Isolation, Identification and Characterization of Two Kinds of Deep-Sea Bacterial Lipopeptides Against Foodborne Pathogens

Vibrio alginolyticus is the pathogen of "Baotou" disease causing serious damage to Gracilariopsis lemaneiformis cultivation in China

In recent years, a disease called "Baotou" has been causing large-scale yield reductions of Gracilariopsis lemaneiformis in China. Interestingly, Vibrio alginolyticus, which once reported to be a probiotic or pathogen for multiple marine organisms, was strongly proved to be the pathogen causing "Baotou" disease in this study. Analysis of 16S rRNA gene profiling revealed that V. alginolyticus was the most abundant and dominant bacterium on the algal thalli suffering from "Baotou" disease, whereas its presence was scarcely detected on healthy thalli. Scanning electron microscope (SEM) analysis revealed that a large number of V. alginolyticus cells were found to be attached to the algal thalli with "Baotou" disease and the rotten thalli acquired by the lab infection treatment. V. alginolyticus could cause the rotten symptoms which were consistent with those of "Baotou" disease. According to Koch's postulates, V. alginolyticus was identified as the pathogen causing "Baotou" disease.IMPORTANCEA highly contagious disease known as "Baotou" disease has persistently triggered significant yield reductions in G. lemaneiformis throughout China. The pathogen of "Baotou" disease was isolated and identified as V. alginolyticus in this study. Interestingly, V. alginolyticus was once reported to be a probiotic to Saccharina japonica, and pathogen to Haliotis diversicolor, Paleopneustes cristatus, Ruditapes decussatus, and Litopenaeus vannamei. The study indicates that V. alginolyticus play a significant role in the competition or co-existence between G. lemaneiformis, S. japonica, abalone, sea urchin, bivalve, and shrimp.

Antimicrobial Activity of Bacillus Cyclic Lipopeptides and Their Role in the Host Adaptive Response to Changes in Environmental Conditions

Bacillus cyclic lipopeptides (CLP), part of the three main families-surfactins, iturins, and fengycins-are secondary metabolites with a unique chemical structure that includes both peptide and lipid components. Being amphiphilic compounds, CLPs exhibit antimicrobial activity in vitro, damaging the membranes of microorganisms. However, the concentrations of CLPs used in vitro are difficult to achieve in natural conditions. Therefore, in a natural environment, alternative mechanisms of antimicrobial action by CLPs are more likely, such as inducing apoptosis in fungal cells, preventing microbial adhesion to the substrate, and promoting the death of phytopathogens by stimulating plant immune responses. In addition, CLPs in low concentrations act as signaling molecules of Bacillus's own metabolism, and when environmental conditions change, they form an adaptive response of the host bacterium. Namely, they trigger the differentiation of the bacterial population into various specialized cell types: competent cells, flagellated cells, matrix producers, and spores. In this review, we have summarized the current understanding of the antimicrobial action of Bacillus CLPs under both experimental and natural conditions. We have also shown the relationship between some regulatory pathways involved in CLP biosynthesis and bacterial cell differentiation, as well as the role of CLPs as signaling molecules that determine changes in the physiological state of Bacillus subpopulations in response to shifts in environmental conditions.

Screening of Antagonistic Bacteria against Three Aquatic Pathogens and Characterization of Lipopeptides in Bacillus cereus BA09

Screening for antagonistic bacteria on aquatic pathogens and identification of antagonistic ingredients are essential to reduce the use of chemicals in aquaculture. In this study, strain BA09, subsequently identified as Bacillus cereus, simultaneously displayed strong antagonistic effects on Edwardsiella tarda, Vibrio harveyi, and Streptococcus anisopliae in the initial screening and rescreening. In addition, the methanol extract of BA09 was subjected to antibacterial activity verification and one-dimensional (1D) reversed-phase liquid chromatography (RPLC) preparation. A total of 27 fractions were collected, 6 of which were subjected to two-dimensional (2D) RPLC separation and tracked as antibacterial. A total of 14 lipopeptides that included 9 fengycin homologs, 3 bacillomycin homologs, and 2 surfactin homologs were identified by tandem high-resolution mass spectrometry. Through characterization of the antibacterial substance in Bacillus cereus BA09, which simultaneously inhibited E. tarda, V. harveyi, and S. agalactiae, the current study provides a theoretical basis for the development of antibacterial drugs in aquaculture.

