Bacterial Volatile Organic Compounds Research Articles

Bacterial volatile organic compounds supplemented with light‐emitting diode lighting enhance antioxidant capacity and delay senescence in broccoli

Bacterial volatile organic compounds supplemented with light‐emitting diode lighting enhance antioxidant capacity and delay senescence in broccoli

Plant growth promoting signatory volatiles emitted by a drought-tolerant bacterium Bacillus altitudinis FD48 and its role in moisture stress alleviation in rice (Oryza sativa L.)

Increasing evidence implies that bacterial volatile organic compounds (bVOCs) play a significant role in plant-microbe interaction. Plant associated bacteria produces plant growth modulating volatiles elicits induced systemic tolerance (IST) in plants against a multitude of abiotic stress. Induction of IST and plant growth promotion by signatory bVOCs of Bacillus altitudinis FD48 against drought are reported in this study. The rice seedlings exposed to bVOCs blends of FD48 showed a one-fold increase in whole plant biomass and auxin content (3 µmol g-1 FW) under induced moisture stress. The effect of bVOCs highly depends on the inoculum load. Higher inoculum quantity (100 µL) is detrimental to plant growth. bVOCs produced by FD48 profiled at different growth intervals in GC–MS-ATD revealed a total of 40 bioactive compounds both under stress (PEG 6000) and non-stressed conditions. Interestingly, potential plant growth-promoting compounds such as 1-Hexanol, 2,3-butanediol, dimethyl disulfide, benzene, butanoic acid, pentadecane, and acetic acid are more pronounced. Few compounds produced under non-stress were found to increase during stress (example, 2,3-Butanediol, and acetic acid). This study unraveled the significant biosynthetic pathways induced by FD48 bVOC blends, such as pyruvate metabolism, tryptophan metabolism, sulfur metabolism, fatty acid biosynthesis, and ethanol degradation that anchors in abating moisture stress. Hence, it can be concluded that PGPB B. altitudinis FD48 produced bVOCs could be potential orchestrators of induced systemic tolerance in plants against moisture stress.

Bacterial Volatile Organic Compounds as a Strategy to Increase Drought Tolerance in Maize (Zea mays L.): Influence on Plant Biochemistry.

Maize is highly susceptible to drought, which affects growth and yield. This study investigated how bacterial volatile organic compounds (BVOCs) affect maize drought tolerance. Drought reduced shoot size but increased root length, an adaptation for accessing deeper soil moisture. BVOCs from strain D12 significantly increased root length and shoot growth under drought conditions. Drought also altered root biochemistry, decreasing enzyme activity, and increased osmolyte levels. BVOCs from strains F11 and FS4-14 further increased osmolyte levels but did not protect membranes from oxidative damage, while BVOCs from strains D12 and D7 strains reduced osmolyte levels and cell damage. In shoots, drought increased the levels of osmolytes and oxidative stress markers. BVOCs from FS4-14 had minimal effects on shoot biochemistry. BVOCs from D12 and F11 partially restored metabolic activity but did not reduce cell damage. BVOCs from D7 reduced metabolic activity and cell damage. These results suggest that BVOCs can modulate the biochemical response of maize to drought, with some strains evidencing the potential to enhance drought tolerance.

Single-Walled Carbon Nanotube Based Field-Effect Transistors Functionalized with Odorant Receptors for Biosensing Applications

Bioelectronic detection of volatile organic compounds (VOC) has been demonstrated in a wide range of applications, with early reports focusing on environmental exogenous VOCs due to their adverse effects on human health; bacterial VOC signatures used as disease biomarkers, etc. Carbon nanotube field-effect transistors (CNT-FETs) are considered promising devices for bioelectronic detection due to their sensitivity, robustness and compatibility with microelectronic fabrication technologies. However, implementation of CNT-FETs still faces many challenges. Current VOCs detection approaches rely on separation techniques coupled with mass spectrometry, necessitating expensive equipment, labor-intensive preparation steps and trained personnel, and are not suitable for field use. Overcoming the challenges in the development of bioelectronic CNT-FET sensors holds great promise for high-throughput screening of VOCs.The natural insect odorant receptors (ORs) are highly attractive probes, potentially allowing ultrahigh specificity for VOC biosensing. Insect ORs exhibit remarkable sensitivities and the ability to selectively detect a vast number of VOCs. We have recently developed OR-functionalized carbon nanotube field-effect transistor (FET) devices. These FET devices comprise an isolated single-walled carbon nanotube (SWCNT), serving as the conducting channel material, functionalized with multiple insect ORs. These hybrid devices are capable of transducing ligand binding into an electronic current, without amplification. The insect OR-functionalized CNT FET were applied for the detection of several markers of environmental and clinical importance including Indole, Skatole, Octen-3-ol and Nonyl-aldehyde.We have developed a bioelectronic assay platform that contains a chip with 61 ORs-functionalized CNT-FET devices. Our unique functionalization method is based on directing the attachment of a native ORs-containing nanovesicles (which we have exogenously expressed and functionally characterized) to a generated CNT-FET point defect. Target binding-induced ionic current and conformational changes of ORs affect an electric field that modulates the device conductance.

