Aerobic and respirative growth of heterofermentative lactic acid bacteria: A screening study

Synthesis and Isolation of Phenol- and Thiol-Derived Epicatechin Adducts Prepared from Avocado Peel Procyanidins Using Centrifugal Partition Chromatography and the Evaluation of Their Antimicrobial and Antioxidant Activity

Polyphenols from agro-food waste represent a valuable source of bioactive molecules that can be recovered to be used for their functional properties. Another option is to use them as starting material to generate molecules with new and better properties through semi-synthesis. A proanthocyanidin-rich (PACs) extract from avocado peels was used to prepare several semi-synthetic derivatives of epicatechin by acid cleavage in the presence of phenol and thiol nucleophiles. The adducts formed by this reaction were successfully purified using one-step centrifugal partition chromatography (CPC) and identified by chromatographic and spectroscopic methods. The nine derivatives showed a concentration-dependent free radical scavenging activity in the DPPH assay. All compounds were also tested against a panel of pathogenic bacterial strains formed by Listeria monocytogenes (ATCC 7644 and 19115), Staphylococcus aureus (ATCC 9144), Escherichia coli (ATCC 11775 and 25922), and Salmonella enterica (ATCC 13076). In addition, adducts were tested against two no-pathogenic strains, Limosilactobacillus fermentum UCO-979C and Lacticaseibacillus rhamnosus UCO-25A. Overall, thiol-derived adducts displayed antimicrobial properties and, in some specific cases, inhibited biofilm formation, particularly in Listeria monocytogenes (ATCC 7644). Interestingly, phenolic adducts were inactive against all the strains and could not inhibit its biofilm formation. Moreover, depending on the structure, in specific cases, biofilm formation was strongly promoted. These findings contribute to demonstrating that CPC is a powerful tool to isolate new semi-synthetic molecules using avocado peels as starting material for PACc extraction. These compounds represent new lead molecules with antioxidant and antimicrobial activity.

Recent Advances in the Understanding of Stress Resistance Mechanisms in Probiotics: Relevance for the Design of Functional Food Systems

In recent years, more and more scientific community, food producers, and food industry show increased interest in functional foods containing probiotics, which is a big challenge. The consumption of probiotics in the context of a balanced diet through the consumption of functional foods or through the intake of pharmaceutical preparations has proven to contribute to the improvement of human health, even contributing to the prevention of diseases. In order for probiotics to be considered suitable for consumption, they must contain a minimum concentration of viable cells, namely, at least 107 colony forming units of beneficial microbes per gram. Ensuring the viability of bacterial cells until the moment of consumption is the overriding priority of functional probiotic food manufacturers. Probiotic bacteria are subject to stress conditions not only during food manufacturing but also during gastrointestinal passage, which limit or even compromise their functionality. This paper first examines all the stressful conditions faced by probiotic cells in their production stages and related to the conditions present in the bioreactor fermentation and drying processes as well as factors related to the food matrix and storage. The stress situations faced by probiotic microorganisms during the gastrointestinal transit especially during stomach and intestinal residence are also analyzed. In order to understand the adaptation mechanisms of probiotic bacteria to gastrointestinal stress, intrinsic and adaptive mechanisms identified in probiotic strains in response to acid stress and to bile and bile acid stress are analyzed. In addition, improvement strategies for multiple stress tolerance of lactic acid bacteria through directions dealing with stress, accumulation of metabolites, use of protectants, and regulation of technological parameters are examined. Finally, the definition of postbiotics, inanimate microorganisms and/or their components conferring health benefits, is also introduced. Postbiotics include cell lysates, enzymes, and cell wall fragments derived from probiotic bacteria and may represent an alternative to the use of probiotics, when they do not tolerate stressful conditions.

Roles of flavoprotein oxidase and the exogenous heme- and quinone-dependent respiratory chain in lactic acid bacteria

Lactic acid bacteria (LAB) are a type of bacteria that convert carbohydrates into lactate through fermentation metabolism. While LAB mainly acquire energy through this anaerobic process, they also have oxygen-consuming systems, one of which is flavoprotein oxidase and the other is exogenous heme- or heme- and quinone-dependent respiratory metabolism. Over the past two decades, research has contributed to the understanding of the roles of these oxidase machineries, confirming their suspected roles and uncovering novel functions. This review presents the roles of these oxidase machineries, which are anticipated to be critical for the future applications of LAB in industry and comprehending the virulence of pathogenic streptococci.