Exploring plant growth promoting traits and biocontrol potential of new isolated Bacillus subtilis BS-2301 strain in suppressing Sclerotinia sclerotiorum through various mechanisms

Sclerotinia sclerotiorum (Lib.) de Bary is the causative agent of stem white mold disease which severely reduces major crop productivity including soybean and rapeseed worldwide. The current study aimed to explore plant growth-promoting traits and biocontrol of new isolated Bacillus subtilis BS-2301 to suppress S. sclerotiorum through various mechanisms. The results indicated that the BS-2301 exhibited strong biocontrol potential against S. sclerotiorum up to 74% both in dual culture and partition plate experiments. The BS-2301 and its crude extract significantly suppressed S. sclerotiorum growth involving excessive reactive oxygen species (ROS) production in mycelia for rapid death. Furthermore, the treated hyphae produced low oxalic acid (OA), a crucial pathogenicity factor of S. sclerotiorum. The SEM and TEM microscopy of S. sclerotiorum showed severe damage in terms of cell wall, cell membrane breakage, cytoplasm displacement, and organelles disintegration compared to control. The pathogenicity of S. sclerotiorum exposed to BS-2301 had less disease progression potential on soybean leaves in the detached leaf assay experiment. Remarkably, the strain also demonstrated broad-range antagonistic activity with 70%, and 68% inhibition rates against Phytophthora sojae and Fusarium oxysporum, respectively. Furthermore, the strain exhibits multiple plant growth-promoting and disease-prevention traits, including the production of indole-3-acetic acid (IAA), siderophores, amylases, cellulases and proteases as well as harboring calcium phosphate decomposition activity. In comparison to the control, the BS-2301 also showed great potential for enhancing soybean seedlings growth for different parameters, including shoot length 31.23%, root length 29.87%, total fresh weight 33.45%, and total dry weight 27.56%. The antioxidant enzymes like CAT, POD, SOD and APX under BS-2301 treatment were up-regulated in S. sclerotiorum infected plants along with the positive regulation of defense-related genes (PR1-2, PR10, PAL1, AOS, CHS, and PDF1.2). These findings demonstrate that the BS-2301 strain possesses a notable broad-spectrum biocontrol potential against different phytopathogens and provides new insight in suppressing S. sclerotiorum through various mechanisms. Therefore, BS-2301 will be helpful in the development of biofertilizers for sustainable agricultural practices.

Antibacterial insights into alternariol and its derivative alternariol monomethyl ether produced by a marine fungus

Alternaria alternata FB1 is a marine fungus identified as a candidate for plastic degradation in our previous study. This fungus has been recently shown to produce secondary metabolites with significant antimicrobial activity against various pathogens, including methicillin-resistant Staphylococcus aureus (MRSA) and the notorious aquaculture pathogen Vibrio anguillarum. The antibacterial compounds were purified and identified as alternariol (AOH) and its derivative, alternariol monomethyl ether (AME). We found that AOH and AME primarily inhibited pathogenic bacteria (MRSA or V. anguillarum) by disordering cell division and some other key physiological and biochemical processes. We further demonstrated that AOH could effectively inhibit the unwinding activity of MRSA topoisomerases, which are closely related to cell division and are the potential action target of AOH. The antibacterial activities of AOH and AME were verified by using zebrafish as the in vivo model. Notably, AOH and AME did not significantly affect the viability of normal human liver cells at concentrations that effectively inhibited MRSA or V. anguillarum. Finally, we developed the genetic operation system of A. alternata FB1 and blocked the biosynthesis of AME by knocking out omtI (encoding an O-methyl transferase), which facilitated A. alternata FB1 to only produce AOH. The development of this system in the marine fungus will accelerate the discovery of novel natural products and further bioactivity study.IMPORTANCEMore and more scientific reports indicate that alternariol (AOH) and its derivative alternariol monomethyl ether (AME) exhibit antibacterial activities. However, limited exploration of their detailed antibacterial mechanisms has been performed. In the present study, the antibacterial mechanisms of AOH and AME produced by the marine fungus Alternaria alternata FB1 were disclosed in vitro and in vivo. Given their low toxicity on the normal human liver cell line under the concentrations exhibiting significant antibacterial activity against different pathogens, AOH and AME are proposed to be good candidates for developing promising antibiotics against methicillin-resistant Staphylococcus aureus and Vibrio anguillarum. We also succeeded in blocking the biosynthesis of AME, which facilitated us to easily obtain pure AOH. Moreover, based on our previous results, A. alternata FB1 was shown to enable polyethylene degradation.

Characterization of a Novel Antimicrobial Peptide Bacipeptin against Foodborne Pathogens

The increasing emergence of multidrug-resistant pathogens and development of biopreservatives in food industries has increased the demand of novel and safe antimicrobial agents. In this study, a marine bacterial strain Bacillus licheniformis M1 was isolated and exhibited obvious antimicrobial activities against foodborne pathogens, especially against methicillin-resistant Staphylococcus aureus. The antimicrobial agent was purified and identified as a novel antimicrobial peptide, which was designated as bacipeptin, and the corresponding mechanism was further investigated by electron microscopy observation and transcriptomic analysis with biochemical validation. The results showed that bacipeptin could reduce the virulence of methicillin-resistant Staphylococcus aureus and exerted its antimicrobial activity by interfering with histidine metabolism, inducing the accumulation of reactive oxygen species and down-regulating genes related to Na+/H+ antiporter and the cell wall, thus causing damage to the cell wall and membrane. Overall, our study provides a novel natural product against foodborne pathogens and discloses the corresponding action mechanism.

