A Strong Antifungal Activity of 7-O-Succinyl Macrolactin A vs Macrolactin A from Bacillus amyloliquefaciens ELI149

Biocontrol mechanism of Bacillus siamensis sp. QN2MO-1 against tomato fusarium wilt disease during fruit postharvest and planting

Tomato fusarium wilt caused by Fusarium oxysporum f. sp. lycopersici (Fol) is a highly destructive disease, resulting in severe economic losses of global tomato production annually. An eco-friendly alternative to chemical fungicide using biological control agents (BCAs) is urgently needed. Here, Bacillus siamensis QN2MO-1 was isolated from Noli fruit and had a strong antagonistic activity against Fol in vitro and in vivo. Strain QN2MO-1 also exhibited a broad-spectrum antifungal activity against the selected 14 phytopathogenic fungi. The crude protein produced by strain QN2MO-1 could inhibit the spore germination of Fol and destroy the spore structure. It was closely related with the generation of chitinase and β-1,3-glucanase secreted by strain QN2MO-1. In a pot experiment, the application of B. siamensis QN2MO-1 effectively alleviated the yellowing and wilting symptoms of tomato plants. The disease index and incidence rate were decreased by 72.72% and 80.96%, respectively. The rhizospheric soil in tomato plants owed a high abundance of microbial community. Moreover, strain QN2MO-1 also enhanced the plant growth and improved the fruit quality of tomato. Therefore, B. siamensis QN2MO-1 will be explored as a potential biocontrol agent and biofertilizer.

Characterization of Bacillus velezensis 32a metabolites and their synergistic bioactivity against crown gall disease

Crown gall disease caused by Agrobacterium tumefaciens is considered to be the main bacterial threat of stone fruit plants in Mediterranean countries. In a previous study, Bacillus velezensis strain 32a was isolated from Tunisian rhizosphere soil and revealed high antagonistic potential against A. tumefaciens strains. In order to better characterize the antagonistic activity of this strain against this important plant pathogen, the production of secondary metabolites was analyzed using liquid chromatography coupled with mass spectrometry. The results revealed the production of different compounds identified as surfactins, fengycins, iturins and bacillibactin belonging to the lipopeptide group, three polyketides (macrolactins, oxydifficidin and bacillaenes), bacilysin and its chlorinated derivative; chlorotetaine. The involvement of lipopeptides in this antagonistic activity was ruled out by performing agar and broth dilution tests with pure molecules. Thus, the construction of B. velezensis 32a mutants defective in polyketides and bacilysin biosynthesis and their antagonistic activity was performed and compared to a set of derivative mutants of a comparable strain, B. velezensis GA1. The defective difficidin mutants (△dfnA and △dfnD) were unable to inhibit the growth of A. tumefaciens, indicating the high-level contribution of difficidin in the antagonism process. While the macrolactin deficient mutant (∆mlnA) slightly decreased the activity, suggesting a synergetic effect with difficidin. Remarkably, the mutant △dhbC only deficient in bacillibactin production showed significant reduction in its capacity to inhibit the growth of Agrobacterium.Taken collectively, our results showed the strong synergetic effect of difficidin and macrolactins and the significant implication of siderophore to manage crown gall disease.

The Prospect of Hydrolytic Enzymes from Bacillus Species in the Biological Control of Pests and Diseases in Forest and Fruit Tree Production

Plant diseases and insect pest damage cause tremendous losses in forestry and fruit tree production. Even though chemical pesticides have been effective in the control of plant diseases and insect pests for several decades, they are increasingly becoming undesirable due to their toxic residues that affect human life, animals, and the environment, as well as the growing challenge of pesticide resistance. In this study, we review the potential of hydrolytic enzymes from Bacillus species such as chitinases, β-1,3-glucanases, proteases, lipases, amylases, and cellulases in the biological control of phytopathogens and insect pests, which could be a more sustainable alternative to chemical pesticides. This study highlights the application potential of the hydrolytic enzymes from different Bacillus sp. as effective biocontrol alternatives against phytopathogens/insect pests through the degradation of cell wall/insect cuticles, which are mainly composed of structural polysaccharides like chitins, β-glucans, glycoproteins, and lipids. This study demonstrates the prospects for applying hydrolytic enzymes from Bacillus sp. as effective biopesticides in forest and fruit tree production, their mode of biocidal activity and dual antimicrobial/insecticidal potential, which indicates a great prospect for the simultaneous biocontrol of pests/diseases. Further research should focus on optimizing the production of hydrolytic enzymes, and the antimicrobial/insecticidal synergism of different Bacillus sp. which could facilitate the simultaneous biocontrol of pests and diseases in forest and fruit tree production.