Development of high cell density Limosilactobacillus reuteri KUB-AC5 for cell factory using oxidative stress reduction approach

BACKGROUND

Expression systems for lactic acid bacteria have been developed for metabolic engineering applications as well as for food-grade recombinant protein production. But the industrial applications of lactic acid bacteria as cell factories have been limited due to low biomass formation resulted in low efficiency of biomanufacturing process. Limosilactobacillus reuteri KUB-AC5 is a safe probiotic lactic acid bacterium that has been proven as a gut health enhancer, which could be developed as a mucosal delivery vehicle for vaccines or therapeutic proteins, or as expression host for cell factory applications. Similar to many lactic acid bacteria, its oxygen sensitivity is a key factor that limits cell growth and causes low biomass production. The aim of this study is to overcome the oxidative stress in L. reuteri KUB-AC5. Several genes involved in oxidative and anti-oxidative stress were investigated, and strain improvement for higher cell densities despite oxidative stress was performed using genetic engineering.

RESULTS

An in-silico study showed that L. reuteri KUB-AC5 genome possesses an incomplete respiratory chain lacking four menaquinone biosynthesis genes as well as a complete biosynthesis pathway for the production of the precursor. The presence of an oxygen consuming enzyme, NADH oxidase (Nox), leads to high ROS formation in aerobic cultivation, resulting in strong growth reduction to approximately 25% compared to anaerobic cultivation. Recombinant strains expressing the ROS scavenging enzymes Mn-catalase and Mn-superoxide dismutase were successfully constructed using the pSIP expression system. The Mn-catalase and Mn-SOD-expressing strains produced activities of 873 U/ml and 1213 U/ml and could minimize the ROS formation in the cell, resulting in fourfold and sevenfold higher biomass formation, respectively.

CONCLUSIONS

Expression of Mn-catalase and Mn-SOD in L. reuteri KUB-AC5 successfully reduced oxidative stress and enhanced growth. This finding could be applied for other lactic acid bacteria that are subject to oxidative stress and will be beneficial for applications of lactic acid bacteria for cell factory applications.

Biological activities and applications of exopolysaccharides produced by lactic acid bacteria: a mini-review

Exopolysaccharides (EPSs) are naturally occurring high-molecular-weight carbohydrates that have been widely studied for their biological activities, including antioxidant, immunomodulatory, anticancer and gut microbiota regulation activities. Polysaccharides are abundant in nature and can be derived from animals, plants, algae, and microorganisms, but among polysaccharides with potential uses, EPSs from microorganisms have the advantages of a short production cycle, high yield, and independence of production from season and climate and thus have broad prospects. While the safety of the producing microorganism can represent a problem in application of microbial EPSs, lactic acid bacteria (LAB) have been used by humans for thousands of years, and they and their products are generally recognized as safe. This makes LAB excellent sources for exopolysaccharides. EPS-producing LAB are readily found in nature. Through screening of strains, optimization of culture conditions, and improvement of the growth medium, the yield of EPSs from LAB can be increased and the scope of application broadened. This review summarizes EPSs from LAB in terms of structure, function and applications, as well as yield optimization, and introduces recent research on the biological activities and practical applications of LAB EPSs, aiming to provide references for researchers in related areas.

High Production of γ-Aminobutyric Acid by Activating the xyl Operon of Lactobacillus brevis

γ-Aminobutyric acid (GABA) is an inhibitory neurotransmitter with important physiological functions such as sleep assistance and anti-depression. In this study, we developed a fermentation process for the high-efficiency production of GABA by Lactobacillus brevis (Lb. brevis) CE701. First, xylose was found as the optimal carbon source that could improve the GABA production and OD₆₀₀ in shake flasks to 40.35 g/L and 8.64, respectively, which were 1.78-fold and 1.67-fold of the glucose. Subsequently, the analysis of the carbon source metabolic pathway indicated that xylose activated the expression of the xyl operon, and xylose metabolism produced more ATP and organic acids than glucose, which significantly promoted the growth and GABA production of Lb. brevis CE701. Then, an efficient GABA fermentation process was developed by optimizing the medium components using response surface methodology. Finally, the production of GABA reached 176.04 g/L in a 5 L fermenter, which was 336% higher than that in a shake flask. This work enables the efficient synthesis of GABA using xylose, which will provide guidance for the industrial production of GABA.

Journey of the Probiotic Bacteria: Survival of the Fittest

This review aims to bring a more general view of the technological and biological challenges regarding production and use of probiotic bacteria in promoting human health. After a brief description of the current concepts, the challenges for the production at an industrial level are presented from the physiology of the central metabolism to the ability to face the main forms of stress in the industrial process. Once produced, these cells are processed to be commercialized in suspension or dried forms or added to food matrices. At this stage, the maintenance of cell viability and vitality is of paramount for the quality of the product. Powder products requires the development of strategies that ensure the integrity of components and cellular functions that allow complete recovery of cells at the time of consumption. Finally, once consumed, probiotic cells must face a very powerful set of physicochemical mechanisms within the body, which include enzymes, antibacterial molecules and sudden changes in pH. Understanding the action of these agents and the induction of cellular tolerance mechanisms is fundamental for the selection of increasingly efficient strains in order to survive from production to colonization of the intestinal tract and to promote the desired health benefits.

