Production Of Antimicrobial Compounds Research Articles

Genomic Analysis of Endophytic Bacillus-Related Strains Isolated from the Medicinal Plant Origanum vulgare L. Revealed the Presence of Metabolic Pathways Involved in the Biosynthesis of Bioactive Compounds.

Multidrug-resistant pathogens represent a serious threat to human health. The inefficacy of traditional antibiotic drugs could be surmounted through the exploitation of natural bioactive compounds of which medicinal plants are a great reservoir. The finding that bacteria living inside plant tissues, (i.e., the endophytic bacterial microbiome) can influence the synthesis of the aforementioned compounds leads to the necessity of unraveling the mechanisms involved in the determination of this symbiotic relationship. Here, we report the genome sequence of four endophytic bacterial strains isolated from the medicinal plant Origanum vulgare L. and able to antagonize the growth of opportunistic pathogens of cystic fibrosis patients. The in silico analysis revealed the presence of gene clusters involved in the production of antimicrobial compounds, such as paeninodin, paenilarvins, polymyxin, and paenicidin A. Endophytes’ adaptation to the plant microenvironment was evaluated through the analysis of the presence of antibiotic resistance genes in the four genomes. The diesel fuel degrading potential was also tested. Strains grew in minimum media supplemented with diesel fuel, but no n-alkanes degradation genes were found in their genomes, suggesting that diesel fuel degradation might occur through other steps involving enzymes catalyzing the oxidation of aromatic compounds.

Biological Control Activity of Plant Growth Promoting Rhizobacteria Burkholderia contaminans AY001 against Tomato Fusarium Wilt and Bacterial Speck Diseases.

Simple SummaryBurkholderia contaminans belongs to B. cepacia complex (Bcc), those of which are found in various environmental conditions. In this study, a novel strain AY001 of B. contaminans (AY001) was identified from the rhizosphere soil sample. AY001 showed (i) various plant growth-promoting rhizobacteria (PGPR)-related traits, (ii) antagonistic activity against different plant pathogenic fungi, (iii) suppressive activity against tomato Fusarium wilt disease, (iv) induced systemic acquired resistance (ISR)-triggering activity, and (v) production of various antimicrobial and plant immune-inducing secondary metabolites. These results suggest that AY001 is, indeed, a successful PGPR, and it can be practically used in tomato cultivation to alleviate biotic and abiotic stresses. However, further safety studies on the use of AY001 will be needed to ensure its safe use in the Agricultural system.Plant growth promoting rhizobacteria (PGPR) is not only enhancing plant growth, but also inducing resistance against a broad range of pathogens, thus providing effective strategies to substitute chemical products. In this study, Burkholderia contaminans AY001 (AY001) is isolated based on its broad-spectrum antifungal activity. AY001 not only inhibited fungal pathogen growth in dual culture and culture filtrate assays, but also showed various PGPR traits, such as nitrogen fixation, phosphate solubilization, extracellular protease production, zinc solubilization and indole-3-acetic acid (IAA) biosynthesis activities. Indeed, AY001 treatment significantly enhanced growth of tomato plants and enhanced resistance against two distinct pathogens, F. oxysporum f.sp. lycopersici and Pseudomonas syringae pv. tomato. Real-time qPCR analyses revealed that AY001 treatment induced jasmonic acid/ethylene-dependent defense-related gene expression, suggesting its Induced Systemic Resistance (ISR)-eliciting activity. Gas chromatography–mass spectrometry (GC-MS) analysis of culture filtrate of AY001 revealed production of antimicrobial compounds, including di(2-ethylhexyl) phthalate and pyrrolo [1,2-a]pyrazine-1,4-dione, hexahydro-3-(phenylmethyl). Taken together, our newly isolated AY001 showed promising PGPR and ISR activities in tomato plants, suggesting its potential use as a biofertilizer and biocontrol agent.