Up-regulation of growth-related gene expression in tobacco by volatile compounds released by Bacillus velezensis WSW007

In order to investigate the mechanism of plant growth promoting (PGP) effects of strain Bacillus velezensis WSW007, its PGP traits and production of volatile organic compounds (VOCs) were tested. The effects of VOCs produced by strain WSW007 on plant growth were observed by co-culturing this strain with tobacco seedlings in I-plates. Meanwhile, the effects of VOCs on tobacco gene expression were analysed by a transcriptome analysis and VOCs were identified by solid phase micro extraction coupled with gas chromatography-mass spectrometry (SPME-GC-MS) analysis. As results, strains WSW007 produced acetic acid and siderophore, and could solubilize phosphate; while it also significantly increased the fresh weight of tobacco seedlings via production of VOCs. In transcriptome analysis, plants co-cultured with strain WSW007 presented the highest up-regulated expression for the genes involved in plant growth and development processes, implying that the bacterial VOCs played a role as regulator of plant gene expression. Conclusively, the up-regulation in expression of growth- and development-related genes via VOCs production is an important PGP mechanism in strain B. velezensis WSW007.

Cooperative effects of three preservatives on physiological quality, endophytic bacterial community and volatile organic compounds of postharvest Codonopsis pilosula var. modesta roots.

Codonopsis pilosula var. modesta (CPVM) is a famous medicinal and edible plant of Campanulaceae. However, fresh CPVM roots (FCPVR) are prone to softening, browning and spoilage after concentrated harvesting in the main production area of Gansu Province, China in autumn, which poses great challenges to their large-scale storage and modern processing. In this study, effects of chitosan (CS), natamycin (NA) and modified atmosphere agent (MA) on the postharvest quality of FCPVR were first investigated. The roots after different treatments were stored at 4 °C and relative humidity of 75 ± 5% for 100 days. Their overall quality changes were evaluated from three perspectives: physiological quality, endophytic bacterial community and volatile organic compounds. The clustering heatmap and principal component analysis results indicated that CS (2 g kg-1), NA (0.5 g kg-1) and MA (5 g) had a synergistic effect on physiological quality. The roots in the CS + NA + MA group maintained better physiological state, effective components and antioxidant capacity throughout the storage process. On this basis, compared with room temperature storage, the relative abundance of the main spoilage bacterium Pseudomonas in the CS + NA + MA group roots decreased by 44% on the 100th day of storage. Furthermore, after CS + NA + MA composite treatment, the roots produced richer esters with fruit aroma during low-temperature storage. The CS + NA + MA composite treatment could maintain the physiological quality and flavor of FCPVR, inhibit spoilage by microbial contamination and maintain the optimal quality during low-temperature storage for up to 100 days. © 2024 Society of Chemical Industry.

Characteristics of changes in volatile organic compounds and bacterial communities in physically preserved pigeon breast meat