The Production of γ-Aminobutyric Acid from Free and Immobilized Cells of Levilactobacillus brevis Cultivated in Anaerobic and Aerobic Conditions

γ-aminobutyric acid (GABA) has several beneficial effects on human health. GABA may be produced via chemical synthesis or through microbial metabolism, and Levilactobacillus brevis is recognized as a GABA-producing species. In this study, 11 Lvb. brevis strains were screened for GABA production, and the best producers were selected to verify the effect of aerobic (AE) and respiratory (RS) cultivations on growth parameters, biomass, and GABA accumulation. Lvb. brevis LB12 was then used to evaluate the combined effect of the incubation atmosphere (anaerobiosis vs. aerobiosis), cell protection (free vs. immobilized cells), and cell recycling (fresh vs. starved cells) on GABA production. Glutamate (GLU) consumption and GABA accumulation were detected by Thin-layer Chromatography (TLC) and RP-HPLC analyses. The ability to produce GABA was widespread among the strains. AE and RS growth improved biomass production, but oxygen availability impaired GLU to GABA conversion, and the anaerobically growing cells had the highest GABA productivity. Immobilized strains had lower efficiency in both GLU uptake and conversion compared to free cells, probably due to the poor diffusion in alginate beads. The use of resting cells allowed further GABA production without the cultivation step, but cell activity was exhausted after three cycles of reutilization. Lvb. brevis LB12 is an excellent GABA producer, and AE cultivation can be exploited to improve the final cell density; however, the conditions for boosting GLU to GABA conversion and cell regeneration need to be further investigated.

Redefining methods for augmenting lactic acid bacteria robustness and phenyllactic acid biocatalysis: Integration valorizes simplicity

The production of phenyllactic acid (PLA) has been reported by several researchers, but so far, no mention has been made of augmented PLA production using an orchestrated assembly of simple techniques integrated to improve lactic acid bacteria (LAB) metabolism for the same. This review summarizes sequentially tailoring LAB growth and metabolism for augmented PLA catalysis through several strategies like monitoring LAB sustenance by choosing appropriate starter PLA-producing LAB strains isolated from natural environments, with desirably fastidious growth rates, properties like acidification, proteolysis, bacteriophage-resistance, aromatic/texturing-features, etc.; entrapping chosen LAB strains in novel cryogels and/or co-cultivating two/more LAB strains to improve their biotransformation potential and promote growth dependency/sustainability; adopting adaptive evolution methods designed to improve LAB strains under selection pressure inducing desired phenotypes tolerant to stress factors like heat, salt, acid, and solvent; monitoring physico-chemical LAB fermentation factors like temperature, pH, dissolved oxygen content, enzymes, and cofactors for PLA biosynthesis; and modulating purification/downstream processes to extract substantial PLA yields. This review paper serves as a comprehensive preliminary guide that can evoke a strategic experimental plan to produce industrial-scale PLA yields using simple techniques orchestrated together in the pursuit of conserving time, effort, and resources.

Traditional Fermented Foods from Ecuador: A Review with a Focus on Microbial Diversity

The development of early civilizations was greatly associated with populations’ ability to exploit natural resources. The development of methods for food preservation was one of the pillars for the economy of early societies. In Ecuador, food fermentation significantly contributed to social advances and fermented foods were considered exclusive to the elite or for religious ceremonies. With the advancement of the scientific research on bioprocesses, together with the implementation of novel sequencing tools for the accurate identification of microorganisms, potential health benefits and the formation of flavor and aroma compounds in fermented foods are progressively being described. This review focuses on describing traditional fermented foods from Ecuador, including cacao and coffee as well as less popular fermented foods. It is important to provide new knowledge associated with nutritional and health benefits of the traditional fermented foods.

Probiotic Effects and Metabolic Products of Enterococcus faecalis LD33 with Respiration Capacity

Respiration metabolism could improve the long-term survival of lactic acid bacteria (LAB); however, its effect on potential probiotic traits of LAB was not reported. The difference made by Enterococcus faecalis LD33 that was cultured under respiration-permissive and fermentation conditions, such as the biomass, metabolites, antimicrobial activity, tolerance to acid and bile salt, adhesion capabilities, and the ability to inhibit the proliferation of cancer cells were studied. Under a respiration-permissive condition, the final biomass of the culture was about twice as compared to that of fermentation condition. When the metabolites were measured, glucose was exhausted within 8 h. Two-folds of acetic acid, triple of both acetoin and diacetyl, and less than half of lactic acid, were accumulated under the respiratory-permissive condition. No discrimination of growth inhibition on Salmonella enterica serovar Typhimurium ATCC 14028 and Shigella sonnei ATCC 25931 was observed when Enterococcus faecalis LD33 was cultured under both conditions; however, under respiration-permissive condition, the strain presented significant antimicrobial activities to Listeria monocytogenes ATCC19111 and Staphylococcus aureus ATCC6538P. Enterococcus faecalis LD33 displayed relatively strong bile salt tolerance and adherence capability but weaker acid tolerance when undergoing respiration metabolism. There was no significant difference in the anti-cancer effect of the viable bacterial cells on both growth modes; however, the supernatant showed a higher inhibition effect on HT-29 cells than the live bacteria, and there was no significant difference between the supernatant and the 5-Fluorouracil (7 μg/mL). Consequently, the Enterococcus faecalis LD33 undergoing respiration metabolism could bring higher biomass, more flavor metabolites, and better antimicrobial and anti-cancer activities. This study extends our knowledge of respiratory metabolism in LAB and its impact on probiotic traits. E. faecalis LD33 qualifies as a suitable strain against foodborne pathogens, cancer therapy, and eventual application in the food and pharmaceutical industries.

