods, whereas less than 12 compounds were detected in extracts obtained using the SXE method. Both the date variety and extraction method affected the phenolic profile of date flesh extract (p ≤ 0.05). The apparent viscosity, surface color, and bioactive properties of yogurt were affected by both date flesh extracts and storage duration in varied magnitudes (p ≤ 0.05). The incorporation of date flesh extracts into yogurt formulations increased the total phenolic content (TPC), DPPH antiradical activity, viscosity, and redness (a*) and decreased the lightness (L*) and yellowness (b*) of the developed product (p ≤ 0.05). The elongation of storage time progressively (p ≤ 0.05) reduced the pH, TPC, DPPH antiradical activity, bacterial counts, and L* and b* values and increased the acidity, syneresis, viscosity, and a* values with few exceptions. Date flesh extracts can improve the health quality of yogurt without major influence on the sensory attributes while stored at 4 °C.

Adding glucose delays the conversion of ethanol and acetic acid to caproic acid in Lacrimispora celerecrescens JSJ-1

Caproic acid is an important fatty acid with diverse applications. In this study, the biomass growth and metabolites of Lacrimispora celerecrescens JSJ-1 were investigated under different carbon sources (ethanol, starch, sucrose, and glucose), with a focus on the effect of the coexistence of glucose and ethanol on the synthesis of caproic acid. The results showed that starch, glucose, and sucrose all contributed to the biomass of L. celerecrescens JSJ-1. Under the three carbon sources, L. celerecrescens JSJ-1 produced acetic acid, butyric acid, lactic acid, ethanol, and butanol, but caproic acid was not produced. Ethanol was the optimal substrate for the production of caproic acid. When glucose and ethanol coexisted, the generation time of caproic acid was delayed compared with that in ethanol sodium acetate medium (ES medium). This was because glucose was preferentially consumed over ethanol. Lactic acid was generated as a result of glucose consumption, which led to a significant decrease in pH from 6.45 to 4.68. The low pH (< 5) inhibited the synthesis of caproic acid. Then, the strain's usage of lactic acid and the reaction between CaCO₃ and lactic acid caused the pH to increase. L. celerecrescens JSJ-1 did not start producing caproic acid using ethanol and acetic acid until the pH increased to 5.8. This research enriches the knowledge regarding the metabolism of L. celerecrescens JSJ-1 and provides guidelines for the industrial production of caproic acid by using L. celerecrescences JSJ-1. KEY POINTS: • Ethanol is the optimal substrate for the synthesis of caproic acid by Lacrimispora celerecrescens JSJ-1. • Lacrimispora celerecrescens JSJ-1 produced lactic acid rapidly when it used glucose, causing a sharp drop in pH. • pH is a crucial factor affecting the synthesis of caproic acid from ethanol by Lacrimispora celerecrescens JSJ-1.

Lactose Content and Selected Quality Parameters of Sheep Milk Fermented Beverages during Storage

The aim of the research was to evaluate lactose content and rheological, physical, chemical, and organoleptic parameters during the storage of fermented beverages made from sheep's milk. The research was carried out on natural, probiotic, and Greek-type yogurts, as well as kefir. The products were made using the thermostat method from the milk of 42 East Frisian sheep in the middle lactation period, in duplicate. Lactose contents, active and titratable acidity, color by the L*a*b*C*h* system, and rheological parameters (hardness, consistency, consistency, and viscosity) were tested, and organoleptic assessments were carried out on the first, seventh, fourteenth, and twenty-first days of storing the drinks at 4 °C. Of all drinks, the highest reduction in lactose after 21 days of storage was found to occur in kefir (52% reduction) and, among the yogurts, in the Greek yogurt (41% reduction). The product with the lowest lactose content, regardless of the storage period, was kefir. This indicates that kefir is more suitable than yogurt for people with partial lactose intolerance. Effects of both inoculation type and beverage storage time were shown to exist for all parameters. It was also found that kefirs suffered deterioration in most rheological parameters and, in general organoleptic evaluation in the final period of storage. Based on our analysis, the optimal storage time for natural yogurts and sheep's milk kefirs at 4 °C was 21 and 14 days, respectively.

Antitumor mechanisms of an exopolysaccharide from Lactobacillus fermentum on HT-29 cells and HT-29 tumor-bearing mice

We have obtained an exopolysaccharide (YL-11 EPS) produced by Lactobacillus fermentum YL-11 isolated from fermented milk and confirmed that it can effectively inhibit colon cancer HT-29 cells proliferation in vitro. The aim of this study is to study anti-colon cancer effect in vivo and its possible mechanisms. Animal assays indicated YL-11 EPS treatment significantly suppressed the growth of HT-29 tumor xenograft without exhibiting obvious negative effects on normal cells. Cell experiments demonstrated YL-11 EPS treatment up regulated the ratio of Bax/Bcl-2 and induced the decrease in mitochondrial membrane potential and improved the expression of cleaved caspases-3 and cleaved PARP proteins, and finally induced HT-29 cells apoptosis, suggesting the involvement of mitochondrial pathway. Moreover, YL-11 EPS can block the PI3K/AKT signaling pathway and arrest the cell cycle in G1-phase to exert its anti-colon cancer activity. Overall, YL-11 EPS can be explored as a potential nutraceutical to prevent colorectal cancer.