To understand the relationship between changes in aroma and bacteria in pigeon breast meat (PBM) during preservation, bacterial communities and volatile compounds in PBM were analyzed using high-throughput sequencing and gas chromatography–ion mobility spectrometry. Analyses of total viable bacteria counts revealed that modified atmospheric packaging (MAP) and electron beam irradiation (EBI) could be used to extend the shelf-life of PBM to 10 d and 15 d, respectively. Furthermore, Lactococcus spp. and Psychrobacter spp. were the dominant bacterial genera of the MAP and EBI groups, respectively. The results of the study revealed 91 volatile organic compounds, one of which, butanal, was the most intense volatile organic compound while being an important source of aroma differences between the physical preservation techniques. Alpha-terpinolene, acetoin-M, gamma-butyrolactone, 1-hexanol-M, and 2,6-dimethyl-4-heptanone may be markers of PBM spoilage. During preservation, the MA group (treatment with 50 % CO2 + 50 % N2) demonstrated greater stabilization of PBM aroma. A Spearman correlation analysis showed that Lactococcus spp., Psychrobacter spp., and Pseudomonas spp. were the dominant bacterial genera of PBM during preservation and were closely related to an increase in the intensity of anisole, 2-methyl-3-furanthiol, and 5-methyl-2-furanmethanol, respectively. Lactococcus spp. and Psychrobacter spp. play crucial roles in the sensory degradation of PBM. In this study, we analyzed the changes in bacterial genera and volatile organic compounds of PBM under different physical preservation techniques to identify a suitable method for preserving PBM and evaluating its freshness.

Screening of core microorganisms in healthy and diseased peaches and effect evaluation of biocontrol bacteria (Burkholderia sp.)

Biological antagonists serve as the most important green alternatives to chemical fungicides, a class of microorganism that inhibits the growth of pathogenic fungi to reduce fruit incidence. In this paper, healthy and diseased peach fruit was selected for amplicon sequencing of the epiphytic microbiota on their surface to obtain a comprehensive understanding. Community structure, diversity and LefSe analysis were performed to screen Acetobacter, Muribaculaceae and Burkholderia as the core bacteria, Mycosphaerella, Penicillium and Alternaria as the core fungi, they showed significant differences and were highly enriched. Two strains fungi (Penicillium K3 and N1) and one strain antagonistic bacteria (Burkholderia J2) were isolated. The in intro test results indicated the bacterial suspension, fermentation broth and volatile organic compounds of antagonistic bacteria J2 were able to significantly inhibit pathogen growth. In vivo experiments, peach was stored at 28 °C for 6 days after different treatments, and samples were taken every day. It was found that Burkholderia J2 enhanced peach resistance by increasing the activities of antioxidant-related enzymes such as SOD, POD, PAL, PPO, GR, MDHAR, and DHAR. The results improved that Burkholderia J2 has great biocontrol potential and could be used as a candidate strain for green control of blue mold.

Qualitative and quantitative rapid detection of VOCs differentially released by VAP-associated bacteria using PTR-MS and FGC-PTR-MS.

Ventilator-associated pneumonia (VAP) is a prevalent disease caused by microbial infection, resulting in significant morbidity and mortality within the intensive care unit (ICU). The rapid and accurate identification of pathogenic bacteria causing VAP can assist clinicians in formulating timely treatment plans. In this study, we attempted to differentiate bacterial species in VAP by utilizing the volatile organic compounds (VOCs) released by pathogens. We cultured 6 common bacteria in VAP in vitro, including Acinetobacter baumannii, Enterobacter cloacae, Escherichia coli, Pseudomonas aeruginosa, Stenotrophomonas maltophilia, and Staphylococcus aureus, which covered most cases of VAP infection in clinic. After the VOCs released by bacteria were collected in sampling bags, they were quantitatively detected by a proton transfer reaction-mass spectrometry (PTR-MS), and the characteristic ions were qualitatively analyzed through a fast gas chromatography-proton transfer reaction-mass spectrometry (FGC-PTR-MS). After conducting principal component analysis (PCA) and analysis of similarities (ANOSIM), we discovered that the VOCs released by 6 bacteria exhibited differentiation following 3 h of quantitative cultivation in vitro. Additionally, we further investigated the variations in the types and concentrations of bacterial VOCs. The results showed that by utilizing the differences in types of VOCs, 6 bacteria could be classified into 5 sets, except for A. baumannii and E. cloacae which were indistinguishable. Furthermore, we observed significant variations in the concentration ratio of acetaldehyde and methyl mercaptan released by A. baumannii and E. cloacae. In conclusion, the VOCs released by bacteria could effectively differentiate the 6 pathogens commonly associated with VAP, which was expected to assist doctors in formulating treatment plans in time and improve the survival rate of patients.

Gut microbiota functional profiling in autism spectrum disorders: bacterial VOCs and related metabolic pathways acting as disease biomarkers and predictors.