Co-production of surfactin and fengycin by Bacillus subtilis BBW1542 isolated from marine sediment: a promising biocontrol agent against foodborne pathogens

Pathogenic bacteria contaminations and related diseases in food industries is an urgent issue to solve. The present study aimed to explore natural food biopreservatives from microorganisms. Using dilution-plate method, a strain BBW1542 with antimicrobial activities against various foodborne pathogenic bacteria was isolated from the seabed silt of Beibu Gulf, which was identified as Bacillus subtilis by the morphological observation and 16S rDNA sequences. The antimicrobial substances of B. subtilis BBW1542 exhibited an excellent stability under cool/heat treatment, UV irradiation, acid/alkali treatment, and protease hydrolysis. The genome sequencing analysis and antiSMASH prediction indicated that B. subtilis BBW1542 contained the gene cluster encoding lipopeptides and bacteriocin subtilosin A. MALDI-TOF-MS analysis showed that the lipopeptides from B. subtilis BBW1542 contained C14 and C15 surfactin homologues, together with fengycin homologues of C18 fengycin A/C16 fengycin B and C19 fengycin A/C17 fengycin B. In silico analysis showed that an eight-gene (sboA-albABCDEFG) operon was involved in the biosynthesis of subtilosin A in B. subtilis BBW1542, and the encoded subtilosin A presented an evident closed-loop structure containing 35 amino acids with a molecular weight of 3425.94 Da. Overall, the antagonistic B. subtilis BBW1542 displayed significant resource value and offered a promising alternative in development of food biopreservation.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13197-023-05864-3.

An insight into the utilization of microbial biosurfactants pertaining to their industrial applications in the food sector

Microbial biosurfactants surpass synthetic alternatives due to their biodegradability, minimal toxicity, selective properties, and efficacy across a wide range of environmental conditions. Owing to their remarkable advantages, biosurfactants employability as effective emulsifiers and stabilizers, antimicrobial and antioxidant attributes, rendering them for integration into food preservation, processing, formulations, and packaging. The biosurfactants can also be derived from various types of food wastes. Biosurfactants are harnessed across multiple sectors within the food industry, ranging from condiments (mayonnaise) to baked goods (bread, muffins, loaves, cookies, and dough), and extending into the dairy industry (cheese, yogurt, and fermented milk). Additionally, their impact reaches the beverage industry, poultry feed, seafood products like tuna, as well as meat processing and instant foods, collectively redefining each sector's landscape. This review thoroughly explores the multifaceted utilization of biosurfactants within the food industry as emulsifiers, antimicrobial, antiadhesive, antibiofilm agents, shelf-life enhancers, texture modifiers, and foaming agents.

Antibacterial Activity of Surfactin and Synergistic Effect with Conventional Antibiotics Against Methicillin-Resistant Staphylococcus aureus Isolated from Patients with Diabetic Foot Ulcers

Introduction

The prevalence of diabetic foot ulcers (DFUs) is increasing, leading to a huge financial burden and human suffering. Furthermore, antibiotic resistance is an urgent problem in the realm of clinical practice. Antimicrobial peptides are an effective and feasible strategy for combating infections caused by drug-resistant bacteria. Therefore, we investigated the in vitro antimicrobial ability of the lipopeptide surfactin, either alone or in combination with conventional antibiotics, against the standard and clinical strains of Staphylococcus aureus, including methicillin-resistant S. aureus (MRSA), isolated from patients with DFUs.

Methods

The minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) of surfactin on the selected strains were evaluated by a microbroth dilution technique. The growth curves of the selected strains with and without surfactin were measured, and transmission electron microscopy was used to observe the structure of surfactin-treated bacterial cells. The biofilm inhibitory abilities of surfactin were assessed by crystal violet staining. The antimicrobial interactions between surfactin and conventional antibiotics were established using a checkerboard assay, as well as determining the mutant prevention concentration. The inhibitory effect of surfactin on penicillinase was tested by iodometry.

Results

The MIC and MBC values of surfactin ranged from 512 to 1024 µg/mL and 1024 to 2048 µg/mL, respectively. Moreover, surfactin significantly prevented the S. aureus biofilm formation and displayed limited toxicity on human red blood cells. The synergies between surfactin and ampicillin, oxacillin, and tetracycline against S. aureus were revealed. In vitro resistance was not readily produced by surfactin. The action of surfactin may be by disrupting bacterial cell membranes and inhibiting penicillinase.

