Volatile organic compounds of Bacillus atrophaeus HAB-5 inhibit the growth of Colletotrichum gloeosporioides

Comparative genomic analysis of Bacillus atrophaeus HAB-5 reveals genes associated with antimicrobial and plant growth-promoting activities

Bacillus atrophaeus HAB-5 is a plant growth-promoting rhizobacterium (PGPR) that exhibits several biotechnological traits, such as enhancing plant growth, colonizing the rhizosphere, and engaging in biocontrol activities. In this study, we conducted whole-genome sequencing of B. atrophaeus HAB-5 using the single-molecule real-time (SMRT) sequencing platform by Pacific Biosciences (PacBio; United States), which has a circular chromosome with a total length of 4,083,597 bp and a G + C content of 44.21%. The comparative genomic analysis of B. atrophaeus HAB-5 with other strains, Bacillus amyloliquefaciens DSM7, B. atrophaeus SRCM101359, Bacillus velezensis FZB42, B. velezensis HAB-2, and Bacillus subtilis 168, revealed that these strains share 2,465 CDSs, while 599 CDSs are exclusive to the B. atrophaeus HAB-5 strain. Many gene clusters in the B. atrophaeus HAB-5 genome are associated with the production of antimicrobial lipopeptides and polypeptides. These gene clusters comprise distinct enzymes that encode three NRPs, two Transat-Pks, one terpene, one lanthipeptide, one T3PKS, one Ripp, and one thiopeptide. In addition to the likely IAA-producing genes (trpA, trpB, trpC, trpD, trpE, trpS, ywkB, miaA, and nadE), there are probable genes that produce volatile chemicals (acoA, acoB, acoR, acuB, and acuC). Moreover, HAB-5 contained genes linked to iron transportation (fbpA, fetB, feuC, feuB, feuA, and fecD), sulfur metabolism (cysC, sat, cysK, cysS, and sulP), phosphorus solubilization (ispH, pstA, pstC, pstS, pstB, gltP, and phoH), and nitrogen fixation (nif3-like, gltP, gltX, glnR, glnA, nadR, nirB, nirD, nasD, narl, narH, narJ, and nark). In conclusion, this study provides a comprehensive genomic analysis of B. atrophaeus HAB-5, pinpointing the genes and genomic regions linked to the antimicrobial properties of the strain. These findings advance our knowledge of the genetic basis of the antimicrobial properties of B. atrophaeus and imply that HAB-5 may employ a variety of commercial biopesticides and biofertilizers as a substitute strategy to increase agricultural output and manage a variety of plant diseases.

Uncovering the antifungal activities of wild apple-associated bacteria against two canker-causing fungi, Cytospora mali and C. parasitica

Cytospora canker has become a devastating disease of apple species worldwide, and in severe cases, it may cause dieback of entire trees. The aim of this study was to characterize the diversity of cultivable bacteria from the wild apple microbiota and to determine their antifungal ability against the canker-causing pathogenic fungi Cytospora mali and C. parasitica. Five bacterial strains belonging to the species Bacillus amyloliquefaciens, B. atrophaeus, B. methylotrophicus, B. mojavensis, and Pseudomonas synxantha showed strong antagonistic effects against pathogenic fungi. Therefore, since the abovementioned Bacillus species produce known antifungal compounds, we characterized the antifungal compounds produced by Ps. synxantha. Bacteria grown on nutritional liquid medium were dehydrated, and the active compound from the crude extract was isolated and analysed via a range of chromatographic processes. High-performance liquid chromatography, mass spectrometry, and nuclear magnetic resonance analyses revealed a bioactive antifungal compound, phenazine-1-carboxylic acid (PCA). The minimum inhibitory concentration (MIC) demonstrated that PCA inhibited mycelial growth, with a MIC of 10 mg mL-1. The results suggested that PCA could be used as a potential compound to control C. mali and C. malicola, and it is a potential alternative for postharvest control of canker disease.