The Effect of Respiration, pH, and Citrate Co-Metabolism on the Growth, Metabolite Production and Enzymatic Activities of Leuconostoc mesenteroides subsp. cremoris E30

Leuconostoc mesenteroides includes strains used as starter and/or adjunct cultures for the production of several fermented foods. In this study, the effect of anaerobic and respiratory cultivations, as well as of citrate supplementation and different pH values, was evaluated on growth, biomass, metabolite, and enzymatic activities (pyruvate oxidase, POX; NADH-dependent oxidase, NOX; NADH-dependent peroxidase, NPR) of Leuconostoc mesenteroides subsp. cremoris E30. We compared the respiration-increased growth rate and biomass production of Leuc. mesenteroides E30 to anaerobic cultivation. A supplementation of citrate impaired the growth rate of the respiratory cells. As expected, anaerobic cultures did not consume oxygen, and a similar trend in oxygen uptake was observed in respiratory cultures. The aerobic incubation caused changes in the metabolic pattern, reducing the production of ethanol in favour of acetic acid. Citrate was already exhausted in the exponential phase and did not affect the yields in acetic acid and ethanol. NOX activity increased in the presence of oxygen, while catalase was also detected in the absence of hemin. The absence of H₂O₂ suggested its degradation by NPR and catalase. Respiratory cultivation provided benefits (increase in growth rate, biomass, and activity in antioxidant enzymes) for Leuc. mesenteroides E30. Therefore, the exploitation of respiratory phenotypes may be useful for the formulation of competitive starter or adjunct cultures.

Analysis and Reconstitution of the Menaquinone Biosynthesis Pathway in Lactiplantibacillus plantarum and Lentilactibacillus buchneri

In Lactococcus lactis and some other lactic acid bacteria, respiratory metabolism has been reported upon supplementation with only heme, leading to enhanced biomass formation, reduced acidification, resistance to oxygen, and improved long-term storage. Genes encoding a complete respiratory chain with all components were found in genomes of L. lactis and Leuconostoc mesenteroides, but menaquinone biosynthesis was found to be incomplete in Lactobacillaceae (except L. mesenteroides). Lactiplantibacillus plantarum has only two genes (menA, menG) encoding enzymes in the biosynthetic pathway (out of eight), and Lentilactobacillus buchneri has only four (menA, menB, menE, and menG). We constructed knock-out strains of L. lactis defective in menA, menB, menE, and menG (encoding the last steps in the pathway) and complemented these by expression of the extant genes from Lactipl. plantarum and Lent. buchneri to verify their functionality. Three of the Lactipl. plantarum biosynthesis genes, lpmenA1, lpmenG1, and lpmenG2, as well as lbmenB and lbmenG from Lent. buchneri, reconstituted menaquinone production and respiratory growth in the deficient L. lactis strains when supplemented with heme. We then reconstituted the incomplete menaquinone biosynthesis pathway in Lactipl. plantarum by expressing six genes from L. lactis homologous to the missing genes in a synthetic operon with two inducible promoters. Higher biomass formation was observed in Lactipl. plantarum carrying this operon, with an OD₆₀₀ increase from 3.0 to 5.0 upon induction.

Effects of carbon concentration, oxygen, and controlled pH on the engineering strain Lactiplantibacillus casei E1 in the production of bioethanol from sugarcane molasses