Biodiversity and succession of lactic microbiota involved in Brazilian buffalo mozzarella cheese production

The biodiversity and succession of lactic acid bacteria (LAB) involved in the production and storage of Brazilian buffalo mozzarella cheese were evaluated. The isolates were characterized by Gram staining and catalase test, by the ability to grow at different conditions: temperatures, pH, concentrations of NaCl, and production of CO₂ from glucose. The biodiversity and succession of 152 LAB isolated during cheese production were evaluated by 16S rRNA gene sequencing, Random Amplified Polymorphic DNA (RAPD-PCR), and Restriction Fragment Length Polymorphism (RFLP-PCR) techniques. Most of the strains grow well at 30 °C and are tolerant to 6.5% of NaCl, and in general, the best pH for growing was 9.6. Leuconostoc mesenteroides, Lacticaseibacillus casei, Limosilactobacillus fermentum, and Enterococcus sp. were prevalent and present in almost all steps of production. The LAB strains are typically found in the traditional Italian cheese, except the Leuconostoc citreum species. Sixty clusters were obtained by RAPD-PCR with 85% of similarity (114 isolates) while most of the LAB was clustered with 100% of similarity by the RFLP-PCR technique. The applied techniques enabled a valuable elucidation of the LAB biodiversity and succession, contributing to a better understanding of the specific microbial cultures with a technological aptitude of this cheese.

High-efficiency genome editing based on endogenous CRISPR-Cas system enhances cell growth and lactic acid production in Pediococcus acidilactici

Pediococcus acidilactici is commonly used for pediocin production and lactic acid fermentation. However, high-efficiency genome editing tool is unavailable for this species. In this study, we constructed the endogenous subtype II-A CRISPR-Cas system-based genome interference plasmids which carried a "Repeat-Spacer-Repeat" cassette in the pMG36e shuttle vector. These plasmids exhibited self-interference activities in P. acidilactici LA412. Then, the genome-editing plasmids were constructed by cloning the upstream/downstream donor DNA into the corresponding interference plasmids to exert high-efficiency markerless gene deletion, gene integration, and point mutation in P. acidilactici LA412. We found that endogenous CRISPR-mediated depletion of the native plasmids enhanced the cell growth, and integration of a L-lactate dehydrogenase gene into the chromosome both enhanced cell growth and lactic acid production. IMPORTANCE A rapid and precise genome editing tool will promote the practical application of Pediococcus acidilactici, one type of lactic acid bacteria with excellent stress tolerance and probiotic characteristics. This study established a high-efficiency endogenous CRISPR-Cas system-based genome editing tool for P. acidilactici and achieved different genetic manipulations, including gene deletion, gene insertion, mononucleotide mutation, and endogenous plasmid depletion. The engineered strain edited by this tool showed significant advantages in cell growth and lactic acid fermentation. Therefore, our tool can satisfy the requirements for genetic manipulations of P. acidilactici, thus making it a sophisticated chassis species for synthetic biology and bioindustry.

Metabolomic profile of milk fermented with Streptococcus thermophilus cocultured with Bifidobacterium animalis ssp. lactis, Lactiplantibacillus plantarum, or both during storage

In this study, the microbial interactions among cocultures of Streptococcus thermophilus (St) with potential probiotics of Bifidobacterium animalis ssp. lactis (Ba) and Lactiplantibacillus plantarum (Lp) in fermented milk were investigated during a storage period of 21 d at 4°C, in terms of acidifying activity (pH and titratable acidity), viable counts, and metabolites. A nontargeted metabolomics approach based on ultra-high-performance liquid chromatography coupled with mass spectrometry was employed for mapping the global metabolite profiles of fermented milk. Probiotic strains cocultured with St accelerated milk acidification, and improved the microbial viability compared with the single culture of St. The St-Ba/Lp treatment manifested a higher bacteria viability and acidification ability in comparison with the St-Ba or the St-Lp treatment. Relative quantitation of 179 significant metabolites was identified, including nucleosides, AA, short peptides, organic acids, lipid derivatives, carbohydrates, carbonyl compounds, and compounds related to energy metabolism. The principal component analysis indicated that St treatment and coculture treatments displayed a complete distinction in metabolite profiles, and Lp had a larger effect than Ba on metabolic profiles of fermented milk produced by cofermentation with St during storage. The heat map in combination with hierarchical cluster analysis showed that the abundance of metabolites significantly varied with the starter cultures over the storage, and high abundance of metabolites was observed in either St or coculture samples. The St-Ba/Lp treatment showed relatively high abundance for the vast majority of metabolites. These findings suggest that the profile of the metabolites characterizing fermented milk samples may depend on the starter cultures, and incorporation of probiotics may considerably influence the metabolomic activities of fermented milks.