Identification of bioactive compounds of Bacillus velezensis HNA3 that contribute to its dual effects as plant growth promoter and biocontrol against post-harvested fungi

IMPORTANCE

The current study is an extension to our previous work on the plant growth-promoting rhizobacteria (PGPR) Bacillus velezensis HNA3 strain, which comes to confirm and reveals the huge stock of active secondary metabolites produced by HNA3. HNA3-emitted volatile organic compounds (VOCs) have demonstrated the capacity to impede the growth of phytopathogens affecting some fruits and vegetables, even in the absence of direct contact. Additionally, these volatiles enhanced soybean seed germination by breaking seed dormancy and inducing root system development. Furthermore, they promoted seedling growth, giving it prominence in soybean cultivation. The relevance of active volatiles derives from the fact that they can be developed as natural-safe biocontrol agents and plant promoters. This research validates the remarkable bioactivities exhibited by the Bacillus velezensis HNA3 and their potential applications in agriculture as an inoculant, encompassing biocontrol, plant growth promotion, and seed germination activities, thereby offering a safer alternative to hazardous chemicals.

Assessment of soil yeasts Papiliotrema laurentii S-08 and Saitozyma podzolica S-77 for plant growth promotion and biocontrol of Fusarium wilt of brinjal

AIM

This study aimed to determine the efficacy of the soil yeasts Papiliotrema laurentii S-08 and Saitozyma podzolica S-77 for plant growth promotion (PGP) and biocontrol of wilt disease in brinjal plants while applying yeasts individually or as a consortium in pot experiments.

METHODS AND RESULTS

The yeasts were tested for various PGP characteristics and antagonistic activity against the phytopathogen Fusarium oxysporum f. sp. melongenae. Both the yeast isolates demonstrated some PGP attributes as well as inhibited the growth of the phytopathogen. A gas chromatography-mass spectrometry analysis of the yeast metabolites revealed the presence of several antifungal compounds. The pot experiment performed under nursery conditions showed that applying these yeasts, individually or in consortium, decreased the percent disease incidence in brinjal seedlings while significantly enhancing their growth parameters.

CONCLUSION

Papiliotrema laurentii S-08 and S. podzolica S-77 can be used in brinjal plants as plant growth promoters and also as biocontrol agents against the brinjal wilt disease.

Exploration of the Biocontrol Activity of Bacillus atrophaeus Strain HF1 against Pear Valsa Canker Caused by Valsa pyri

Valsa pyri-induced pear Valsa canker is among the most prevalent diseases to impact pear quality and yields. Biocontrol strategies to control plant disease represent an attractive alternative to the application of fungicides. In this study, the potential utility of Bacillus atrophaeus strain HF1 was assessed as a biocontrol agent against pear Valsa canker. Strain HF1 suppressed V. pyri mycelium growth by 61.20% and induced the development of malformed hyphae. Both culture filtrate and volatile organic compounds (VOCs) derived from strain HF1 were able to antagonize V. pyri growth. Treatment with strain HF1-derived culture filtrate or VOCs also induced the destruction of hyphal cell membranes. Headspace mixtures prepared from strain HF1 were analyzed, leading to the identification of 27 potential VOCs. Of the thirteen pure chemicals tested, iberverin, hexanoic acid, and 2-methylvaleraldehyde exhibited the strongest antifungal effects on V. pyri, with respective EC50 values of 0.30, 6.65, and 74.07 μL L-1. Fumigation treatment of pear twigs with each of these three compounds was also sufficient to prevent the development of pear Valsa canker. As such, these results demonstrate that B. atrophaeus strain HF1 and the volatile compounds iberverin, hexanoic acid, and 2-methylvaleraldehyde exhibit promise as novel candidate biocontrol agents against pear Valsa canker.

Extraction and characterization of bioactive secondary metabolites from lactic acid bacteria and evaluating their antifungal and antiaflatoxigenic activity

Aflatoxins are toxic carcinogens and mutagens formed by some moulds, specifically Aspergillus spp. Therefore, this study aimed to extract and identify bioactive secondary metabolites from Lactobacillus species, to evaluate their efficacy in reducing fungal growth and aflatoxin production and to investigate their toxicity. The bioactive secondary metabolites of Lactobacillus species showed variable degrees of antifungal activity, whereas L. rhamnosus ethyl acetate extract No. 5 exhibited the highest antifungal activity and, thus, was selected for further identification studies. Data revealed that L. rhamnosus ethyl acetate extract No. 5 produced various organic acids, volatile organic compounds and polyphenols, displayed antifungal activity against A. flavus, and triggered morphological changes in fungal conidiophores and conidiospores. L. rhamnosus ethyl acetate extract No. 5 at a 9 mg/mL concentration reduced AFB₁ production by 99.98%. When the effect of L. rhamnosus ethyl acetate extract No. 5 on brine shrimp mortality was studied, the extract attained a 100% mortality at a concentration of 400 µg/mL, with an IC₅₀ of 230 µg/mL. Meanwhile, a mouse bioassay was performed to assess the toxicity of L. rhamnosus ethyl acetate extract No. 5, whereas there were no harmful effects or symptoms in mice injected with L. rhamnosus ethyl acetate extract at concentrations of 1, 3, 5, 7, and 9 mg/kg body weight.