Autism spectrum disorder (ASD) is a multifactorial neurodevelopmental disorder. Major interplays between the gastrointestinal (GI) tract and the central nervous system (CNS) seem to be driven by gut microbiota (GM). Herein, we provide a GM functional characterization, based on GM metabolomics, mapping of bacterial biochemical pathways, and anamnestic, clinical, and nutritional patient metadata. Fecal samples collected from children with ASD and neurotypical children were analyzed by gas-chromatography mass spectrometry coupled with solid phase microextraction (GC-MS/SPME) to determine volatile organic compounds (VOCs) associated with the metataxonomic approach by 16S rRNA gene sequencing. Multivariate and univariate statistical analyses assessed differential VOC profiles and relationships with ASD anamnestic and clinical features for biomarker discovery. Multiple web-based and machine learning (ML) models identified metabolic predictors of disease and network analyses correlated GM ecological and metabolic patterns. The GM core volatilome for all ASD patients was characterized by a high concentration of 1-pentanol, 1-butanol, phenyl ethyl alcohol; benzeneacetaldehyde, octadecanal, tetradecanal; methyl isobutyl ketone, 2-hexanone, acetone; acetic, propanoic, 3-methyl-butanoic and 2-methyl-propanoic acids; indole and skatole; and o-cymene. Patients were stratified based on age, GI symptoms, and ASD severity symptoms. Disease risk prediction allowed us to associate butanoic acid with subjects older than 5 years, indole with the absence of GI symptoms and low disease severity, propanoic acid with the ASD risk group, and p-cymene with ASD symptoms, all based on the predictive CBCL-EXT scale. The HistGradientBoostingClassifier model classified ASD patients vs. CTRLs by an accuracy of 89%, based on methyl isobutyl ketone, benzeneacetaldehyde, phenyl ethyl alcohol, ethanol, butanoic acid, octadecane, acetic acid, skatole, and tetradecanal features. LogisticRegression models corroborated methyl isobutyl ketone, benzeneacetaldehyde, phenyl ethyl alcohol, skatole, and acetic acid as ASD predictors. Our results will aid the development of advanced clinical decision support systems (CDSSs), assisted by ML models, for advanced ASD-personalized medicine, based on omics data integrated into electronic health/medical records. Furthermore, new ASD screening strategies based on GM-related predictors could be used to improve ASD risk assessment by uncovering novel ASD onset and risk predictors.

Dimethylpolysulfides production as the major mechanism behind wheat fungal pathogen biocontrol, by Arthrobacter and Microbacterium actinomycetes.

Volatile organic compounds (VOCs) produced by plant-associated microorganisms could serve as natural biocontrol compounds. In this work, we investigate the potential of wheat rhizosphere Microbacterium and Arthrobacter actinomycetes to inhibit the growth of major wheat phytopathogenic fungi, Fusarium graminearum and Zymoseptoria tritici via production of antifungal VOCs. A correlative analysis between fungal-growth inhibition versus bacterial volatilomic profiles identified four dimethylpolysulfide (DMPS) VOCs as the main active compounds behind their biocontrol potential. Subsequent inhibition assays reveal that DMTriS (dimethyltrisulfide) exhibits the strongest inhibition effects, then dimethyldisulfide (DMDiS). Further investigation focuses on the mechanisms behind F. graminearum-growth inhibition by the most active strain Microbacterium JM188 in dual culture. Surprisingly, nine interaction-induced VOCs, including two fungal sesquiterpenes, were exclusively detected in dual cultures, suggesting a complex interplay between microbial VOC production and sensing. More importantly, all JM188 VOCs, including antifungal DMPS, were less abundant in dual cultures, suggesting an uptake of bacterial VOCs by the fungus. Quantification of pure DMPS after confrontation with F. graminearum confirmed complete and partial uptake by the fungus of DMTriS and DMDiS VOCs, respectively, suggesting a potential link between the fungal uptake level of bacterial VOCs and their toxicity. Finally, we demonstrated that F. graminearum growth inhibition leads to a complete depletion of DON (deoxynivalenol) carcinogenic mycotoxins, highlighting a significant modulation of fungal metabolism. Collectively, these results pinpoint DMPS as broad-range fungal-inhibiting VOCs produced by rhizosphere Microbacterium and Arthrobacter, and emphasize the extensive VOC-mediated interplay between bacterial biocontrol agents and fungal pathogens.IMPORTANCEAs the management of wheat fungal diseases becomes increasingly challenging, the use of bacterial agents with biocontrol potential against the two major wheat phytopathogens, Fusarium graminearum and Zymoseptoria tritici, may prove to be an interesting alternative to conventional pest management. Here, we have shown that dimethylpolysulfide volatiles are ubiquitously and predominantly produced by wheat-associated Microbacterium and Arthrobacter actinomycetes, displaying antifungal activity against both pathogens. By limiting pathogen growth and DON virulence factor production, the use of such DMPS-producing strains as soil biocontrol inoculants could limit the supply of pathogen inocula in soil and plant residues, providing an attractive alternative to dimethyldisulfide fumigant, which has many non-targeted toxicities. Notably, this study demonstrates the importance of bacterial volatile organic compound uptake by inhibited F. graminearum, providing new insights for the study of volatiles-mediated toxicity mechanisms within bacteria-fungus signaling crosstalk.