Improving Human Health with Milk Fat Globule Membrane, Lactic Acid Bacteria, and Bifidobacteria

The milk fat globule membrane (MFGM), the component that surrounds fat globules in milk, and its constituents have gained significant attention for their gut function, immune-boosting properties, and cognitive-development roles. The MFGM can directly interact with probiotic bacteria, such as bifidobacteria and lactic acid bacteria (LAB), through interactions with bacterial surface proteins. With these interactions in mind, increasing evidence supports a synergistic effect between MFGM and probiotics to benefit human health at all ages. This important synergy affects the survival and adhesion of probiotic bacteria through gastrointestinal transit, mucosal immunity, and neurocognitive behavior in developing infants. In this review, we highlight the current understanding of the co-supplementation of MFGM and probiotics with a specific emphasis on their interactions and colocalization in dairy foods, supporting in vivo and clinical evidence, and current and future potential applications.

Lactic acid bacteria-derived α-glucans: From enzymatic synthesis to miscellaneous applications

Lactic acid bacteria (LAB) are capable of producing a variety of exopolysaccharide α-glucans, such as dextran, mutan, reuteran, and alternan. Their structural diversity allows LAB-derived α-glucans to hold vast commercial value and application potential in the food, cosmetic, medical, and biotechnology fields, garnering much attention in recent years. Glycoside Hydrolase 70 family (GH70) enzymes are efficient tools for the biosynthesis of α-glucans with various sizes, linkage compositions, and degrees of branching, using renewable and low-cost sucrose and starch as substrates. To date, plenty of various LAB-derived GH70 glucansucrases (especially dextransucrase) have been biochemically characterized to synthesize α-glucans from sucrose with a variety of structural organizations. This review mainly aimed at the biotechnological synthesis of α-glucans using GH70 family enzymes and their diverse (potential) applications. The purification, structural analysis and physicochemical properties of α-glucan polysaccharides were reviewed in detail. Synchronously, some new insights and future perspectives of LAB-derived α-glucans enzymatic synthesis and applications were also discussed. To expand the range of applications, the physicochemical properties and bioactivities of LAB-derived α-glucans, other than dextran, should be further explored. Additionally, screening novel GH70 subfamily starch-acting enzymes is conducive to expanding the repertoire of α-glucans.

Advancing mechanistic understanding and bioengineering of probiotic lactobacilli and bifidobacteria by genome editing

Typical traditional probiotics lactobacilli and bifidobacteria are gaining great interest to be developed as living diagnostics and therapeutics for improving human health. However, the mechanistic basis underlying their inherent health beneficial property remain incompletely understood which can slow down the translational pipeline in the functional food and pharmaceutical field. Efficient genome editing will advance the understanding of the molecular mechanism of the probiotics' physiological properties and their interaction with the host and the host microbiota, thereby further promote the development of next-generation designer probiotics with improved robustness and tailored functionalities. With the expansion of genome editing strategies such as CRISPR-Cas-based tools and IPSD assisted genome engineering as well as other synthetic biology technologies, the research and application of these health-promoting bacteria for the food and pharmaceutical industry will be further enhanced.

Potentiality of Self-Cloned Lactobacillus plantarum Taj-Apis362 for Enhancing GABA Production in Yogurt under Glucose Induction: Optimization and Its Cardiovascular Effect on Spontaneous Hypertensive Rats

The current study evaluated the γ-aminobutyric acid (GABA) producing ability from three novel strains of lactic acid bacteria (L. plantarum Taj-Apis362, assigned as UPMC90, UPMC91, and UPMC1065) co-cultured with starter culture in a yogurt. A combination of UPMC90 + UPMC91 with starter culture symbiotically revealed the most prominent GABA-producing effect. Response surface methodology revealed the optimized fermentation conditions at 39.0 °C, 7.25 h, and 11.5 mM glutamate substrate concentration to produce GABA-rich yogurt (29.96 mg/100 g) with desirable pH (3.93) and water-holding capacity (63.06%). At 2% glucose to replace pyridoxal-5-phosphate (PLP), a cofactor typically needed during GABA production, GABA content was further enhanced to 59.00 mg/100 g. In vivo study using this sample revealed a blood pressure-lowering efficacy at 0.1 mg/kg GABA dosage (equivalent to 30 mg/kg GABA-rich yogurt) in spontaneously hypertensive rats. An improved method to produce GABA-rich yogurt has been established, involving shorter fermentation time and lower glutamate concentration than previous work, along with glucose induction that omits the use of costly PLP, fostering the potential of developing a GABA-rich functional dairy product through natural fermentation with desirable product quality and antihypertensive property.