Volatile Organic Compounds of Bacillus velezensis GJ-7 against Meloidogyne hapla through Multiple Prevention and Control Modes

The Bacillus velezensis GJ-7 strain isolated from the rhizosphere soil of Panax notoginseng showed high nematicidal activity and therefore has been considered a biological control agent that could act against the root-knot nematode Meloidogyne hapla. However, little was known about whether the GJ-7 strain could produce volatile organic compounds (VOCs) that were effective in biocontrol against M. hapla. In this study, we evaluated the nematicidal activity of VOCs produced by the fermentation of GJ-7 in three-compartment Petri dishes. The results revealed that the mortality rates of M. hapla J2s were 85% at 24 h and 97.1% at 48 h after treatment with the VOCs produced during GJ-7 fermentation. Subsequently, the VOCs produced by the GJ-7 strain were identified through solid-phase micro-extraction gas chromatography mass spectrometry (SPME-GC/MS). Six characteristic VOCs from the GJ-7 strain fermentation broth were identified, including 3-methyl-1-butanol, 3-methyl-2-pentanone, 5-methyl-2-hexanone, 2-heptanone, 2,5-dimethylpyrazine, and 6-methyl-2-heptanone. The in vitro experimental results from 24-well culture plates showed that the six volatiles had direct-contact nematicidal activity against M. hapla J2s and inhibition activity against egg hatching. In addition, 3-methyl-1-butanol and 2-heptanone showed significant fumigation effects on M. hapla J2s and eggs. Furthermore, all six of the VOCs repelled M. hapla J2 juveniles in 2% water agar Petri plates. The above data suggested that the VOCs of B. velezensis GJ-7 acted against M. hapla through multiple prevention and control modes (including direct-contact nematicidal activity, fumigant activity, and repellent activity), and therefore could be considered as potential biocontrol agents against root-knot nematodes.

Bacillus VOCs in the Context of Biological Control

A contemporary agricultural production system relying on heavy usage of agrochemicals represents a questionable outlook for sustainable food supply in the future. The visible negative environmental impacts and unforeseen consequences to human and animal health have been requiring a shift towards the novel eco-friendly alternatives for chemical pesticides for a while now. Microbial-based biocontrol agents have shown a promising potential for plant disease management. The bacteria of the genus Bacillus have been among the most exploited microbial active components due to several highly efficient mechanisms of action against plant pathogens, as well as a palette of additional plant-beneficial mechanisms, together with their suitable properties for microbial biopesticide formulations. Among other bioactive metabolites, volatile organic compounds (VOCs) have been investigated for their biocontrol applications, exhibiting the main advantage of long-distance effect without the necessity for direct contact with plants or pathogens. The aim of this study is to give an overview of the state-of-the-art in the field of Bacillus-based VOCs, especially in terms of their antibacterial, antifungal, and nematicidal action as the main segments determining their potential for biocontrol applications in sustainable agriculture.

Production of Bacillus subtilis soil isolate as biocontrol agent under bioreactor conditions