Antimicrobial Potentials of Bacillus Subtilis Isolated from Soil and Water

In this study, an attempt was made to extract antimicrobial substance from the soil isolate of Bacillus subtilis and explore its antimicrobial activity on some selected bacteria. The bacteria were typed using RAPD-PCR method to identify organism to strain level, using 16sr RNA A1A13 specific primers. Results revealed bands of different sizes identical in both strains of Bacillus subtilis. The antimicrobial potential of the soil isolate was determined by disc diffusion assay. The production of antimicrobial compound was recorded maximum at the late exponential growth phase. The filtered concentrate of the crude extract was used to determine antimicrobial activity on some selected gram positive and gram – negative bacteria. Partial purification of the crude extract was carried out using High pressure Liquid chromatography (HPLC), Zymogram analysis and TLC were performed to determine the preliminary biochemical nature. The molecular weight of the antimicrobial peptide was determined to be less than 2.5k Da. The presence of antibiotic Bacilysocin was discovered from the extract. The minimum inhibitory concentration MIC of the purified fraction showed (MIC) ranging from 0.5 to 10µg/ml for Staphylococcus aureus and Streptococcus pyogenes. The MIC for the gram–negative was 0ug/ml. The crude extracts showed antimicrobial activity only on gram positive selected bacteria strains, while the purified extract showed antimicrobial activity on both the gram-positive and gram-negative with more activity on the gram-positive. The zones of inhibition for crude extract 1 on Staphylococcus auerus were 8.0mm, 5.0mm, 3.0mm, 1.0mm; extract 2: 15.0mm, 10.0mm, 9.0mm, 7.0mm, and 6.0mm, 4.0mm, 3.0mm, 1.0mm respectively for extract 3. for Streptococcus pyogenes the zones of inhibition range from 2.0mm to 9.0mm, for extracts 1 and 2:10.0mm to 3.0m and10.0mm to 1.0mm for extract 3.for the gram negative test organisms, the zones of inhibition were 0.0mm for all the extracts tested against it.

Combining in vitro and in vivo screening to identify efficient Pseudomonas biocontrol strains against the phytopathogenic bacterium Ralstonia solanacearum.

Although plant pathogens are traditionally controlled using synthetic agrochemicals, the availability of commercial bactericides is still limited. One potential control strategy could be the use of plant growth‐promoting bacteria (PGPB) to suppress pathogens via resource competition or the production of antimicrobial compounds. This study aimed to conduct in vitro and in vivo screening of eight Pseudomonas strains against Ralstonia solanacearum (the causative agent of bacterial wilt) and to investigate underlying mechanisms of potential pathogen suppression. We found that inhibitory effects were Pseudomonas strain‐specific, with strain CHA0 showing the highest pathogen suppression. Genomic screening identified 2,4‐diacetylphloroglucinol, pyoluteorin, and orfamides A and B secondary metabolite clusters in the genomes of the most inhibitory strains, which were investigated further. Although all these compounds suppressed R. solanacearum growth, only orfamide A was produced in the growth media based on mass spectrometry. Moreover, orfamide variants extracted from Pseudomonas cultures showed high pathogen suppression. Using the “Micro‐Tom” tomato cultivar, it was found that CHA0 could reduce bacterial wilt disease incidence with one of the two tested pathogen strains. Together, these findings suggest that a better understanding of Pseudomonas–Ralstonia interactions in the rhizosphere is required to successfully translate in vitro findings into agricultural applications.

Current Status of Probiotics as Supplements in the Prevention and Treatment of Infectious Diseases.

Probiotics play an important role against infectious pathogens via their effects on the epithelium, the production of antimicrobial compounds, and competitive exclusion. Administration of probiotic supplements may reduce the risk of infectious diseases and the use of antibiotics, hence contributing to a reduction or a delay of the development of multi-resistant bacteria. Infection is a constant concern for people who experience recurrent infections, and antibiotic treatment usually fails due to antibiotic resistance. Therefore, an infection can lead to severe illness and hospitalization if left untreated. A growing number of studies have demonstrated promising results for a variety of probiotic strains used to prevent or treat acute and recurrent infectious diseases, but additional standardized clinical research is needed.