CRISPR-Cas-mediated gene editing in lactic acid bacteria

The high efficiency, convenience and diversity of clustered regular interspaced short palindromic repeats (CRISPR)/CRISPR-associated (Cas) systems are driving a technological revolution in the gene editing of lactic acid bacteria (LAB). Cas-RNA cassettes have been adopted as tools to perform gene deletion, insertion and point mutation in several species of LAB. In this article, we describe the basic mechanisms of the CRISPR-Cas system, and the current gene editing methods available, focusing on the CRISPR-Cas models developed for LAB. We also compare the different types of CRISPR-Cas-based genomic manipulations classified according to the different Cas proteins and the type of recombineering, and discuss the rapidly evolving landscape of CRISPR-Cas application in LAB.

Complete genome sequence analysis of a strain Lactobacillus pentosus ZFM94 and its probiotic characteristics

Lactic acid bacteria have been attracting increased attentions recent years because of harboring probiotic properties. In present study, a Lactobacillus pentosus strain ZFM94 was screened from healthy infant feces and its probiotic characteristics were investigated. We found that ZFM94 was resistant to environmental stresses (temperature, pH and NaCl), tolerant to gastrointestinal juice and bile salts, with inhibitory action against pathogens and capacity of folate production etc. Additionally, complete genome sequence of the strain was analyzed to highlight the probiotic features at genetic level. Genomic characteristics along with the experimental studies is critically important for building an appropriate probiotic profile of novel strains. Genes that correspond to phenotypes mentioned above were identified. Moreover, genes potentially related to its adaptation, such as carbon metabolism and carbohydrate transporter, carbohydrate-active enzymes, and a novel gene cluster RaS-RiPPs, were also revealed. Together, ZFM94 could be considered as a potential probiotic candidate.

Safety Aspects of Genetically Modified Lactic Acid Bacteria

Lactic acid bacteria (LAB) have a long history of use in the food industry. Some species are part of the normal human microbiota and have beneficial properties for human health. Their long-standing use and considerable biotechnological potential have led to the development of various systems for their engineering. Together with novel approaches such as CRISPR-Cas, the established systems for engineering now allow significant improvements to LAB strains. Nevertheless, genetically modified LAB (GM-LAB) still encounter disapproval and are under extensive regulatory requirements. This review presents data on the prospects for LAB to obtain 'generally recognized as safe' (GRAS) status. Genetic modification of LAB is discussed, together with problems that can arise from their engineering, including their dissemination into the environment and the spread of antibiotic resistance markers. Possible solutions that would allow the use of GM-LAB are described, such as biocontainment, alternative selection markers, and use of homologous DNA. The use of GM-LAB as cell factories in closed systems that prevent their environmental release is the least problematic aspect, and this is also discussed.

Genetic and Biochemical Characterization of an Exopolysaccharide With in vitro Antitumoral Activity Produced by Lactobacillus fermentum YL-11

In the present study, the whole genome sequence of Lactobacillus fermentum YL-11, a novel exopolysaccharide (EPS)-producing lactic acid bacteria (LAB) strain isolated from fermented milk, was determined. Genetic information and the synthetic mechanism of the EPS in L. fermentum YL-11 were identified based on bioinformatic analysis of the complete genome. The purified EPS of YL-11 mainly comprised galactose (48.0%), glucose (30.3%), mannose (11.8%), and arabinose (6.0%). In vitro, the EPS from YL-11 exhibited inhibition activity against HT-29 and Caco-2 colon cancer cells, suggesting that EPS from strain YL-11 might be used as an antitumoral agent. EPS at 600 and 800 μg/mL achieved inhibition rates of 46.5 ± 3.5% and 45.6 ± 6.1% to HT-29 cells, respectively. The genomic information about L. fermentum YL-11 and the antitumoral activity of YL-11 EPS provide a theoretical foundation for the future application of EPS in the food and pharmaceutical industries.