Antifungal Effect of Bacillus velezensis ZN-S10 against Plant Pathogen Colletotrichum changpingense and Its Inhibition Mechanism

In order to optimize crop production and mitigate the adverse impacts associated with the utilization of chemical agents, it is necessary to explore new biocontrol agents. Bacillus velezensis has been widely studied as a biocontrol agent because of its efficient and ecofriendly plant disease control mechanisms. This study shows that the strain ZN-S10 effectively reduces the area of leaf spots caused by the pathogen Colletotrichum changpingense ZAFU0163-1, which affects conidia production and germination, inhibits mycelium growth, and induces mycelium deformation. In antifungal experiments with crude extracts, we observed a delay in the cell cycle of conidia, which may be responsible for the inhibition of conidial germination. Among the bioactive metabolites detected through integrated LC-MS- and GC-MS-based untargeted metabolomics, 7-O-Succinyl macrolactin A, telocinobufagin, and surfactin A may be the main antifungal metabolites of strain ZN-S10. The presence of 7-O-Succinyl macrolactin A could explain the cell damage in germ tubes. This is the first report of telocinobufagin detected in B. velezensis. These results are significant for understanding the inhibitory mechanisms employed by B. velezensis and should serve as a reference in the production of biocontrol agents.

Bacillus spp. as Bio-factories for Antifungal Secondary Metabolites: Innovation Beyond Whole Organism Formulations

Several fungi act as parasites for crops causing huge annual crop losses at both pre- and post-harvest stages. For years, chemical fungicides were the solution; however, their wide use has caused environmental contamination and human health problems. For this reason, the use of biofungicides has been in practice as a green solution against fungal phytopathogens. In the context of a more sustainable agriculture, microbial biofungicides have the largest share among the commercial biocontrol products that are available in the market. Precisely, the genus Bacillus has been largely studied for the management of plant pathogenic fungi because they offer a chemically diverse arsenal of antifungal secondary metabolites, which have spawned a heightened industrial engrossment of it as a biopesticide. In this sense, it is indispensable to know the wide arsenal that Bacillus genus has to apply these products for sustainable agriculture. Having this idea in our minds, in this review, secondary metabolites from Bacillus having antifungal activity are chemically and structurally described giving details of their action against several phytopathogens. Knowing the current status of Bacillus secreted antifungals is the base for the goal to apply these in agriculture and it is addressed in depth in the second part of this review.

Macrolactin a mediated biocontrol of Fusarium oxysporum and Rhizoctonia solani infestation on Amaranthus hypochondriacus by Bacillus subtilis BS-58

Plant diseases are one of the main hurdles for successful crop production and sustainable agriculture development world-wide. Though several chemical measures are available to manage crop diseases, many of them have serious side effects on humans, animals and the environment. Therefore, the use of such chemicals must be limited by using effective and eco-friendly alternatives. In view of the same, we found a Bacillus subtilis BS-58 as a good antagonist towards the two most devastating phytopathogens, i.e., Fusarium oxysporum and Rhizoctonia solani. Both the pathogens attack several agricultural crops (including amaranth) and induce a variety of infections in them. The findings of scanning electron microscopy (SEM) in this study suggested that B. subtilis BS-58 could inhibit the growth of both the pathogenic fungi by various means such as perforation, cell wall lysis, and cytoplasmic disintegration in the fungal hyphae. Thin-layer chromatography, LC–MS and FT-IR data revealed the antifungal metabolite to be macrolactin A with a molecular weight of 402 Da. Presence of the mln gene in the bacterial genome further endorsed that the antifungal metabolite produced by BS-58 was macrolactin A. Pot trial conducted in the present study showed that seed treatment by BS-58 effectively reduced seedling mortality (54.00 and 43.76%) in amaranth, when grown in pathogen infested soil (F. oxysporum and R. solani, respectively), when compared to their respective negative controls. Data also revealed that the disease suppression ability of BS-58 was almost equivalent to the recommended fungicide, carbendazim. SEM analysis of roots of the seedlings recovered from pathogenic attack substantiated the hyphal disintegration by BS-58 and prevention of amaranth crop. The findings of this study conclude that macrolactin A produced by B. subtilis BS-58 is responsible for the inhibition of both the phytopathogens and the suppression of the diseases caused by them. Being native and target specific, such strains under suitable conditions, may result in ample production of antibiotic and better suppression of the disease.