Sugarcane molasses are considered a potential source for bioethanol's commercial production because of its availability and low market price. It contains high concentrations of fermentable sugars that can be directly metabolized by microbial fermentation. Heterofermentative lactic acid bacteria, especially Lactiplantibacillus casei, have a high potential to be a biocatalyst in ethanol production that they are characterized by strong abilities of carbohydrate metabolism, ethanol synthesis, and high alcohol tolerance. This study aimed to evaluate the feasibility of producing ethanol by Lactiplantibacillus casei used the ethanologen engineering strain L. casei E1 as a starter culture and cane molasses as substrate medium. The effects of environmental factors on the metabolism of L. casei E1 were analyzed by high-performance liquid chromatography (HPLC) system, and the gene expression of key enzymes in carbon source metabolism was detected using quantitative real-time PCR (RT-qPCR). Results showed that the strain could grow well, ferment sugar quickly in cane molasses. By fermenting this bacterium anaerobically at 37 °C for 36 h incubation in 5 °BX molasses when the fermenter's pH was controlled at 6.0, ethanol yield reached 13.77 g/L, and carbohydrate utilization percentage was 78.60%. RT-qPCR results verified the strain preferentially ferment glucose and fructose of molasses to ethanol at the molecular level. In addition, the metabolism of sugars, especially fructose, would be inhibited by elevating acidity. Our findings support the theoretical basis for exploring Lactic acid bacteria as a starter culture for converting sugarcane molasses into ethanol.

Co-inoculation of yeasts starters: A strategy to improve quality of low altitude Arabica coffee

The study aimed to improve the quality of dry-processed coffee grown at low altitudes through yeast inoculation, using three species (Saccharomyces cerevisiae CCMA 0543, Torulaspora delbrueckii CCMA 0684, and Candida parapsilosis CCMA 0544) singly and with co-inoculation for fermentation. Important chemical compounds and groups were analyzed by liquid and gas chromatography and Fourier-transform infrared spectroscopy (FTIR). The inoculated coffees with yeast populations around 10⁶ cell/g obtained the highest scores, and the co-inoculation with C. parapsilosis CCMA 0544 and T. delbrueckii CCMA 0684 had the highest score in the sensory analysis (85). Different descriptors were observed in each treatment, and body, flavor, balance, and aftertaste are strongly related to C. parapsilosis CCMA 0544. The fermentation process improved the quality of low-altitude coffees, and the combination of non-Saccharomyces yeasts (C. parapsilosis CCMA 0544 and T. delbrueckii CCMA 0684) was the most indicated as starter cultures.

Role of milk and honey in the tolerance of lactobacilli to oxidative stress

In the development of functional probiotic food, the carrier matrices should be carefully selected and optimized to ensure the highest levels of probiotic survival in the symbiotic food along storage. Because milk and honey food matrices are rich in antioxidant substances, the aim of the research was to evaluate their effect in protecting lactobacilli from reactive oxygen species (ROS) generated by the addition of hydrogen peroxide. Viability assays were performed with and without the addition of H₂O₂, in three different matrices: 0.9% peptone saline, 5% honey, or 12% reconstituted skim milk. The milk matrix provided protection for the Lacticaseibacillus paracasei DTA83 and Lacticaseibacillus rhamnosus DTA76. However, this protective effect was not observed in the survival of Lactobacillus acidophilus La 5. Honey solution did not maintain the viability of probiotic microorganisms exposed to hydrogen peroxide and, on the contrary, caused a significant reduction in the population of L. rhamnosus DTA76 (p < 0.001). Lower membrane lipid peroxidation due to H₂O₂ exposure was observed in L. acidophilus La 5 and L. rhamnosus DTA76, but this marker showed no relation with viability. It was concluded: (i) lactobacilli from the Lacticaseibacillus genus were the ones that benefited most from the lactic environment; (ii) the absence of the protective effect of honey was possibly due to the presence of Fe²⁺ which reacts with H₂O₂ to produce hydroxyl radicals; and (iii) cell viability did not correlate with membrane lipid peroxidation, and it is not a good marker to evaluate this type of damage in cells of different microorganisms.

Hydrogen sulfide toxicity in the gut environment: Meta-analysis of sulfate-reducing and lactic acid bacteria in inflammatory processes

BACKGROUND

Hydrogen sulfide is the final product of sulfate-reducing bacteria metabolism. Its high concentration in the gut can affect adversely bowel environment and intestinal microbiota by toxicity and pH lowering.

AIM OF REVIEW

The aim of the review was to give observations related to the properties of bacterial communities inhabiting the gut, with the emphasis on sulfate-reducing bacteria and lactic acid bacteria.

KEY SCIENTIFIC CONCEPTS OF REVIEW

The conduction of meta-analysis was another goal, since it gave statistical observation of the relevant studies. The review literature consisted of more than 160 studies, published from 1945 to 2019. Meta-analysis included 16 studies and they were chosen from the Web of Science database. The systematic review gave important information about the development of gut inflammation, with emphasis on sulfate-reducing and lactic acid bacteria. Oppositely from sulfate-reducing bacteria, probiotic properties of lactic acid bacteria are effective inhibitors against inflammatory bowel disease development, including ulcerative colitis. These facts were confirmed by the conducted meta-analysis. The results and observations gained from the systematic review represent the emphasized importance of gut microbiota for bowel inflammation. On the other side, it should be stated that more studies in the future will provide even better confirmations.