CRISPR/Cas9-Assisted Seamless Genome Editing in Lactobacillus plantarum and Its Application in N-Acetylglucosamine Production

Lactobacillus plantarum is a potential starter and health-promoting probiotic bacterium. Effective, precise, and diverse genome editing of Lactobacillus plantarum without introducing exogenous genes or plasmids is of great importance. In this study, CRISPR/Cas9-assisted double-stranded DNA (dsDNA) and single-stranded DNA (ssDNA) recombineering was established in L. plantarum WCFS1 to seamlessly edit the genome, including gene knockouts, insertions, and point mutations. To optimize our editing method, phosphorothioate modification was used to improve the dsDNA insertion, and adenine-specific methyltransferase was used to improve the ssDNA recombination efficiency. These strategies were applied to engineer L. plantarum WCFS1 toward producing N-acetylglucosamine (GlcNAc). nagB was truncated to eliminate the reverse reaction of fructose-6-phosphate (F6P) to glucosamine 6-phosphate (GlcN-6P). Riboswitch replacement and point mutation in glmS1 were introduced to relieve feedback repression. The resulting strain produced 797.3 mg/liter GlcNAc without introducing exogenous genes or plasmids. This strategy may contribute to the available methods for precise and diverse genetic engineering in lactic acid bacteria and boost strain engineering for more applications.IMPORTANCE CRISPR/Cas9-assisted recombineering is restricted in lactic acid bacteria because of the lack of available antibiotics and vectors. In this study, a seamless genome editing method was carried out in Lactobacillus plantarum using CRISPR/Cas9-assisted double-stranded DNA (dsDNA) and single-stranded DNA (ssDNA) recombineering, and recombination efficiency was effectively improved by endogenous adenine-specific methyltransferase overexpression. L. plantarum WCFS1 produced 797.3 mg/liter N-acetylglucosamine (GlcNAc) through reinforcement of the GlcNAc pathway, without introducing exogenous genes or plasmids. This seamless editing strategy, combined with the potential exogenous GlcNAc-producing pathway, makes this strain an attractive candidate for industrial use in the future.

Genome editing of lactic acid bacteria: opportunities for food, feed, pharma and biotech

This mini-review provides a perspective of traditional, emerging and future applications of lactic acid bacteria (LAB) and how genome editing tools can be used to overcome current challenges in all these applications. It also describes available tools and how these can be further developed, and takes current legislation into account. Genome editing tools are necessary for the construction of strains for new applications and products, but can also play a crucial role in traditional ones, such as food and probiotics, as a research tool for gaining mechanistic insights and discovering new properties. Traditionally, recombinant DNA techniques for LAB have strongly focused on being food-grade, but they lack speed and the number of genetically tractable strains is still rather limited. Further tool development will enable rapid construction of multiple mutants or mutant libraries on a genomic level in a wide variety of LAB strains. We also propose an iterative Design–Build–Test–Learn workflow cycle for LAB cell factory development based on systems biology, with ‘cell factory’ expanding beyond its traditional meaning of production strains and making use of genome editing tools to advance LAB understanding, applications and strain development.

Alternative strategies for lignocellulose fermentation through lactic acid bacteria: the state of the art and perspectives

Lactic acid bacteria (LAB) have a long history in industrial processes as food starters and biocontrol agents, and also as producers of high-value compounds. Lactic acid, their main product, is among the most requested chemicals because of its multiple applications, including the synthesis of biodegradable plastic polymers. Moreover, LAB are attractive candidates for the production of ethanol, polyhydroalkanoates, sweeteners and exopolysaccharides. LAB generally have complex nutritional requirements. Furthermore, they cannot directly ferment inexpensive feedstocks such as lignocellulose. This significantly increases the cost of LAB fermentation and hinders its application in the production of high volumes of low-cost chemicals. Different strategies have been explored to extend LAB fermentation to lignocellulosic biomass. Fermentation of lignocellulose hydrolysates by LAB has been frequently reported and is the most mature technology. However, current economic constraints of this strategy have driven research for alternative approaches. Co-cultivation of LAB with native cellulolytic microorganisms may reduce the high cost of exogenous cellulase supplementation. Special attention is given in this review to the construction of recombinant cellulolytic LAB by metabolic engineering, which may generate strains able to directly ferment plant biomass. The state of the art of these strategies is illustrated along with perspectives of their applications to industrial second generation biorefinery processes.

Culture fermentation of Lactobacillus in traditional pickled gherkins: Microbial development, chemical, biogenic amine and metabolite analysis

Fermented cucumber pickles are the lactic acid fermentation products formed through the influence of microorganisms present in the environment. This study investigated the impacts of starter cultures, namely, Lactobacillus plantarum, Lactobacillus pentosus and Lactobacillus paraplantarum, typically utilized for the fermentation of traditional pickled gherkins, on fermentation process. The chemical (pH, total acidity and salt) and microbiological (total mesophilic aerobic bacteria, lactic acid bacteria and yeast-mould) changes were observed against the control sample during fermentation process. Moreover, the amounts of biogenic amines (BAs) and metabolites formed as a consequence of fermentation were determined using HPLC. It was found that the chemical analyses provided similar results for all the samples. The amount of total mesophilic aerobic bacteria and yeast-mould colonies in pickle sample containing L. plantarum 49 strain appeared to reduce significantly. The amount of BAs was the lowest for the pickle samples where L. plantarum strains were added. The amount of BAs was below the toxic value that could affect human health. More BAs were synthesized as the fermentation period increased. Lactate was seen to exist in the samples when pyruvate was present, and acetoin was converted into 2.3-butanediol during the fermentation period. It was concluded that the pickle sample for which L. plantarum 49 strain was used displayed a better fermentation profile (i.e., metabolite and biogenic amines) than the remaining samples. Producing a more delicious and reliable product using such characteristics of L. plantarum strains in pickled gherkins is believed to significantly contribute to the food industry.