Sources of Antifungal Drugs

Due to their eukaryotic heritage, the differences between a fungal pathogen's molecular makeup and its human host are small. Therefore, the discovery and subsequent development of novel antifungal drugs are extremely challenging. Nevertheless, since the 1940s, researchers have successfully uncovered potent candidates from natural or synthetic sources. Analogs and novel formulations of these drugs enhanced the pharmacological parameters and improved overall drug efficiency. These compounds ultimately became the founding members of novel drug classes and were successfully applied in clinical settings, offering valuable and efficient treatment of mycosis for decades. Currently, only five different antifungal drug classes exist, all characterized by a unique mode of action; these are polyenes, pyrimidine analogs, azoles, allylamines, and echinocandins. The latter, being the latest addition to the antifungal armamentarium, was introduced over two decades ago. As a result of this limited arsenal, antifungal resistance development has exponentially increased and, with it, a growing healthcare crisis. In this review, we discuss the original sources of antifungal compounds, either natural or synthetic. Additionally, we summarize the existing drug classes, potential novel candidates in the clinical pipeline, and emerging non-traditional treatment options.

Biocontrol and plant growth promoting traits of two avocado rhizobacteria are orchestrated by the emission of diffusible and volatile compounds

Avocado (Persea americana Mill.) is a tree crop of great social and economic importance. However, the crop productivity is hindered by fast-spreading diseases, which calls for the search of new biocontrol alternatives to mitigate the impact of avocado phytopathogens. Our objectives were to evaluate the antimicrobial activity of diffusible and volatile organic compounds (VOCs) produced by two avocado rhizobacteria (Bacillus A8a and HA) against phytopathogens Fusarium solani, Fusarium kuroshium, and Phytophthora cinnamomi, and assess their plant growth promoting effect in Arabidopsis thaliana. We found that, in vitro, VOCs emitted by both bacterial strains inhibited mycelial growth of the tested pathogens by at least 20%. Identification of bacterial VOCs by gas chromatography coupled to mass spectrometry (GC-MS) showed a predominance of ketones, alcohols and nitrogenous compounds, previously reported for their antimicrobial activity. Bacterial organic extracts obtained with ethyl acetate significantly reduced mycelial growth of F. solani, F. kuroshium, and P. cinnamomi, the highest inhibition being displayed by those from strain A8a (32, 77, and 100% inhibition, respectively). Tentative identifications carried out by liquid chromatography coupled to accurate mass spectrometry of diffusible metabolites in the bacterial extracts, evidenced the presence of some polyketides such as macrolactins and difficidin, hybrid peptides including bacillaene, and non-ribosomal peptides such as bacilysin, which have also been described in Bacillus spp. for antimicrobial activities. The plant growth regulator indole-3-acetic acid was also identified in the bacterial extracts. In vitro assays showed that VOCs from strain HA and diffusible compounds from strain A8a modified root development and increased fresh weight of A. thaliana. These compounds differentially activated several hormonal signaling pathways involved in development and defense responses in A. thaliana, such as auxin, jasmonic acid (JA) and salicylic acid (SA); genetic analyses suggested that developmental stimulation of the root system architecture by strain A8a was mediated by the auxin signaling pathway. Furthermore, both strains were able to enhance plant growth and decreased the symptoms of Fusarium wilt in A. thaliana when soil-inoculated. Collectively, our results evidence the potential of these two rhizobacterial strains and their metabolites as biocontrol agents of avocado pathogens and as biofertilizers.

Bacillus Metabolites: Compounds, Identification and Anti-Candida albicans Mechanisms

Candida albicans seriously threatens human health, especially for immunosuppressed groups. The antifungal agents mainly include azoles, polyenes and echinocandins. However, the few types of existing antifungal drugs and their resistance make it necessary to develop new antifungal drugs. Bacillus and its metabolites has antifungal activity against pathogenic fungi. This review introduces the application of Bacillus metabolites in the control of C. albicans in recent years. Firstly, several compounds produced by Bacillus spp. are listed. Then the isolation and identification techniques of Bacillus metabolites in recent years are described, including high-precision separation technology and omics technology for the separation of similar components of Bacillus metabolites. The mechanisms of Bacillus metabolites against C. albicans are distinguished from the inhibition of pathogenic fungi and inhibition of the fungal virulence factors. The purpose of this review is to systematically summarize the recent studies on the inhibition of pathogenic fungi by Bacillus metabolites. The review is expected to become the reference for the control of pathogenic fungi such as C. albicans and the application of Bacillus metabolites in the future.