This study aimed to investigate the biomass production of Bacillus subtilis in flask and bioreactor conditions. It is necessary to carry the culture from the shake flask to the pH, air, temperature and stirring controlled bioreactor in order to reduce the working time and increase the production efficiency and product quality. In this study, Bacillus was isolated from soil and grown under flask and bioreactor conditions as biocontrol agent against Botrytis cinerea and Fusarium oxysporum. In this process, a pH value of 7.5, 100% O₂ saturation, 30% dissolved O₂, at the temperature of 37 °C, total flow of 0.1 Lmin^-1 and mixing speed of 150 min^-1 were preferred for optimal concerning high production yield of B. subtilis in bioreactor. To test whether B. subtilis has antifungal activity on the growth of B. cinerea and F. oxysporum, a dual culture assay in a PDA medium was carried out. Ultimately, high biomass production in a short incubation period by reaching 2.2 µg/mL after 9 h in the bioreactor. It was observed that the bacteria produced in the bioreactor cultivation grew stronger and showed high antifungal activity which resulted 33.33% inhibition percentage against B. cinerea. It was concluded that B. subtilis can be used as a green-fungicide against B. cinerea and F. oxysporum, and bacterial metabolites from B. subtilis could pave the way for the development of next generation green/biopesticides.

Fumigant Activity of Bacterial Volatile Organic Compounds against the Nematodes Caenorhabditis elegans and Meloidogyne incognita

Plant-parasitic nematodes infect a diversity of crops, resulting in severe economic losses in agriculture. Microbial volatile organic compounds (VOCs) are potential agents to control plant-parasitic nematodes and other pests. In this study, VOCs emitted by a dozen bacterial strains were analyzed using solid-phase microextraction followed by gas chromatography-mass spectrometry. Fumigant toxicity of selected VOCs, including dimethyl disulfide (DMDS), 2-butanone, 2-pentanone, 2-nonanone, 2-undecanone, anisole, 2,5-dimethylfuran, glyoxylic acid, and S-methyl thioacetate (MTA) was then tested against Caenorhabditis elegans. DMDS and MTA exhibited much stronger fumigant toxicity than the others. Probit analysis suggested that the values of LC₅₀ were 8.57 and 1.43 μg/cm³ air for DMDS and MTA, respectively. MTA also showed stronger fumigant toxicity than DMDS against the root-knot nematode Meloidogyne incognita, suggesting the application potential of MTA.

Microbial volatile organic compounds: Antifungal mechanisms, applications, and challenges

The fungal decay of fresh fruits and vegetables annually generates substantial global economic losses. The utilization of conventional synthetic fungicides is damaging to the environment and human health. Recently, the biological control of post-harvest fruit and vegetable diseases via antagonistic microorganisms has become an attractive possible substitution for synthetic fungicides. Numerous studies have confirmed the potential of volatile organic compounds (VOCs) for post-harvest disease management. Moreover, VOC emission is a predominant antifungal mechanism of antagonistic microorganisms. As such, it is of great significance to discuss and explore the antifungal mechanisms of microbial VOCs for commercial application. This review summarizes the main sources of microbial VOCs in the post-harvest treatment and control of fruit and vegetable diseases. Recent advances in the elucidation of antifungal VOC mechanisms are emphasized, and the applications of VOCs produced from antagonistic microorganisms are described. Finally, the current prospects and challenges associated with microbial VOCs are considered.

Antifungal Potential of Bacillus velezensis CE 100 for the Control of Different Colletotrichum Species through Isolation of Active Dipeptide, Cyclo-(D-phenylalanyl-D-prolyl)

Colletotrichum species are important fungal pathogens causing anthracnose of tropical and subtropical fruit and vegetable crops. Dual culture assay indicated that Bacillus velezensis CE 100 was a strong antagonist against C. acutatum, C. coccodes, C. dematium, and C. gloeosporioides. The volatile organic compounds produced by B. velezensis CE 100 affected mycelial growth of Colletotrichum species tested in our study and caused twisted hyphal structures of all these fungal species. Chloroform crude compounds of B. velezensis CE 100 inhibited four Colletotrichum species in a concentration-dependent manner and induced severe damage in hyphal morphology of these fungal pathogens, including swelling, bulging, and multiple branching. Moreover, the active cyclic dipeptide, cyclo-(D-phenylalanyl-D-prolyl), was isolated from chloroform crude extract and identified by nuclear magnetic resonance (NMR) and mass spectrometry. The inhibitory effect of cyclo-(D-phenylalanyl-D-prolyl) on conidial germination of C. gloeosporioides occurred in a concentration-dependent manner. The conidial germination rate was completely inhibited by a concentration of 3 mg/mL of cyclo-(D-phenylalanyl-D-prolyl). Scanning electron micrographs revealed that the exposure to cyclic dipeptide resulted in seriously deformed hyphae and conidia with shriveled surfaces in dipeptide-treated C. gloeosporioides. Therefore, active dipeptide-producing B. velezensis CE 100 is a promising biocontrol agent for Colletotrichum species causing anthracnose.