Lentilactobacillus kribbianus sp. nov., isolated from the small intestine of a mini pig

A Gram-stain-positive, facultative anaerobic, rod-shaped bacteria isolated from the small intestine of a mini pig was designated as strain YH-lac9^T. 16S rRNA gene sequence analysis revealed that the strain belongs to the genus Lentilactobacillus and is closely related to Lentilactobacillus senioris JCM 17472^T, Lentilactobacillus rapi JCM 15042^T and Lentilactobacillus diolivorans JCM 13927^T, with 97.6, 96.2 and 95.7 % sequence similarity, respectively. Analysis of housekeeping gene sequences (pheS and recA) revealed that the strain formed a sub-cluster with L. senioris, supporting the results of 16S rRNA gene sequences analysis. The average nucleotide identity value for YH-lac9^T and the most closely related strain is 74.1 %. The main fatty acids are C_18 : 1ω9c, summed feature 7, C_16 : 0 and summed feature 8. The G+C content of the genomic DNA is 37.8 mol%. In view of its chemotaxonomic, phenotypic and phylogenetic properties, YH-lac9^T (=KCTC 25005=JCM 33997) represents a novel taxon. The name Lentilactobacillus kribbianus sp. nov. is proposed.

Evaluation of the bioprotectivity of Lactobacillus binary/ternary cultures in yogurt

The attempts toward addition of biocontrol agents in dairy products have gained popularity. Here, we worked on analysing the antifungal activity of binary and ternary combinations of three Lactic Acid Bacteria (LAB) against five spoilage yeasts in yogurt. The yogurt samples were characterized in terms of pH, acidity, WHC, textural parameters, viscosity, survivability and antifungal activity of LAB and sensorial properties during cold storage. The results showed that the inoculation of LAB in yogurt gave rise in significant reduction of pH throughout cold storage while titrable acidity and WHC decreased (p < .05). Inoculation of LAB resulted in significant increase in hardness and adhesiveness while springiness remained constant. On the other hand, apparent viscosity of all samples experienced a profound increase up to the 10th day of storage followed by a reduction trend for the rest of storage period. Analysis of inhibitory activity of LAB showed an efficient barrier against all five yeasts, in which the most activity was recorded for Lactobacillus reuteri followed by Lactobacillus acidophilus. On the other hand, the most resistance yeast was Kluyveromyces marxianus followed by Rhodotorula mucilaginosa. Sensorial analysis revealed that addition of LAB in yogurt brought about a profound improvement in textural quality of samples. Inoculation of LAB cultures in yogurt at 5% (v/v) not only could improve the physicochemical and sensorial properties of yogurt, but also could introduce a strategy toward substituting of chemical preservatives with biocontrol agents.

Respiratory Physiology of Lactococcus lactis in Chemostat Cultures and Its Effect on Cellular Robustness in Frozen and Freeze-Dried Starter Cultures

Lactococcus lactis is used in large quantities by the food and biotechnology industries. L. lactis can use oxygen for respiration if heme is supplied in the growth medium. This has been extensively studied in batch cultures using various mutants, but quantitative studies of how the cell growth affects respiratory metabolism, energetics, and cell quality are surprisingly scarce. Our results demonstrate that the respiratory metabolism of L. lactis is remarkably flexible and can be modulated by controlling the specific growth rate. We also link the physiological state of cells during cultivation to the quality of frozen or freeze-dried cells, which is relevant to the industry that may lack understanding of such relationships. This study extends our knowledge of respiratory metabolism in L. lactis and its impact on frozen and freeze-dried starter culture products, and it illustrates the influence of cultivation conditions and microbial physiology on the quality of starter cultures.

In this study, we used chemostat cultures to analyze the quantitative effects of the specific growth rate and respiration on the metabolism in Lactococcus lactis CHCC2862 and on the downstream robustness of cells after freezing or freeze-drying. Under anaerobic conditions, metabolism remained homofermentative, although biomass yields varied with the dilution rate (D). In contrast, metabolism shifted with the dilution rate under respiration-permissive conditions. At D = 0.1 h⁻¹, no lactate was produced, while lactate formation increased with higher dilution rates. Thus, a clear metabolic shift was observed, from flavor-forming respiratory metabolism at low specific growth rates to mixed-acid respiro-fermentative metabolism at higher specific growth rates. Quantitative analysis of the respiratory activity, lactose uptake rate, and metabolite production rates showed that aerobic acetoin formation provided most of the NADH consumed in respiration. Moreover, the maintenance-associated lactose consumption under respiration-permissive conditions was only 10% of the anaerobic value, either due to higher respiratory yield of ATP on consumed lactose or due to lower maintenance-related ATP demand. The cultivation conditions also affected the quality of the starter cultures produced. Cells harvested under respiration-permissive conditions at D = 0.1 h⁻¹ were less robust after freeze-drying and had lower acidification activity for subsequent milk acidification, whereas respiration-permissive conditions at the higher dilution rates led to robust cells that performed equally well or better than anaerobic cells.