Tailoring of microbes for the production of high value plant-derived compounds: From pathway engineering to fermentative production

Plant natural products have been an attracting platform for the isolation of various active drugs and other bioactives. However large-scale extraction of these compounds is affected by the difficulty in mass cultivation of these plants and absence of strategies for successful extraction. Even though, synthesis by chemical method is an alternative method; it is less efficient as their chemical structure is highly complex which involve enantio-selectivity. Thus an alternate bio-system for heterologous production of plant natural products using microbes has emerged. Advent of various omics, synthetic and metabolic engineering strategies revolutionised the field of heterologous plant metabolite production. In this context, various engineering methods taken to synthesise plant natural products are described with an additional focus to fermentation strategies.

Short communication: Biogenic amine formation during fermentation in functional sheep milk yogurts

The present study evaluated biogenic amine (BA) content during the fermentation period in functional sheep milk yogurts. Four treatments were prepared and assessed: natural (NSY), prebiotic (PreSY), probiotic (ProSY), and synbiotic (SynSY). Biogenic amines (putrescine, cadaverine, spermidine, spermine, and tyramine), proteolysis activity, and pH were measured during each hour of fermentation. Grumixama pulp was added to all formulations as a technological strategy and potential substrate for bacteria during fermentation. The yogurt and probiotic bacteria were viable (≥7 log cfu·mL^-1) on d 0. The pH levels of the functional sheep milk yogurts had a more pronounced decrease than did the control of NSY. However, all yogurt samples underwent gradual decreases in pH until final fermentation. Proteolytic activity remained constant in all treatments during fermentation. The NSY, PreSY, ProSY, and SynSY presented the same behavior for all BA, with differences in concentration. Putrescine, cadaverine, and spermidine contents decreased, whereas spermine remained constant and tyramine increased. We conclude that fermentation of functional sheep milk yogurts can produce tyramine.

Production of clovamide and its analogues in Saccharomyces cerevisiae and Lactococcus lactis

Clovamide and its analogues are N-hydroxycinnamoyl-L-amino acids (HAA) that exhibit antioxidant activities. For environmental and economic reasons, biological synthesis of these plant-derived metabolites has garnered interest. In this study, we exploited HDT1, a BAHD acyltransferase recently isolated from red clover, for the production of clovamide and derivatives in S. cerevisiae and L. lactis. HDT1 catalyses the transfer of hydroxycinnamoyl-coenzyme A (CoA) onto aromatic amino acids. Therefore, by heterologously co-expressing HDT1 with 4-coumarate:CoA ligase (4CL), we succeeded in the biological production of clovamide and more than 20 other HAA, including halogenated ones, upon feeding the engineered micro-organisms with various combinations of cinnamates and amino acids. To the best of our knowledge, this is the first report on the biological synthesis of HAA and, more generally, on the synthesis of plant-derived antioxidant phenolic compounds in L. lactis. The production of these health beneficial metabolites in Generally Recognized As Safe (GRAS) micro-organisms such as S. cerevisiae and L. lactis provides new options for their delivery as therapeutics. SIGNIFICANCE AND IMPACT OF THE STUDY: N-hydroxycinnamoyl-L-amino acids such as clovamide are bioactive plant-derived phenolic compounds with health beneficial effects. Relying on chemical synthesis or direct extraction from plant sources for the supply of these valuable molecules poses challenges to environmental sustainability. As an alternative route, this work demonstrates the potential for biological synthesis of N-hydroxycinnamoyl-L-amino acids using engineered microbial hosts such as Saccharomyces cerevisiae and Lactococcus lactis. Besides being more eco-friendly, this approach should also provide more structurally diverse compounds and offer new methods for their delivery to the human body.