Evaluation of Modulatory Activities of Lactobacillus crispatus Strains in the Context of the Vaginal Microbiota.

ABSTRACTIt has been widely reported that members of the genus Lactobacillus dominate the vaginal microbiota, which is represented by the most prevalent species Lactobacillus crispatus, Lactobacillus jensenii, Lactobacillus gasseri, and Lactobacillus iners. L. crispatus is furthermore considered an important microbial biomarker due to its professed beneficial implications on vaginal health. In order to identify molecular mechanisms responsible for health-promoting activities that are believed to be elicited by L. crispatus, we performed in silico investigations of the intraspecies biodiversity of vaginal microbiomes followed by in vitro experiments involving various L. crispatus strains along with other vaginal Lactobacillus species mentioned above. Specifically, we assessed their antibacterial activities against a variety of pathogenic microorganisms that are associated with vaginal infections. Moreover, coculture experiments of L. crispatus strains showing the most antibacterial activity against different pathogens revealed distinct ecological fitness and competitive properties with regard to other microbial colonizers. Interestingly, we observed that even phylogenetically closely related L. crispatus strains possess unique features in terms of their antimicrobial activities and associated competitive abilities, which suggests that they exert marked competition and evolutionary pressure within their specific environmental niche.IMPORTANCE The human vaginal microbiota includes all microorganisms that colonize the vaginal tract. In this context, a vaginal microbiota dominated by Lactobacillus and specifically by Lactobacillus crispatus is considered a hallmark of health. The role of L. crispatus in maintaining host health is linked to its modulatory activity toward other members of the vaginal ecosystem and toward the host. In this study, in vitro experiments followed by genetic analyses of the mechanisms used by L. crispatus in colonizing the vaginal ecological niche, particularly in the production of different antimicrobial compounds, were evaluated, highlighting some intriguing novel aspects concerning the genetic variability of this species. Our results indicate that this species has adapted to its niche and may still undergo adaptation to enhance its competitiveness for niche colonization.

Potentials of Endophytes of Andrographis Paniculata for the Production of Plant Growth Promoters, Enzymes and Antimicrobial Compounds

In the present study, 9 bacterial and 6 fungal endophytes were isolated from surface sterilized leaf, stem and root samples of the medicinal plant - Andrographis paniculata (Kalmegh). The endophytes were screened for plant growth promoting traits (IAA, phosphate solubilization and N2 fixation), enzymes (cellulase and amylase) and antimicrobial compounds against 3 potent human pathogens- E. coli, Staphylococcus sp. and Vibrio sp. The majority of the isolated endophytes produced the phytohormone - IAA (ranging 2-45μg/ml), and 1 endophyte solubilized phosphate and fixed N2. All the fungal endophytes possessed cellulase and amylase activity. In the preliminary screening, 4 bacterial and 4 fungal endophytic isolates extract showed antagonistic activity against the 3 potent human pathogens which are known causative agents of urinary tract, skin and gastrointestinal tract infections, respectively. The endophytes of A. paniculata exhibiting broad and specific antimicrobial activity make them ideal candidates in medical purposes. SAARC J. Agric., 19(2): 157-170 (2021)

Genomic characterization and production of antimicrobial lipopeptides by Bacillus velezensis P45 growing on feather by-products.

To investigate the potential of novel Bacillus velezensis P45 as an eco-friendly alternative for bioprocessing poultry by-products into valuable antimicrobial products. The complete genome of B. velezensis P45 was sequenced using the Illumina MiSeq platform, showing 4455 protein and 98 RNA coding sequences according to the annotation on the RAST server. Moreover, the genome contains eight gene clusters for the production of antimicrobial secondary metabolites and 25 putative protease-related genes, which can be related to feather-degrading activity. Then, in vitro tests were performed to determine the production of antimicrobial compounds using feather, feather meal and brain-heart infusion (BHI) cultures. Antimicrobial activity was observed in feather meal and BHI media, reaching 800 and 3200AUml-1 against Listeria monocytogenes respectively. Mass spectrometry analysis indicates the production of antimicrobial lipopeptides surfactin, fengycin and iturin. The biotechnological potential of B. velezensis P45 was deciphered through genome analysis and in vitro studies. This strain produced antimicrobial lipopeptides growing on feather meal, a low-cost substrate. The production of antimicrobial peptides by this keratinolytic strain may represent a sustainable alternative for recycling by-products from poultry industry. Furthermore, whole B. velezensis P45genome sequence was obtained and deposited.