Genomic insights on fighting bacterial wilt by a novel Bacillus amyloliquefaciens strain Cas02

Bacterial wilt, caused by the Ralstonia solanacearum, can infect several economically important crops. However, the management strategies available to control this disease are limited. Plant growth-promoting rhizobacteria (PGPR) have been considered promising biocontrol agents. In this study, Bacillus amyloliquefaciens strain Cas02 was isolated from the rhizosphere soil of healthy tobacco plants and evaluated for its effect on plant growth promotion and bacterial wilt suppression. Strain Cas02 exhibited several growth-promoting-related features including siderophore production, cellulase activity, protease activity, ammonia production and catalase activity. Moreover, strain Cas02 showed a significant inhibitory growth effect on R. solanacearum, and its active substances were separated and identified to be macrolactin A and macrolactin W by HPLC-DAD-ESI-MS/MS. Both greenhouse and field experiments demonstrated a good performance of Cas02 in plant growth promotion and bacterial wilt suppression. To explore the underlying genetic mechanisms, complete genome sequencing was performed and the gene clusters responsible for antibacterial metabolites expression were identified. Overall, these findings suggest that the strain Cas02 could be a potential biocontrol agent in bacterial wilt management and a source of antimicrobial compounds for further exploitation.

Macrolactin A as a Novel Inhibitory Agent for SARS-CoV-2 Mpro: Bioinformatics Approach

COVID-19 is a disease that puts most of the world on lockdown and the search for therapeutic drugs is still ongoing. Therefore, this study used in silico screening to identify natural bioactive compounds from fruits, herbaceous plants, and marine invertebrates that are able to inhibit protease activity in SARS-CoV-2 (PDB: 6LU7). We have used extensive screening strategies such as drug likeliness, antiviral activity value prediction, molecular docking, ADME, molecular dynamics (MD) simulation, and MM/GBSA. A total of 17 compounds were shortlisted using Lipinski’s rule in which 5 compounds showed significant predicted antiviral activity values. Among these 5, only 2 compounds, Macrolactin A and Stachyflin, showed good binding energy of −9.22 and −8.00 kcal/mol, respectively, within the binding pocket of the M^pro catalytic residues (HIS 41 and CYS 145). These two compounds were further analyzed to determine their ADME properties. The ADME evaluation of these 2 compounds suggested that they could be effective in developing therapeutic drugs to be used in clinical trials. MD simulations showed that protein–ligand complexes of Macrolactin A and Stachyflin with the target receptor (6LU7) were stable for 100 nanoseconds. The MM/GBSA calculations of M^pro–Macrolactin A complex indicated higher binding free energy (−42.58 ± 6.35 kcal/mol). Dynamic cross-correlation matrix (DCCM) and principal component analysis (PCA) on the residual movement in the MD trajectories further confirmed the stability of Macrolactin A bound state with 6LU7. In conclusion, this study showed that marine natural compound Macrolactin A could be an effective therapeutic inhibitor against SARS-CoV-2 protease (6LU7). Additional in vitro and in vivo validations are strongly needed to determine the efficacy and therapeutic dose of Macrolactin A in biological systems.

Recent advancements for microorganisms and their natural compounds useful in agriculture

During the past years, microorganisms have been the cause of many problems for human's health. However, today with the development of many techniques of microbiology, the researchers have studied several roles of microorganisms which may help the society. Microbial-based products are expected to play important role in agriculture-enhancing plant production and therefore increasing crop's yieldeswani et al. . Microorganisms can act by several action mechanisms including antibiosis or mechanisms in plant-microbe interactions underlining the dual function of microbial strains toward plant nutrition and protection. The market has increased with the development of microbial-based products. Currently, it is normal to think that microorganisms help us in agriculture by applying them as biological control. In this mini review, we collect the last findings about this topic including very recent literature. KEY POINTS: • Microorganisms play a beneficial role in agriculture by different mechanisms. • One of these mechanisms is the secretion of chemical compounds with different activities.