A genome-scale metabolic network of the aroma bacterium Leuconostoc mesenteroides subsp. cremoris

Leuconostoc mesenteroides subsp. cremoris is an obligate heterolactic fermentative lactic acid bacterium that is mostly used in industrial dairy fermentations. The phosphoketolase pathway (PKP) is a unique feature of the obligate heterolactic fermentation, which leads to the production of lactate, ethanol, and/or acetate, and the final product profile of PKP highly depends on the energetics and redox state of the organism. Another characteristic of the L. mesenteroides subsp. cremoris is the production of aroma compounds in dairy fermentation, such as in cheese production, through the utilization of citrate. Considering its importance in dairy fermentation, a detailed metabolic characterization of the organism is necessary for its more efficient use in the industry. To this aim, a genome-scale metabolic model of dairy-origin L. mesenteroides subsp. cremoris ATCC 19254 (iLM.c559) was reconstructed to explain the energetics and redox state mechanisms of the organism in full detail. The model includes 559 genes governing 1088 reactions between 1129 metabolites, and the reactions cover citrate utilization and citrate-related flavor metabolism. The model was validated by simulating co-metabolism of glucose and citrate and comparing the in silico results to our experimental results. Model simulations further showed that, in co-metabolism of citrate and glucose, no flavor compounds were produced when citrate could stimulate the formation of biomass. Significant amounts of flavor metabolites (e.g., diacetyl and acetoin) were only produced when citrate could not enhance growth, which suggests that flavor formation only occurs under carbon and ATP excess. The effects of aerobic conditions and different carbon sources on product profiles and growth were also investigated using the reconstructed model. The analyses provided further insights for the growth stimulation and flavor formation mechanisms of the organism.

Exploring the contributions of two glutamate decarboxylase isozymes in Lactobacillus brevis to acid resistance and γ-aminobutyric acid production

Background

The glutamate decarboxylase (GAD) system of Lactobacillus brevis involves two isoforms of GAD, GadA and GadB, which catalyze the conversion of L-glutamate to γ-aminobutyric acid (GABA) in a proton-consuming reaction contributing to intracellular pH homeostasis. However, direct experimental evidence for detailed contributions of gad genes to acid tolerance and GABA production is lacking.

Results

Molecular analysis revealed that gadB is cotranscribed in tandem with upstream gadC, and that expression of gadCB is greatly upregulated in response to low ambient pH when cells enter the late exponential growth phase. In contrast, gadA is located away from the other gad genes, and its expression was consistently lower and not induced by mild acid treatment. Analysis of deletion mutations in the gad genes of L. brevis demonstrated a decrease in the level of GAD activity and a concomitant decrease in acid resistance in the order of wild-type> ΔgadA> ΔgadB> ΔgadC> ΔgadAB, indicating that the GAD activity mainly endowed by GadB rather than GadA is an indispensable step in the GadCB mediated acid resistance of this organism. Moreover, engineered strains with higher GAD activities were constructed by overexpressing key GAD system genes. With the proposed two-stage pH and temperature control fed-batch fermentation strategy, GABA production by the engineered strain L. brevis 9530: pNZ8148-gadBC continuously increased reaching a high level of 104.38 ± 3.47 g/L at 72 h.

Conclusions

This is the first report of the detailed contribution of gad genes to acid tolerance and GABA production in L. brevis. Enhanced production of GABA by engineered L. brevis was achieved, and the resulting GABA level was one of the highest among lactic acid bacterial species grown in batch or fed-batch culture.

Electronic supplementary material

The online version of this article (10.1186/s12934-018-1029-1) contains supplementary material, which is available to authorized users.

In vitro Assessment of the Probiotic Properties and Bacteriocinogenic Potential of Pediococcus pentosaceus MZF16 Isolated From Artisanal Tunisian Meat "Dried Ossban"

Pediococcus pentosaceus MZF16 has been isolated from artisanal Tunisian meat so called "Dried Ossban," an original ecological niche, and identified by MALDI-TOF mass spectrometry and 16S rDNA sequencing. This bacterium showed a high tolerance to gastric stress conditions, and toward bile salts. P. pentosaceus MZF16 also demonstrated a hydrophobic surface profile (high adhesion to xylene), autoaggregation, and adhesive abilities to the human intestinal Caco-2/TC7 cell line. These properties may help the bacterium colonizing the gut. Furthermore, MZF16 was found to be resistant to gentamycin and chloramphenicol but did not harbor any transferable resistance determinants and/or virulence genes. The data also demonstrated absence of cytotoxicity of this strain. Conversely, P. pentosaceus MZF16 can slightly stimulate the immune system and enhance the intestinal epithelial barrier function. Moreover, this bacterium has been shown to be highly active against Listeria spp. due to bacteriocin production. Characterization of the bacteriocin by PCR amplification, sequencing and bioinformatic analyses revealed that MZF16 produces a bacteriocin 100% identical to coagulin, a pediocin-like inhibitory substance produced by Bacillus coagulans. To our knowledge, this is the first report that highlights the production of a pediocin 100% identical to coagulin in a Pediococcus strain. As coagulin, pediocin MZF16 has the consensus sequence YYGNGVXCXXXXCXVXXXXA (X denotes any amino acid), which confirms its belonging to class IIa bacteriocins, and its suitability to preserve foods from Listeria monocytogenes development. According to these results, P. pentosaceus MZF16 can be proposed as a probiotic and bioprotective agent for fermented foods, including Tunisian dry meat and sausages. Further investigations will aim to study the behavior of this strain in meat products as a component of functional food.