Microbial Ecology of Sheep Milk, Artisanal Feta, and Kefalograviera Cheeses. Part II: Technological, Safety, and Probiotic Attributes of Lactic Acid Bacteria Isolates.

The aim of the present study was to examine 189 LAB strains belonging to the species Enterococcus faecium, E. faecalis, Lactococcus lactis, Pediococcus pentosaceus, Leuconostoc mesenteroides, Lactiplantibacillus pentosus, Latilactobacillus curvatus, Lp. plantarum, Levilactobacillus brevis, and Weissella paramesenteroides isolated form sheep milk, Feta and Kefalograviera cheeses at different ripening stages, for their technological compatibility with dairy products manufacturing, their activities that may compromise safety of the dairy products as well as their capacity to survive in the human gastrointestinal tract. For that purpose, milk acidification and coagulation capacity, caseinolytic, lipolytic, hemolytic, gelatinolytic, and bile salt hydrolase activity, production of exopolysaccharides, antimicrobial compounds, and biogenic amines, as well as acid and bile salt tolerance and antibiotic susceptibility were examined. The faster acidifying strains were Lc. lactis DRD 2658 and P. pentosaceus DRD 2657 that reduced the pH value of skim milk, within 6 h to 5.97 and 5.92, respectively. Strains able to perform weak caseinolysis were detected in all species assessed. On the contrary, lipolytic activity, production of exopolysaccharides, amino acid decarboxylation, hemolytic, gelatinase, and bile salt hydrolase activity were not detected. Variable susceptibility to the antibiotics examined was detected among LAB strains. However, in the majority of the cases, resistance was evident. None of the strains assessed, managed to survive to exposure at pH value 1. On the contrary, 25.9 and 88.9% of the strains survived after exposure at pH values 2 and 3, respectively; the reduction of the population was larger in the first case. The strains survived well after exposure to bile salts. The strain-dependent character of the properties examined was verified. Many strains, belonging to different species, have presented very interesting properties; however, further examination is needed before their potential use as starter or adjunct cultures.

Commensal Pseudomonas fluorescens Strains Protect Arabidopsis from Closely Related Pseudomonas Pathogens in a Colonization-Dependent Manner.

ABSTRACTPlants form commensal associations with soil microorganisms, creating a root microbiome that provides benefits, including protection against pathogens. While bacteria can inhibit pathogens through the production of antimicrobial compounds in vitro, it is largely unknown how microbiota contribute to pathogen protection in planta. We developed a gnotobiotic model consisting of Arabidopsis thaliana and the opportunistic pathogen Pseudomonas sp. N2C3, to identify mechanisms that determine the outcome of plant-pathogen-microbiome interactions in the rhizosphere. We screened 25 phylogenetically diverse Pseudomonas strains for their ability to protect against N2C3 and found that commensal strains closely related to N2C3, including Pseudomonas sp. WCS365, were more likely to protect against pathogenesis. We used comparative genomics to identify genes unique to the protective strains and found no genes that correlate with protection, suggesting that variable regulation of components of the core Pseudomonas genome may contribute to pathogen protection. We found that commensal colonization level was highly predictive of protection, so we tested deletions in genes required for Arabidopsis rhizosphere colonization. We identified a response regulator colR, and two ColR-dependent genes with predicted roles in membrane modifications (warB and pap2_2), that are required for Pseudomonas-mediated protection from N2C3. We found that WCS365 also protects against the agricultural pathogen Pseudomonas fuscovaginae SE-1, the causal agent of bacterial sheath brown rot of rice, in a ColR-dependent manner. This work establishes a gnotobiotic model to uncover mechanisms by which members of the microbiome can protect hosts from pathogens and informs our understanding of the use of beneficial strains for microbiome engineering in dysbiotic soil systems.