Application Potential of Bacterial Volatile Organic Compounds in the Control of Root-Knot Nematodes

Plant-parasitic nematodes (PPNs) constitute the most damaging group of plant pathogens. Plant infections by root-knot nematodes (RKNs) alone could cause approximately 5% of global crop loss. Conventionally, chemical-based methods are used to control PPNs at the expense of the environment and human health. Accordingly, the development of eco-friendly and safer methods has been urged to supplement or replace chemical-based methods for the control of RKNs. Using microorganisms or their metabolites as biological control agents (BCAs) is a promising approach to controlling RKNs. Among the metabolites, volatile organic compounds (VOCs) have gained increasing attention because of their potential in the control of not only RKNs but also other plant pathogens, such as insects, fungi, and bacteria. This review discusses the biology of RKNs as well as the status of various control strategies. The discovery of VOCs emitted by bacteria from various environmental sources and their application potential as BCAs in controlling RKNs are specifically addressed.

Occurrence, Diversity, and Character of Bacillaceae in the Solid Fermentation Process of Strong Aromatic Liquors

Strong aromatic liquors, also known as strong aromatic Baijiu (SAB) in China, are manufactured by solid fermentation, with a multi-microbe mixing and cooperative fermentation process that uses Daqu as a brewing starter. Bacillaceae have a specific action in food fermentation, such as soybean and wine, and more recent studies have found Bacillaceae play important roles in the SAB making industry. This review describes the diversity, functionality, and influence of Bacillaceae in Daqu, pit mud, Zaopei, Huangshui within making processes of SAB. Furthermore, aromatic flavor components from the Bacillaceae metabolism of SAB are discussed in this review. Ultimately, the resulting improvements and deeper understanding will benefit practical efforts to apply representatives of Bacillaceae in improving the quality of SAB as well as biological control of the micro-ecological environment of brewing.

Biocontrol Activity of Aspergillus terreus ANU-301 against Two Distinct Plant Diseases, Tomato Fusarium Wilt and Potato Soft Rot

To screen antagonistic fungi against plant pathogens, dual culture assay (DCA) and culture filtrate assay (CFA) were performed with unknown soil-born fungi. Among the different fungi isolated and screened from the soil, fungal isolate ANU-301 successfully inhibited growth of different plant pathogenic fungi, Colletotrichum acutatum, Alternaria alternata, and Fusarium oxysporum, in DCA and CFA. Morphological characteristics and rDNA internal transcribed spacer sequence analysis identified ANU-301 as Aspergillus terreus. Inoculation of tomato plants with Fusarium oxysporum f. sp. lycopersici (FOL) induced severe wilting symptom; however, co-inoculation with ANU-301 significantly enhanced resistance of tomato plants against FOL. In addition, culture filtrate (CF) of ANU-301 not only showed bacterial growth inhibition activity against Dickeya chrysanthemi (Dc), but also demonstrated protective effect in potato tuber against soft rot disease. Gas chromatography-tandem mass spectrometry analysis of CF of ANU-301 identified 2,4-bis(1-methyl-1-phenylethyl)-phenol (MPP) as the most abundant compound. MPP inhibited growth of Dc, but not of FOL, in a dose-dependent manner, and protected potato tuber from the soft rot disease induced by Dc. In conclusion, Aspergillus terreus ANU-301 could be used and further tested as a potential biological control agent.

Biocontrol Potential of Endophytic Streptomyces malaysiensis 8ZJF-21 From Medicinal Plant Against Banana Fusarium Wilt Caused by Fusarium oxysporum f. sp. cubense Tropical Race 4