Dynamic and Functional Characteristics of Predominant Species in Industrial Paocai as Revealed by Combined DGGE and Metagenomic Sequencing

The microbial community during the fermentation of industrial paocai, a lactic acid fermented vegetable food, was investigated via combined denaturing gradient gel electrophoresis (DGGE) and metagenomic sequencing. Firmicutes and Proteobacteria were identified as the dominant phyla during the fermentation. DGGE results of the bacterial community analysis showed that many genera were observed during the fermentation of industrial paocai, but the same predominant genus and species were observed: Lactobacillus and Lactobacillus (L.) alimentarius/L. paralimentarius. The abundance of L. alimentarius/L. paralimentarius increased fast during the initial stage of fermentation and approximately remained constant during the later stage. Metagenomic sequencing was used to finally identify the predominant species and their genetic functions. Metabolism was the primary functions of the microbial community in industrial paocai fermentation, including carbohydrate metabolism (CM), overview (OV), amino acid metabolism (AAM), nucleotide metabolism (NM), energy metabolism (EM), etc. The predominant species L. alimentarius and L. paralimentarius were involved in plenty of pathways in metabolism and played different roles in the metabolism of carbohydrate, amino acid, lipid to form flavor compounds during industrial paocai fermentation. This study provided valuable information about the predominant species in industrial paocai and its functional properties, which could enable us to advance our understanding of the fermentation mechanism during fermentation of industrial paocai. Our results will advance the understanding of the microbial roles in the industrial paocai fermentation and provide a theoretical basis for improving the quality of industrial paocai products.

Oral immunization with recombinant Lactococcus lactis NZ9000 expressing human papillomavirus type 16 E7 antigen and evaluation of its immune effects in female C57BL/6 mice

The ORFs of both native and codon-optimized E7 genes were successfully fused to SPusp45 signal peptide and expressed by a nisin-controlled gene expression system in the NZ9000 strains of Lactococcus lactis. Recombinant strains were confirmed by Western blot analysis. To measure immune responses against the E7 antigen, specific-pathogen-free C57BL/6 mice were inoculated with L lactis harboring pNZ8123-rE7 by oral gavage. Then, specific antibodies and cytokines were measured by enzyme-linked immunosorbent assay and enzyme-linked immunospot assay, respectively. Oral administration of L lactis strains expressing rE7 elicited the highest levels of E7-specific antibody and greatest numbers of E7-specific CD4⁺ T helper and CD8⁺ T cell precursors. Our outcomes indicated that the HPV-16 E7 specific IL-2- and IFN-γ-secreting T cells in antigen-stimulated splenocytes and intestinal mucosal lymphocytes were significantly higher than the control groups. Our data also demonstrated that mice vaccinated with recombinant L lactis were able to generate potent protective effects against challenge with the E7-expressing tumor cell line (TC-1). Moreover, L lactis containing pNZ8123-HPV16-optiE7 showed strong therapeutic antitumor effects against established tumors in vivo. These findings demonstrate that recombinant L lactis induce both humoral and cellular immune responses in mice and are therefore recommended for therapeutic treatments in humans after oral administration.

Lactobacillus brevis CGMCC 1306 glutamate decarboxylase: Crystal structure and functional analysis

Glutamate decarboxylase (GAD), which is a unique pyridoxal 5-phosphate (PLP)-dependent enzyme, can catalyze α-decarboxylation of l-glutamate (L-Glu) to γ-aminobutyrate (GABA). The crystal structure of GAD in complex with PLP from Lactobacillus brevis CGMCC 1306 was successfully solved by molecular-replacement, and refined at 2.2 Å resolution to an R_work factor of 18.76% (R_free = 23.08%). The coenzyme pyridoxal 5-phosphate (PLP) forms a Schiff base with the active-site residue Lys279 by continuous electron density map, which is critical for catalysis by PLP-dependent decarboxylase. Gel filtration showed that the active (pH 4.8) and inactive (pH 7.0) forms of GAD are all dimer. The residues (Ser126, Ser127, Cys168, Ile211, Ser276, His278 and Ser321) play important roles in anchoring PLP cofactor inside the active site and supporting its catalytic reactivity. The mutant T215A around the putative substrate pocket displayed an 1.6-fold improvement in catalytic efficiency (k_cat/K_m) compared to the wild-type enzyme (1.227 mM^-1 S^-1 versus 0.777 mM^-1 S^-1), which was the highest activity among all variants tested. The flexible loop (Tyr308-Glu312), which is positioned near the substrate-binding site, is involved in the catalytic reaction, and the conserved residue Tyr308 plays a vital role in decarboxylation of L-Glu.