Biosynthetic Gene Clusters in Bacteria: A Review

Soil is a nutrient-rich environment that harbors billions of microbial species. The diversity of microbes in an environment varies with the change in edaphic factors. To survive these environmental changes, microbes produce secondary metabolites which are not directly associated with their growth and reproduction. Bacterial genomes possess biosynthetic gene clusters (BGC) which regulate the synthesis of these secondary metabolites. These BGCs encode for megasynthases such as nonribosomal peptide synthases (NRPS) and polyketide synthases (PKS) which produce metabolites such as antimicrobial compounds, which are the most common metabolites produced by these megasynthases. They help bacteria to survive in the competitive environment by killing surrounding microbes. As chemical drugs may pose immense damage to human health and the environment, so antibiotics produced by natural sources are of major attention these days. The extraction of antimicrobial compounds from bacterial sources also provides scaffolds for new synthetic drugs. Bacteria maintain strict genetic control over antibiotic production. They have particular quorum sensing pathways that help to trigger the surrounding cells to produce antibiotics when required. Biosynthetic gene clusters need to be explored widely under various culture conditions so that more useful products can be extracted from a single type of bacterium. This review focuses on the secondary metabolite production, extraction, and biosynthetic gene clusters which encode megasynthases responsible for the production of antimicrobial compounds.

Complete Genome Sequences of Five Burkholderia Strains with Biocontrol Activity against Various Lettuce Pathogens.

ABSTRACTNumerous bacterial strains from the Burkholderia cepacia complex display biocontrol activity. Here, we report the complete genome sequences of five Burkholderia strains isolated from soil. Biosynthetic gene clusters responsible for the production of antimicrobial compounds were found in the genome of these strains, which display biocontrol activity against various lettuce pathogens.

Overview of Antimicrobial Compounds Produced from Bacteria

Every year more than 10 million lives are lost due to antimicrobial-resistant infections. Therefore, the development of antimicrobial agents has become a necessity. Natural products are being used as therapeutic agents. Antimicrobial beings are one of the most compelling molecules. Microorganisms produce bioactive compounds with antimicrobial and cytotoxic bioactivity. Fungi and bacteria are mostly used as novel antimicrobial agents. In the years 1940 & 1950 marine bacteria were shown to produce antimicrobial compounds. The first marine antibacterials were identified in the year 1960, and in the year 1970 their antibiotic properties has been reported. The majority of identified compounds were isolated from Actinobacteria & Bacilli. The interesting fact is that the molecules that were active against gram-negative were isolated from Bacilli. It is estimated at least 4-5% of the genome of any strain of the B. subtilus group is devoted to antimicrobial compounds (AMCs) production.

Isolation, Characterization and Antimicrobial Activity of Endophytic Actinobacteria from Medicinal Plants

The current study is aimed to isolate endophytic actinobacteria from various medicinal plants for exploring its antimicrobial activity against clinical pathogens. Out of 50 actinobacterial cultures isolated from eleven medicinal plants, 58 % of the cultures showed antimicrobial activity against at least two out of nine pathogens tested. Strain KCA1 isolated from Phyllanthus niruri leaves showed maximum zone of inhibition against all of the nine pathogens tested. Strain KCA1 exhibited maximum level of antimicrobial metabolite production in solid state fermentation during 9 d of incubation. Three well separated spots were observed when the ethyl acetate extract of strain KCA1 was analyzed through thin-layer chromatography. One spot with retardation factor value 0.82 was found to inhibit Staphylococcus aureus in thin-layer chromatography bioautographic assay. The active fraction eluted from thin-layer chromatography was further characterized by gas chromatography-mass spectral analysis. The mass spectral data revealed the presence of three compounds viz., 2,4-di-tert-butylphenol (C14H22O), 1-hexadecanol (C16H34O) and 1-nonadecene (C19H38) present in major proportions which are responsible for the antimicrobial activity of the strain KCA1. In fermentation experiment, variables such as glucose, yeast extract and sodium chloride were found to influence the antimicrobial compound production. The potential strain KCA1 was identified as Streptomyces sp. on the basis of microscopic, cultural, physiological and 16S ribosomal ribonucleic acid analysis.