Volatile profiling distinguishes Streptococcus pyogenes from other respiratory streptococcal species.

Sore throat is one of the most common complaints encountered in the ambulatory clinical setting. Rapid, culture-independent diagnostic techniques that do not rely on pharyngeal swabs would be highly valuable as a point-of-care strategy to guide outpatient antibiotic treatment. Despite the promise of this approach, efforts to detect volatiles during oropharyngeal infection have yet been limited. In our research study, we sought to evaluate for specific bacterial volatile organic compounds (VOC) biomarkers in isolated cultures in vitro, in order to establish proof-of-concept prior to initial clinical studies of breath biomarkers. A particular challenge for the diagnosis of pharyngitis due to Streptococcus pyogenes is the likelihood that many metabolites may be shared by S. pyogenes and other related oropharyngeal colonizing bacterial species. Therefore, we evaluated whether sufficient metabolic differences are present, which distinguish the volatile metabolome of Group A streptococci from other streptococcal species that also colonize the respiratory mucosa, such as Streptococcus pneumoniae and Streptococcus intermedius. In this work, we identified 27 discriminatory VOCs (q-values < 0.05), composed of aldehydes, alcohols, nitrogen-containing compounds, hydrocarbons, ketones, aromatic compounds, esters, ethers, and carboxylic acid. From this group of volatiles, we identify candidate biomarkers that distinguish S. pyogenes from other species and establish highly produced VOCs that indicate the presence of S. pyogenes in vitro, supporting future breath-based diagnostic testing for streptococcal pharyngitis. IMPORTANCE Acute pharyngitis accounts for approximately 15 million ambulatory care visits in the United States. The most common and important bacterial cause of pharyngitis is Streptococcus pyogenesis, accounting for 15%-30% of pediatric pharyngitis. Distinguishing between bacterial and viral pharyngitis is key to management in US practice. The culture of a specimen obtained by a throat swab is the standard laboratory procedure for the microbiologic confirmation of pharyngitis; however, this method is time-consuming, which delays appropriate treatment. If left untreated, S. pyogenes pharyngitis may lead to local and distant complications. In this study, we characterized the volatile metabolomes of S. pyogenes and other related oropharyngeal colonizing bacterial species. We identify candidate biomarkers that distinguish S. pyogenes from other species and provide evidence to support future breath-based diagnostic testing for streptococcal pharyngitis.

Alleviation of banded leaf and sheath blight disease incidence in maize by bacterial volatile organic compounds and molecular docking of targeted inhibitors in Rhizoctonia solani.