Conclusion

Surfactin appears to be a potential option for the treatment of DFUs infected with MRSA, as it is capable of improving antimicrobial activities and can be used alone or in combination with conventional antibiotics to prevent or postpone the emergence of resistance.

Bacterial and Fungal Biocontrol Agents for Plant Disease Protection: Journey from Lab to Field, Current Status, Challenges, and Global Perspectives

Plants are constantly exposed to various phytopathogens such as fungi, Oomycetes, nematodes, bacteria, and viruses. These pathogens can significantly reduce the productivity of important crops worldwide, with annual crop yield losses ranging from 20% to 40% caused by various pathogenic diseases. While the use of chemical pesticides has been effective at controlling multiple diseases in major crops, excessive use of synthetic chemicals has detrimental effects on the environment and human health, which discourages pesticide application in the agriculture sector. As a result, researchers worldwide have shifted their focus towards alternative eco-friendly strategies to prevent plant diseases. Biocontrol of phytopathogens is a less toxic and safer method that reduces the severity of various crop diseases. A variety of biological control agents (BCAs) are available for use, but further research is needed to identify potential microbes and their natural products with a broad-spectrum antagonistic activity to control crop diseases. This review aims to highlight the importance of biocontrol strategies for managing crop diseases. Furthermore, the role of beneficial microbes in controlling plant diseases and the current status of their biocontrol mechanisms will be summarized. The review will also cover the challenges and the need for the future development of biocontrol methods to ensure efficient crop disease management for sustainable agriculture.

Bacillus atrophaeus NX-12 Utilizes Exosmotic Glycerol from Fusarium oxysporum f. sp. cucumerinum for Fengycin Production

Bacillus strains are widely used as biological control agents to protect plants from fungal pathogens. However, whether Bacillus can exploit fungal pathogens to increase its biocontrol efficacy remains largely unexplored. Here, Bacillus atrophaeus NX-12 showed a high inhibition efficacy against Fusarium oxysporum f. sp. cucumerinum (FOC). The primary extracellular antifungal component of B. atrophaeus NX-12 was identified as fengycin by matrix-assisted laser desorption/ionization-time-of-flight-mass spectrometry (MALDI-TOF-MS) analysis. NX-12-secreted fengycin not only inhibited the germination of FOC spores but also induced the production of reactive oxygen species (ROS) in FOC cells, leading to oxidative stress and the accumulation of glycerol. Additionally, NX-12-secreted fengycin increased FOC cell wall hydrolase activity, leading to cell splitting and the exosmose of accumulated glycerol. The increased exosmose of glycerol further promoted the production of fengycin. Our results showed that in addition to the direct inhibition of FOC, NX-12 can indirectly strengthen its antagonistic efficacy against the pathogen by exploiting the exosmotic glycerol from FOC.

Systemically engineering Bacillus amyloliquefaciens for increasing its antifungal activity and green antifungal lipopeptides production

The biosynthesis of antifungal lipopeptides iturin and fengycin has attracted broad interest; however, there is a bottleneck in its low yield in wild strains. Because the key metabolic mechanisms in the lipopeptides synthesis pathway remain unclear, genetic engineering approaches are all ending up with a single or a few gene modifications. The aim of this study is to develop a systematic engineering approach to improve the antifungal activity and biosynthesis of iturin and fengycin in Bacillus amyloliquefaciens. First, blocking the carbon overflow metabolic pathway to increase precursor supply of the branched-chain amino acids by knockout of bdh, disrupting sporulation to extend the stage for producing antifungal lipopeptides by deletion of kinA, blocking of siderophore synthesis to enhance the availability of amino acids and fatty acids by deletion of dhbF, and increasing Spo0A∼P by deletion of rapA, could improve the antifungal activity by 24%, 10%, 13% and 18%, respectively. Second, the double knockout strain ΔbdhΔkinA, triple knockout strain ΔbdhΔkinAΔdhbF and quadruple knockout strain ΔkinAΔbdhΔdhbFΔrapA could improve the antifungal activity by 38%, 44% and 53%, respectively. Finally, overexpression of sfp in ΔkinAΔbdhΔdhbFΔrapA further increased the antifungal activity by 65%. After purifying iturin and fengycin as standards for quantitative analysis of lipopeptides, we found the iturin titer was 17.0 mg/L in the final engineered strain, which was 3.2-fold of the original strain. After fermentation optimization, the titer of iturin and fengycin reached 31.1 mg/L and 175.3 mg/L in flask, and 123.5 mg/L and 1200.8 mg/L in bioreactor. Compared to the original strain, the iturin and fengycin titer in bioreactor increased by 22.8-fold and 15.9-fold in the final engineered strain, respectively. This study may pave the way for the commercial production of green antifungal lipopeptides, and is also favorable for understanding the regulatory and biosynthetic mechanism of iturin and fengycin.