IMPORTANCELactococcus lactis is used in large quantities by the food and biotechnology industries. L. lactis can use oxygen for respiration if heme is supplied in the growth medium. This has been extensively studied in batch cultures using various mutants, but quantitative studies of how the cell growth affects respiratory metabolism, energetics, and cell quality are surprisingly scarce. Our results demonstrate that the respiratory metabolism of L. lactis is remarkably flexible and can be modulated by controlling the specific growth rate. We also link the physiological state of cells during cultivation to the quality of frozen or freeze-dried cells, which is relevant to the industry that may lack understanding of such relationships. This study extends our knowledge of respiratory metabolism in L. lactis and its impact on frozen and freeze-dried starter culture products, and it illustrates the influence of cultivation conditions and microbial physiology on the quality of starter cultures.

Effects of ccpA gene deficiency in Lactobacillus delbrueckii subsp. bulgaricus under aerobic conditions as assessed by proteomic analysis

Background

Aerobic growth provides benefits in biomass yield and stress tolerance of Lactobacillus delbrueckii subsp. bulgaricus (L. bulgaricus). Catabolite control protein A (CcpA) is a master regulator involved in the aerobic and anaerobic growth, metabolic production and stress response in L. bulgaricus, but its potential molecular mechanisms remains unclear. The aim of this study is to elucidate the role of CcpA in L. bulgaricus in aerobic growth at the proteomic perspective.

Results

The differential proteomic analysis was performed on the L. bulgaricus ATCC11842 and its ccpA inactivated mutant strain using iTRAQ technology. A total of 132 differentially expressed proteins were obtained, among which 58 were up-regulated and 74 were down-regulated. These proteins were mainly involved in the cellular stress response, carbohydrate and energy metabolism, amino acid transport and protein synthesis, genetic information processing. Moreover, inactivation of ccpA negatively affected the expression of key enzymes involved in glycolysis pathway, while it enhanced the expression of proteins related to the pyruvate pathway, supporting the conclusion that CcpA mediated the shift from homolactic fermentation to mixed acid fermentation in L. bulgaricus.

Conclusions

Overall, these results showed that the role of CcpA in L. bulgaricus as a pleiotropic regulator in aerobic metabolism and stress response. This proteomic analysis also provide new insights into the CcpA-mediated regulatory network of L. bulgaricus and potential strategies to improve the production of starter and probiotic cultures based on the metabolic engineering of global regulators.

Hydrogen Sulfide Effects on the Survival of Lactobacilli with Emphasis on the Development of Inflammatory Bowel Diseases

The gut microbiota is a complex component of humans that depends on diet, host genome, and lifestyle. The background: The study purpose is to find relations between nutrition, intestinal lactic acid bacteria (LAB) from various environments (human, animal intestine, and yogurt) and sulfate-reducing microbial communities in the large intestine; to compare kinetic growth parameters of LAB; and to determine their sensitivity to different concentration of hydrogen sulfide produced by intestinal sulfate-reducing bacteria.

METHODS

Microbiological (isolation and identification), biochemical (electrophoresis), molecular biology methods (DNA isolation and PCR analysis), and statistical processing (average and standard error calculations) of the results were used.

THE RESULTS

The toxicity of hydrogen sulfide produced by sulfate-reducing bacteria, the survival of lactic acid bacteria, and minimal inhibitory concentrations (MIC) were determined. The measured hydrogen sulfide sensitivity values were the same for L. paracasei and L. reuteri (MIC > 1.1 mM). In addition, L. plantarum and L.fermentum showed also a similar sensitivity (MIC > 0.45 mM) but significantly (p < 0.05) lower than L.reuteri and L. paracasei (1.1 > 0.45 mM). L. paracasei and L. reuteri are more sensitive to hydrogen sulfide than L. fermentum and L. plantarum. L. pentosus was sensitive to the extremely low concentration of H2S (MIC > 0.15 mM).

CONCLUSIONS

The Lactobacillus species were significantly sensitive to hydrogen sulfide, which is a final metabolite of intestinal sulfate-reducing bacteria. The results are definitely helpful for a better understanding of complicated interaction among intestinal microbiota and nutrition.