Rhizoctonia solani (RS) is a pathogenic fungus that affects maize (Zea mays L.) plants and causes banded leaf and sheath blight (BLSB) with severe consequences leading to significant economic losses. Contrarily, rhizobacteria produce numerous volatile organic compounds (VOCs) that help in devising the environment-friendly mechanism for promoting plant growth and stress alleviation without having physical contact with plants. In the present study, 15 rhizobacterial strains were tested for their antagonism against RS. The antagonistic potential of VOCs of the tested plant growth-promoting rhizobacteria (PGPR) strains ranged from 50% to 80% as compared to the control (without PGPR). Among these 15 strains, the maximum (80%) antagonistic activity was exhibited by Pseudomonas pseudoalcaligenes SRM-16. Thus, the potential of VOCs produced by P. pseudoalcaligenes SRM-16 to alleviate the BLSB disease in maize was evaluated. A pot experiment was conducted under greenhouse conditions to observe the effect of VOCs on disease resistance of BLSB-infected seedlings. Overall, maize seedlings exposed to VOCs showed a significant increase in disease resistance as indicated by a reduced disease score than that of unexposed infected plants. The VOCs-exposed maize exhibited lower (11.6%) disease incidence compared to the non-inoculated maize (14.1%). Moreover, plants exposed to VOCs displayed visible improvements in biomass, photosynthetic pigments, osmoregulation, and plant antioxidant and defense enzyme activities compared to the healthy but unexposed seedlings. Simultaneous application of RS and VOCs enhanced superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), phenylalanine ammonia lyase (PAL), ascorbate peroxidase (APX), and polyphenol oxidase (PPO) activities by 96.7%, 266.6%, 313.7%, 246.6%, 307%, and 149.7%, respectively, in the roots and by 81.6%, 246.4%, 269.5%, 269.6%, 329%, and 137.6%, respectively, in the shoots, relative to those of the control plants. The binding affinity of the VOCs (2-pentylfuran, 2,3-butanediol, and dimethyl disulfide) with CRZ1 and S9 protein receptors of RS was assessed by deploying in silico methods. Overall, 2-pentylfuran exhibited a binding affinity with both the selected receptors of RS, while 2,3-butanediol and dimethyl disulfide were able to bind S9 protein only. Hence, it can be deduced that S9 protein receptors are more likely the target RS receptors of bacterial VOCs to inhibit the proliferation of RS.

Control of post-harvest gray mold (Botrytis cinerea) on grape (Vitis vinifera) and tomato (Solanum lycopersicum) using volatile organic compounds produced by Xenorhabdus nematophila and Photorhabdus laumondii subsp. laumondii

Post-harvest fruit and vegetable rot produced by Botrytis cinerea (Helotiales: Sclerotiniaceae) causes significant reductions in food availability and drastically increases economic losses. The use of microbial-based tools for pathogen management holds promise. In particular, volatile organic compounds (VOCs) emitted by microbes (e.g., bacterial compounds) are becoming increasingly more frequent as an alternative to chemical and physical treatments. In this study, we performed three laboratory experiments to investigate the effects of VOCs emitted by two gram-negative entomopathogenic bacteria, Xenorhabdus nematophila, and Photorhabdus laumondii subsp. laumondii, on the infection and growth of the pathogenic mold B. cinerea on post-harvest red grapes and tomatoes. In addition, we evaluated the preventive effects of these bacterial VOCs against pathogens in post-harvest wounded and intact grapes. Overall, VOCs emitted by X. nematophila and P. laumondii limited the lesion area of B. cinerea to 0.5% and 2.2%, respectively, on the grapes. Similarly, VOCs emitted by X. nematophila and P. laumondii limited the lesion area of B. cinerea to 0.5% and 0.02%, respectively, in tomatoes. In addition, the emission of VOCs by both bacteria showed strong preventive fungal effects. In particular, VOCs emitted by P. laumondii reduced to 13% B. cinerea incidence in damaged grapes exposed to VOCs. Moreover, intact grapes exposed to VOCs emitted by X. nematophila and P. laumondii decreased B. cinerea incidence by 33%. This study provides insightful information about a potential novel bacteria-based tool that can be used as an alternative in the integrated control of post-harvest diseases.

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.

Antifungal Activity and Alleviation of Salt Stress by Volatile Organic Compounds of Native Pseudomonas Obtained from Mentha piperita.