Banana (Musa spp.) is an important fruit crop cultivated in most tropical countries. Banana Fusarium wilt caused by Fusarium oxysporum f. sp. cubense tropical race 4 (Foc TR4) is the most destructive fungal disease. Biocontrol using endophytic microorganisms is considered as a safety and sustainable strategy. Actinomycetes have a potential for the production of diverse metabolites. Isolation of endophytic actinomycetes with high efficiency and broad-spectrum antagonism is key for exploring biocontrol agents. Our previous study showed that a total of 144 endophytic actinomycetes were isolated from different tissues of medicinal plants in Hainan, China. Especially, strain 8ZJF-21 exhibited a broad-spectrum antifungal activity. Its morphological, physiological, and biochemical characteristics were consistent with the genus Streptomyces. The phylogenetic tree demonstrated that strain 8ZJF-21 formed a distinct clade with Streptomyces malaysiensis. Average nucleotide identity (ANI) was 98.49% above the threshold of novel species. The pot experiment revealed that endophytic Streptomyces malaysiensis 8ZJF-21 could improve the plant resistance to Foc TR4 by enhancing the expression levels of defense-related and antioxidant enzyme genes. It also promoted the plant growth by producing several extracellular enzymes and metabolites. Antifungal mechanism assays showed that S. malaysiensis 8ZJF-21 extract inhibited mycelial growth and spore germination of Foc TR4 in vitro. Pathogenic cells occurred cytoplasmic heterogeneity, disappeared organelles, and ruptured ultrastructure. Sequencing and annotation of genome suggested that S. malaysiensis 8ZJF-21 had a potential of producing novel metabolites. Nineteen volatile organic compounds were obtained from the extract by Gas Chromatography-Mass Spectrometry (GC-MS). Hence, endophytic Streptomyces strains will become essential biocontrol agents of modern agricultural practice.

Identification of Volatile Organic Compounds in Extremophilic Bacteria and Their Effective Use in Biocontrol of Postharvest Fungal Phytopathogens

Phytopathogenic fungal growth in postharvest fruits and vegetables is responsible for 20-25% of production losses. Volatile organic compounds (VOCs) have been gaining importance in the food industry as a safe and ecofriendly alternative to pesticides for combating these phytopathogenic fungi. In this study, we analysed the ability of some VOCs produced by strains of the genera Bacillus, Peribacillus, Pseudomonas, Psychrobacillus and Staphylococcus to inhibit the growth of Alternaria alternata, Botrytis cinerea, Fusarium oxysporum, Fusarium solani, Monilinia fructicola, Monilinia laxa and Sclerotinia sclerotiorum, in vitro and in vivo. We analysed bacterial VOCs by using GC/MS and 87 volatile compounds were identified, in particular acetoin, acetic acid, 2,3-butanediol, isopentanol, dimethyl disulphide and isopentyl isobutanoate. In vitro growth inhibition assays and in vivo experiments using cherry fruits showed that the best producers of VOCs, Bacillus atrophaeus L193, Bacillus velezensis XT1 and Psychrobacillus vulpis Z8, exhibited the highest antifungal activity against B. cinerea, M. fructicola and M. laxa, which highlights the potential of these strains to control postharvest diseases. Transmission electron microscopy micrographs of bacterial VOC-treated fungi clearly showed antifungal activity which led to an intense degeneration of cellular components of mycelium and cell death.

Antifungal Activity and Biocontrol Mechanism of Fusicolla violacea J-1 against Soft Rot in Kiwifruit Caused by Alternaria alternata

Alternaria alternata is the main pathogenic species of various crops, including kiwifruit (Actinidia cinensis). In this study, an antagonistic fungus, J-1, with high antifungal activity against A. alternata was isolated from A. cinensis “Hongyang.” The strain J-1 was identified as Fusicolla violacea via morphological identification and DNA sequencing. This study aimed to evaluate the antifungal activity and potential mechanism of the strain J-1 against A. alternata. The strain J-1 exhibited antifungal activity against A. alternata, with an inhibition rate of 66.1% in vitro. Aseptic filtrate (AF) produced by the strain J-1 could suppress the mycelial growth and conidia germination of A. alternata at the inhibition rates of 66.8% and 80%, respectively, as well as suppress the spread of Alternaria rot in fresh kiwifruit. We observed that many clusters of spherical protrusions appeared at the mycelial tips of A. alternata after treatment with 200 mL L⁻¹ AF of J-1. Scanning electron microscopy analysis results showed that the mycelial structures were bent and/or malformed and the surfaces were rough and protuberant. Variations in temperature, pH, and storage time had little effect on the antifungal activity of the AF. Moreover, the AF could damage the integrity of cell membranes and cause intracellular content leakage. Meanwhile, the chitinase and β-1,3-glucanase enzyme activities increased significantly, indicating that the function of A. alternata cell wall was seriously injured. Eleven antimicrobial metabolites were identified by gas chromatography–mass spectrometry (GC–MS). The strain J-I and its AF exhibited well broad-spectrum antifungal activity against Diaporthe eres, Epicoccum sorghinum, Fusarium graminearum, Phomopsis sp., and Botryosphaeria dothidea, with inhibition rates ranging from 34.4% to 75.1% and 42.7% to 75.2%, respectively. Fusicolla violacea J-1 is a potential biocontrol agent against A. alternata and other fungal phytopathogens.