Uncovering the potential of actinobacterium BLH 1-22 isolated from marine sediment as a producer of antibiotics

Actinobacteria have been known as producers of many bioactive compounds. The present study examines ten marine Actinobacterial isolates, aiming to investigate their potential as producers of antimicrobial compounds. The secondary metabolites were extracted from these Actinobacteria using ethyl acetate, and the crude extracts were tested for their bioactivity against Escherichia coli, Bacillus subtilis, and Micrococcus luteus. The antibacterial screening showed that the crude extracts of these Actinobacteria inhibit the growth of indicator strains. The extracts of isolate BLH 1-22 were further analysed using high-performance liquid chromatography (HPLC), which showed potential compounds with peak and retention time similar to the antibiotic standards (i.e., erythromycin, ampicillin, tetracycline, and penicillin). In addition to the HPLC profile, molecular identification showed that the isolate BLH 1-22 was similar to Micromonospora chalcea (99.6%). Further genome characterization of the strain, as well as purification and fractionation of the metabolite extracts, are important to obtain a comprehensive study on the potential of isolate BLH 1-22 as antibiotic compound producers. This study reported the potential of Micromonospora BLH 1-22 isolated from marine sediment. Hence, it also highlighted the potential of Actinobacteria isolated from Indonesian environments for bioprospecting studies.

Biostimulation of Bacteria in Liquid Culture for Identification of New Antimicrobial Compounds.

We hypothesized that environmental microbiomes contain a wide range of bacteria that produce yet uncharacterized antimicrobial compounds (AMCs) that can potentially be used to control pathogens. Over 600 bacterial strains were isolated from soil and food compost samples, and 68 biocontrol bacteria with antimicrobial activity were chosen for further studies based on inhibition assays against a wide range of food and plant pathogens. For further characterization of the bioactive compounds, a new method was established that used living pathogens in a liquid culture to stimulate bacteria to produce high amounts of AMCs in bacterial supernatants. A peptide gel electrophoresis microbial inhibition assay was used to concurrently achieve size separation of the antimicrobial peptides. Fifteen potential bioactive peptides were then further characterized by tandem MS, revealing cold-shock proteins and 50S ribosomal proteins. To identify non-peptidic AMCs, bacterial supernatants were analyzed by HPLC followed by GC/MS. Among the 14 identified bioactive compounds, 3-isobutylhexahydropyrrolo[1,2-a]pyrazine-1,4-dione and 2-acetyl-3-methyl-octahydropyrrolo[1,2-a]piperazine-1,4-dione were identified as new AMCs. Our work suggests that antimicrobial compound production in microbes is enhanced when faced with a threat from other microorganisms, and that this approach can rapidly lead to the development of new antimicrobials with the potential for upscaling.

Anti-microbial Activities of <em>Pseudomonas</em> Metabolites Methanolic Extracts against Different MDR Bacterial Pathogens