As salt stress has a negative impact on plant growth and crop yield, it is very important to identify and develop any available biotechnology which can improve the salt tolerance of plants. Inoculation with plant-growth-promoting rhizobacteria (PGPR) is a proven environmentally friendly biotechnological resource for increasing the salt stress tolerance of plants and has a potential in-field application. In addition, bacterial volatile organic compounds (mVOCs) are signal molecules that may have beneficial roles in the soil-plant-microbiome ecosystem. We investigated the effects of mVOCs emitted by Pseudomona putida SJ46 and SJ04 on Mentha piperita grown under different levels of NaCl stress by evaluating their growth-promoting potential and capacity to increase salt tolerance effects. Furthermore, we evaluated under control and salt stress conditions the biocontrol ability of VOCs emitted by both these strains to inhibit the growth of Alternaria alternata and Sclerotium rolfsii. The VOCs emitted by both strains under control conditions did not lead to an significant improvement in peppermint growth. However, under salt stress conditions (75 or 100 mM NaCl), an amelioration of its physiological status was observed, with this effect being greater at 100 mM NaCl. This led to an enhancement of the number of leaves and nodes and, increased the shoot fresh and root dry weight by approximately twice in relation to control stressed plants. Moreover, the VOCs released by the two bacteria grown in control or saline media showed a significant reduction in the mycelial growth of A. alternata. In contrast, S. rolfsii growth was reduced 40% by the mVOCs released only under control conditions, with no effects being observed under salt stress. We also explored the composition of the bacterial volatile profiles by means of a solid-phase microextraction/gas chromatography-mass spectrometry (SPME/GC-MS) analysis. From the headspace of SJ46, three VOCs were identified: n-octanol, decane and tetradecane. The emission of SJ04 had the same chromatographic profile, with the addition of two more compounds: 1-(N-phenyl carbamyl)-2-morpholino cyclohexene and tridecane. Only compounds that were not present in the headspace of the control groups were recorded. The salt stress conditions where the bacteria were grown did not qualitatively modify the mVOC emissions. Taken together, our results suggest that plant-associated rhizobacterial VOCs play a potentially important role in modulating plant salt tolerance and reducing fungal growth. Thus, biological resources represent novel tools for counteracting the deleterious effects of salt stress and have the potential to be exploited in sustainable agriculture. Nevertheless, future studies are necessary to investigate technological improvements for bacterial VOC application under greenhouse and open field conditions.

Long-term fertilization suppresses rice pathogens by microbial volatile compounds

Microbial volatile organic compounds (VOCs) can suppress plant pathogens. Although fertilization strongly affects soil microbial communities, the influence of fertilization on microbial VOC-mediated suppression of pathogens has not been elucidated. Soil was sampled from a paddy field that had been subjected to the following treatments for 30 years: a no-fertilizer control, mineral fertilization (NPK), NPK combined with rice straw (NPK + S), NPK combined with chicken manure (70% NPK + 30% M). Then, within a laboratory experiment, pathogens were exposed to VOCs without physical contact to assess the impact of VOCs emitted from paddy soils on in vitro growth of the fungal rice pathogens: Pyricularia oryzae and Rhizoctonia solani. The VOCs emitted from soil reduced the mycelial biomass of P. oryzae and R. solani by 36–51% and 10–30%, respectively, compared to that of the control (no soil; no VOCs emission). Overall, the highest suppression of P. oryzae and R. solani was in the NPK and NPK + S soils, which emitted more quinones, phenols, and low alcohols than NPK + M soils. The abundances of quinones and phenols in the soil air were maximal in the NPK-fertilized soil because the low ratio of dissolved organic carbon and Olsen-P increased the population of key species such as Acidobacteriae, Anaerolineae, and Entorrhizomycetes. The abundance of alcohols was minimum in the NPK + S fertilized soil because the high SOC content decreased the population of Sordariomycetes. In conclusion, mineral fertilization affects bacterial and fungal VOC emissions, thereby suppressing the growth of R. solani and P. oryzae.

Bacterial volatile organic compounds (VOCs) promote growth and induce metabolic changes in rice.

Plant growth-promoting bacteria (PGPB) represent an eco-friendly alternative to reduce the use of chemical products while increasing the productivity of economically important crops. The emission of small gaseous signaling molecules from PGPB named volatile organic compounds (VOCs) has emerged as a promising biotechnological tool to promote biomass accumulation in model plants (especially Arabidopsis thaliana) and a few crops, such as tomato, lettuce, and cucumber. Rice (Oryza sativa) is the most essential food crop for more than half of the world's population. However, the use of VOCs to improve this crop performance has not yet been investigated. Here, we evaluated the composition and effects of bacterial VOCs on the growth and metabolism of rice. First, we selected bacterial isolates (IAT P4F9 and E.1b) that increased rice dry shoot biomass by up to 83% in co-cultivation assays performed with different durations of time (7 and 12 days). Metabolic profiles of the plants co-cultivated with these isolates and controls (without bacteria and non-promoter bacteria-1003-S-C1) were investigated via 1H nuclear magnetic resonance. The analysis identified metabolites (e.g., amino acids, sugars, and others) with differential abundance between treatments that might play a role in metabolic pathways, such as protein synthesis, signaling, photosynthesis, energy metabolism, and nitrogen assimilation, involved in rice growth promotion. Interestingly, VOCs from IAT P4F9 displayed a more consistent promotion activity and were also able to increase rice dry shoot biomass in vivo. Molecular identification by sequencing the 16S rRNA gene of the isolates IAT P4F9 and E.1b showed a higher identity with Serratia and Achromobacter species, respectively. Lastly, volatilomes of these and two other non-promoter bacteria (1003-S-C1 and Escherichia coli DH5α) were evaluated through headspace solid-phase microextraction coupled with gas chromatography-mass spectrometry. Compounds belonging to different chemical classes, such as benzenoids, ketones, alcohols, sulfide, alkanes, and pyrazines, were identified. One of these VOCs, nonan-2-one, was validated in vitro as a bioactive compound capable of promoting rice growth. Although further analyses are necessary to properly elucidate the molecular mechanisms, our results suggest that these two bacterial isolates are potential candidates as sources for bioproducts, contributing to a more sustainable agriculture.