Effect of Respiratory Growth on the Metabolite Production and Stress Robustness of Lactobacillus casei N87 Cultivated in Cheese Whey Permeate Medium

Cheese whey permeate (WP) is a low-cost feedstock used for the production of biomass and metabolites from several lactic acid bacteria (LAB) strains. In this study, Lactobacillus casei N87 was cultivated in an optimized WP medium (WPM) to evaluate the effect of anaerobic and respiratory conditions on the growth performances (kinetics, biomass yield), consumption of sugars (lactose, galactose, glucose) and citrate, metabolite production [organic acids, volatile organic compounds (VOCs)] and stress survival (oxidative, heat, freezing, freeze-drying). The transcription of genes involved in the main pathways for pyruvate conversion was quantified through Real Time-PCR to elucidate the metabolic shifts due to respiratory state. Cultivation in WPM induced a diauxic growth in both anaerobic and respiratory conditions, and L. casei N87 effectively consumed the lactose and galactose present in WPM. Genomic information suggested that membrane PTS system and tagatose-6-P pathway mediated the metabolism of lactose and galactose in L. casei N87. Respiration did not affect specific growth rate and biomass production, but significantly altered the pyruvate conversion pathways, reducing lactate accumulation and promoting the formation of acetate, acetoin and diacetyl to ensure the redox balance. Ethanol was not produced under either cultivation. Pyruvate oxidase (pox), acetate kinase (ack), α-acetolactate decarboxylase (ald), acetolactate synthase (als) and oxaloacetate decarboxylase (oad) genes were up-regulated under respiration, while L-lactate dehydrogenase (ldh), pyruvate formate lyase (pfl), pyruvate carboxylase (pyc), and phosphate acetyltransferase (pta) were down regulated by oxygen. Transcription analysis was consistent with metabolite production, confirming that POX-ACK and ALS-ALD were the alternative pathways activated under aerobic cultivation. Respiratory growth affected the production of volatile compounds useful for the development of aroma profile in several fermented foods, and promoted the survival of L. casei N87 to oxidative stresses and long-term storage. This study confirmed that the respiration-based technology coupled with cultivation on low-cost medium may be effectively exploited to produce competitive and functional starter and/or adjunct cultures. Our results, additionally, provided further information on the activation and regulation of metabolic pathways in homofermentative LAB grown under respiratory promoting conditions.

Impact of aerobic and respirative life-style on Lactobacillus casei N87 proteome

Lactic acid bacteria (LAB) are used as starter, adjunct and/or probiotic cultures in fermented foods. Several species are recognized as oxygen-tolerant anaerobes, and aerobic and respiratory cultivations may provide them with physiological and technological benefits. In this light, mechanisms involved in the adaptation to aerobic and respiratory (supplementation with heme and menaquinone) growth conditions of the O₂-tolerant strain Lactobacillus casei N87 were investigated by proteomics. In fact, in this bacterial strain, respiration induced an increase in biomass yield and robustness to oxidative, long-term starvation and freeze-drying stresses, while high concentrations of dissolved O₂ (dO₂ 60%) negatively affected its growth and cell survival. Proteomic results well paralleled with physiological and metabolic features and clearly showed that aerobic life-style led to a higher abundance of several proteins involved in carbohydrate metabolism and stress response mechanisms and, concurrently, impaired the biosynthesis of proteins involved in nucleic acid formation and translation processes, thus providing evidence at molecular level of the significant damage to L.casei N87 fitness. On the contrary, the activation of respiratory pathways due to heme and menaquinone supplementation, led to a decreased amount of chaperones and other stress related proteins. These findings confirmed that respiration reduced oxidative stress condition, allowing to positively modulate the central carbohydrate and energy metabolism and improve growth and stress tolerance features. Results of this study could be potentially functional to develop competitive adjunct and probiotic cultures effectively focused on the improvement of quality of fermented foods and the promotion of human health.

Evolution of Volatile Compounds and Spoilage Bacteria in Smoked Bacon during Refrigeration Using an E-Nose and GC-MS Combined with Partial Least Squares Regression

The changes in the concentration of volatile organic compounds (VOCs) and biogenic amines (BAs) in smoked bacon during 45-day refrigerated storage is investigated using solid-phase micro-extraction coupled with gas chromatography-mass spectrometry and high-performance liquid chromatography. In total, 56 VOCs and 6 BAs were identified and quantified. The possible pathways leading to their formation are analyzed and considered as the potential signs of microbial activity, especially by specific spoilage microorganisms (SSOs). Leuconostoc and Lactobacillus, which levels increased markedly with the extension of storage time, were recognized as SSOs. An electronic nose (e-nose) was employed to determine the changes in concentration of the odor components per sample present within half an hour. Partial least squares regression was then carried out to analyze the correlation between SSO growth, metabolite concentration, BA accumulation, and e-nose response. The results show that ten VOCs (ethanol, 2-furanmethanol, 1-hexanol, 1-propanol, phenol, 2-methoxyphenol, acetic acid, 3-ethyl-2-cyclopenten-1-one, furfural, and ethyl hexanoate) and three BAs (putrescine, cadaverine, and tyramine) can be associated with the growth of SSOs. Thus, they can be adopted as potential indicators to evaluate and monitor the quality of the bacon and develop appropriate detection methods. E-noses can used to recognize odors and diagnose quality of bacon.