Background: A Multiple drug resistance (MDR) in pathogenic bacteria has become a significant public health issue for treatment. Among bacteria, Pseudomonas is another important genus except Streptomyces and Bacillus in production of antimicrobial compounds. The current study aim to to determine the antibacterial activity and preliminary characterization of antibacterial compounds produced by Pseudomonas species such as BB1D11, BN2D41, TG1D11, TR1D41, LH1D11 and TN1D41. Method: The antibacterial activity was checked by using bio autography method as well as agar well diffusion method, against four multiple drug resistant bacteria including three Gram negative bacteria (E.coli, Acinetobacter and Pseudomonas) and one Gram positive bacterium (Methicillin Resistant Staphylococcus aureus). Isolation test showed good activity against all the four MDR bacteria, by producing clear zone of diameter from 2mm up to 20mm. Optimum temperature for growth and antibiotic production of Pseudomonas BB1D11, BN2D41, TG1D11, TR1D41, LH1D11 and TN1D41 was 37C0. It produces more metabolites when subjected to shaking incubation. In thin layer chromatography, the extracts were repeatedly inserted on a silica gel coated plate, which was run in mobile phase. Normal HPLC was perform to reveal the presence of antibacterial compounds. Results: By well diffusion assay a zone of inhibition range from 2-18 mm of diameter against different test bacteria. The components were separated, resulting in the formation of bands with different colors, each showing a different compounds. Biological screening was performed by bio autography, metabolites showed a significant activity at retention factor of 0.89. While HPLC at retention time 2.50-2.90 showed presence of significant antibacterial compounds. Conclusion: Pseudomonas BB1D11, BN2D41, TG1D11, TR1D41, LH1D11 and TN1D41 showed promising anti-microbial activity against different MDR bacteria. It is concluded that HPLC revealed the presence of DAPG at retention time 2.90 which inhibit the growth of MDR bacterial strains.

Antimicrobial compound production by pigment producing endophytic bacterium (Burkholderia sp. GBPI_TWL) isolated from Taxus wallichiana Zucc.

Antimicrobial compound production by pigment producing endophytic bacterium (Burkholderia sp. GBPI_TWL) isolated from Taxus wallichiana Zucc.

The antioxidant activity of Lactic acid bacteria and probiotics: a review

Probiotics and Lactic Acid Bacteria play important roles such as the production of antimicrobial compounds and other metabolites. So they have positive effects on human health. When reactive oxygen species generated in excess or cellular defenses are deficient, biomolecules can be damaged by the oxidative stress process. Various studies have shown that the best way to protect the human body from the effects of oxidation reactions is to avoid them, which can be accomplished by using antioxidants. Due to the damages of synthetic antioxidants, their usage has been discussed. Nowadays natural antioxidants derived from bio-resources have recently gained a lot of attention as a potential replacement for synthetic antioxidants. Probiotic bacteria are thought to defend against oxidative stress by restoring the gut microbiota, according to hypothesis of some scientists. This type of microorganisms indicated their antioxidant activity by producing and increasing antioxidant enzymes, production of secondary metabolites, small hydrolyzed peptides in food, resistance to the presence of hydrogen peroxide, and production of intracellular and extracellular compounds such as Exopolysaccharides. Also, they have shown their positive effect on in vivo models. In conclusion, according to the results of studies, lactic acid bacteria and probiotics are significant sources of natural antioxidants. Therefore, they have important research value and market development potential. Also, it should be noted that the mechanism of antioxidant activity of this group of microorganisms has not been fully investigated, this requires further research.

Mannan Oligosaccharides Application: Multipath Restriction From Aeromonas hydrophila Infection in the Skin Barrier of Grass Carp (Ctenopharyngodon idella).

The objective of this study was to evaluate the efficacy of dietary Mannan oligosaccharides (MOS) supplementation on skin barrier function and the mechanism of on-growing grass carp (Ctenopharyngodon idella). Five hundred forty grass carp were fed for 60 days from the growing stage with six different levels of MOS diets (0, 200, 400, 600, 800, and 1,000 mg kg-1). At the end of the growth trial, the 14-day Aeromonas hydrophila challenge experiment has proceeded. The obtained data indicate that MOS could (1) decline skin lesion morbidity after being challenged by the pathogenic bacteria; (2) maintain physical barrier function via improving antioxidant ability, inhibiting excessive apoptosis, and strengthening the tight junction between the epithelial cell and the related signaling pathway (Nrf2/Keap1, p38MAPK, and MLCK); and (3) regulate immune barrier function by modulating the production of antimicrobial compound and expression of involved cytokines and the related signaling pathway (TOR and NFκB). Finally, we concluded that MOS supplementation reinforced the disease resistance and protected the fish skin barrier function from Aeromonas hydrophila infection.