Bacterial volatile compound-based tools for crop management and quality

Bacteria produce a huge diversity of metabolites, many of which mediate ecological relations. Among these, volatile compounds cause broad-range effects at low doses and, therefore, may be exploited for plant defence strategies and agricultural production, but such applications are still in their early development. Here, we review the latest technologies involving the use of bacterial volatile compounds for phytosanitary inspection, biological control, plant growth promotion, and crop quality. We highlight a variety of effects with a potential applicative interest, based on either live biocontrol and/or biostimulant agents, or the isolated metabolites responsible for the interaction with hosts or competitors. Future agricultural technologies may benefit from the development of new analytical tools to understand bacterial interactions with the environment.

Biocontrol Activity of Bacillus megaterium BM344-1 against Toxigenic Fungi

Mycotoxins are secondary metabolites of some fungal species and represent important contaminants of food and feed. This study aimed to explore the biological control activity of Bacillus megaterium BM344-1 volatile organic compounds (VOCs) on the growth and mycotoxin production of single representatives of the toxigenic species Aspergillus flavus, Aspergillus carbonarius, Penicillium verrucosum, and Fusarium verticillioides. In vitro co-incubation experiments indicated the P. verrucosum isolate as the most sensitive one, with a growth inhibition ratio of 66.7%, followed by A. flavus (29.4%) and F. verticillioides (18.2%). Exposure of A. flavus, P. verrucosum, and F. verticillioides to BM344-1 VOCs resulted in complete inhibition of aflatoxins (AFB₁, AFG₁, and AFG₂), ochratoxin A, and fumonisin B₁ (FB₁) synthesis on artificial media, respectively. In vivo experiments on maize kernels showed 51% inhibition of fungal growth on ears simultaneously infected with A. flavus spores and exposed to BM344-1 volatiles. Likewise, AF synthesis by A. flavus was significantly (p < 0.05) inhibited (25.34 ± 6.72 μg/kg) by bacterial volatiles as compared to that in control maize ears (91.81 ± 29.10 μg/kg). Gas chromatography-tandem mass spectrometry-based analysis of headspace volatiles revealed hexadecanoic acid methyl ester (palmitic acid) and tetracosane as bioactive compounds in the BM344-1 volatilome. Bacterial volatiles have promising potential to control the growth and mycotoxin synthesis of toxigenic fungi and may present valuable aid in the efforts to warrant food and feed safety.

Antifungal effect of volatile organic compounds produced by Pseudomonas chlororaphis subsp. aureofaciens SPS-41 on oxidative stress and mitochondrial dysfunction of Ceratocystis fimbriata

Ceratocystis fimbriata is the pathogen of black rot disease, which widely exists in sweet potato producing areas all over the world. The antifungal activity of volatile organic compounds (VOCs) released by Pseudomonas chlororaphis subsp. aureofaciens SPS-41 against C. fimbriata was reported in our previous study. In this study, we attempted to reveal the underlying antifungal mechanism of SPS-41 volatiles. Our results showed that the VOCs released by SPS-41 caused the morphological change of hyphae, destroyed the integrity of cell membrane, reduced the content of ergosterol, and induced massive accumulation of reactive oxygen species in C. fimbriata cells. Furthermore, SPS-41 fumigation decreased the mitochondrial membrane potential, acetyl-CoA and pyruvate content of C. fimbriata cells, as well as the mitochondrial dehydrogenases activity. In addition, the VOCs generated by SPS-41 reduced the intracellular ATP content and increased the extracellular ATP content of C. fimbriata. In summary, SPS-41 fumigation exerted its antifungal activity by inducing oxidative stress and mitochondrial dysfunction in C. fimbriata.