Bacterial volatiles elicit differential olfactory responses in insect species from the same and different trophic levels.

Insect communities consist of species from several trophic levels that have to forage for suitable resources among and within larger patches of nonresources. To locate their resources, insects use diverse stimuli, including olfactory, visual, acoustic, tactile and gustatory cues. While most research has focused on cues derived from plants and other insects, there is mounting evidence that insects also respond to volatile organic compounds (VOCs) emitted by microorganisms. However, to date little is known about how the olfactory response of insects within and across different trophic levels is affected by bacterial VOCs. In this study, we used Y-tube bioassays and chemical analysis of VOCs to assess how VOCs emitted by bacteria affect the olfactory response of insects of the same and different trophic levels. Experiments were performed using two aphid species (Amphorophora idaei Börner and Myzus persicae var. nicotianae Blackman), three primary parasitoid species (Aphidius colemani Viereck, A. ervi Haliday, and A. matricariae Viereck), and two hyperparasitoid species (Asaphes suspensus Nees and Dendrocerus aphidum Rondani). Olfactory responses were evaluated for three bacterial strains (Bacillus pumilus ST18.16/133, Curtobacterium sp. ST18.16/085, and Staphylococcus saprophyticus ST18.16/160) that were isolated from the habitat of the insects. Results revealed that insects from all trophic levels responded to bacterial volatiles, but olfactory responses varied between and within trophic levels. All bacteria produced the same set of volatile compounds, but often in different relative concentrations. For 11 of these volatiles we found contrasting correlations between their concentration and the behavior of the primary parasitoids and hyperparasitoids. Furthermore, olfactometer experiments on three of these compounds confirmed the contrasting olfactory responses of primary parasitoids and hyperparasitoids. The potential of these findings for the development of novel semiochemical-based strategies to improve biological aphid control has been discussed.

Bacterial volatile organic compounds attenuate pathogen virulence via evolutionary trade-offs.

Volatile organic compounds (VOCs) produced by soil bacteria have been shown to exert plant pathogen biocontrol potential owing to their strong antimicrobial activity. While the impact of VOCs on soil microbial ecology is well established, their effect on plant pathogen evolution is yet poorly understood. Here we experimentally investigated how plant-pathogenic Ralstonia solanacearum bacterium adapts to VOC-mixture produced by a biocontrol Bacillus amyloliquefaciens T-5 bacterium and how these adaptations might affect its virulence. We found that VOC selection led to a clear increase in VOC-tolerance, which was accompanied with cross-tolerance to several antibiotics commonly produced by soil bacteria. The increasing VOC-tolerance led to trade-offs with R. solanacearum virulence, resulting in almost complete loss of pathogenicity in planta. At the genetic level, these phenotypic changes were associated with parallel mutations in genes encoding lipopolysaccharide O-antigen (wecA) and type-4 pilus biosynthesis (pilM), which both have been linked with outer membrane permeability to antimicrobials and plant pathogen virulence. Reverse genetic engineering revealed that both mutations were important, with pilM having a relatively larger negative effect on the virulence, while wecA having a relatively larger effect on increased antimicrobial tolerance. Together, our results suggest that microbial VOCs are important drivers of bacterial evolution and could potentially be used in biocontrol to select for less virulent pathogens via evolutionary trade-offs.