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Diffusible and volatile organic compounds produced by avocado rhizobacteria exhibit antifungal effects against Fusarium kuroshium.

ABSTRACT: Rhizobacteria emit bioactive metabolites with antifungal properties that could be used for biocontrol of fungal diseases. In this study, we evaluated the potential of diffusible and volatile organic compounds (VOCs) emitted by avocado rhizobacteria to inhibit the growth of Fusarium kuroshium, one of the causal agents of Fusarium dieback (FD) in avocado. Three bacterial isolates (INECOL-6004, INECOL-6005, and INECOL-6006), belonging to the Bacillus genus, were selected based on their capacity to inhibit several avocado fungal pathogens, and tested in antagonism assays against F. kuroshium. The three bacterial isolates significantly inhibited F. kuroshium mycelial growth by up to 48%. The composition of bacterial diffusible compounds was characterized by the analysis of EtOAc and n-BuOH extracts by using ultra-performance liquid chromatography (UPLC) coupled to high-resolution mass spectrometry (HRMS). The three bacterial isolates produced cyclo-lipopeptides belonging to the iturin, fengycin, and surfactin families. The antifungal activity of n-BuOH extracts was larger than that of EtOAc extracts, probably due to the greater relative abundance of fengycin in the former than in the latter. In addition, isolates INECOL-6004 and INECOL-6006 significantly inhibited F. kuroshium mycelial growth through VOC emission by up to 69.88%. The analysis of their VOC profiles by solid phase micro-extraction (SPME) coupled to gas chromatography and mass spectrometry (GC-MS) revealed the presence of ketones and pyrazine compounds, particularly of 2-nonanone, which was not detected in the VOC profile of isolate INECOL-6005. These results emphasize the need to further investigate the antifungal activity of each bioactive compound for the development of new formulations against fungal phytopathogens.

SUBMITTER: Guevara-Avendano E

PROVIDER: S-EPMC7455650 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:To obtain biocontrol agents for suppression of food-deteriorating fungi during storage of agricultural products, bacteria producing volatile organic compounds (VOCs) with strong antifungal activity were screened and isolated from various environmental samples. Among 136 bacterial isolates, strain TM-R showed the strongest and broadest antifungal activity. Based on physiological and genetical characterization, the bacterium was identified as Bacillus pumilus. The effects of VOCs produced by the bacterium, which was grown on four types of agar media (nutrient, Trypto-Soya, Luria-Bertani, and TM Enterprise), were examined against six species of fungi (Alternaria alternata, Aspergillus niger, Cladosporium cladosporioides, Curvularia lunata, Fusarium oxysporum, and Penicillium italicum) in both small- and large-scale tests (plate and 12-L tests, respectively). In the plate test, the bacterium markedly suppressed the mycelial growth of five fungi (Alternaria alternata, Cladosporium cladosporioides, Curvularia lunata, F. oxysporum, and P. italicum) but promoted the growth of Aspergillus niger. In the 12-L test, the degree of growth inhibitiondecreased; however, the bacterium grown on TMEA still exhibited the strongest inhibition, especially against P. italicum (growth inhibition rate of 93%). Surprisingly, the growth of Aspergillus niger was promoted even more strongly (-36%) by the bacterium on TMEA than in the plate test (-9%). Twenty-two of 32 VOCs detected by GC-MS were identified using three databases (NIST 2011, AromaOffice, and AroChemBase). The species and concentration of detected VOCs differed greatly among growth media. To identify causative antifungal VOCs, we estimated the correlation between growth inhibition of P. italicum by the bacterium grown on each of the four media vs. the relative abundance of individual VOCs. As a result, four VOCs (methyl isobutyl ketone, ethanol, 5-methyl-2-heptanone, and S-(-)-2-methylbutylamine) were determined to be the predominant antifungal VOCs. To the best of our knowledge, this study is the first to specify causative antifungal VOCs using such an approach.

Project description:Bacterial metabolites have been observed to be important in new drug formulation for both plant, animals and human beings. The aim of this study was to identify the different bioactive compounds found in three rhizobacterial isolates (B. amyloliquefaciens, B. thuringiensis and Bacillus sp.) from the rhizosphere of Bambara groundnut and to assay for their antibacterial properties. Gas chromatography mass spectrometry (GC-MS) was used to carry out the analysis using seven extraction solvents. In the GC-MS analysis, 68 compounds were identified based on peak area percentage, retention time and structure. From the bioactive compounds in B. amyloliquefaciens and B. thuringiensis, the peak area percentage shows that dimethylfuvene from ethyl acetate extraction had the highest relative abundance with 89.11% while Formic acid 2-methylpropyl ester from hexane extraction had the lowest with 6.25%. Others are tridecane, acetic acid butyl ester, paraldehyde, s-(+)-1,2 propanediol, tropone, phthalan and p-xylene with relative abundance of 61.72%, 60.41%, 83.79%, 71.53%, 24.06%, 86.72% and 64.33% respectively. These extracts inhibited the growth of the four test organisms, Bacillus cereus, Pseudomonas aeruginosa, Micrococcus cryophilus and Enterococcus feacalis. Butanol extract from B. amyloliquefaciens had 28 mm zone of inhibition against B. cereus compared to 18 mm and 16 mm by Bacillus sp. and B. thuringiensis respectively. Its zone of inhibition was 24 mm zone against M. cryophilus compared to 12 mm and 19 mm by Bacillus sp. and B. thuringiensis respectively. Butanol extract from B. thuringiensis suppressed E. feacalis and P. aeruginosa having 23 mm and 26 mm zones of inhibition respectively. This was higher compared to Bacillus sp. and B. amyloliquefaciens having 18 mm/15 mm and 21 mm/15 mm against E. feacalis and P. aeruginosa respectively. Hexane and ethyl acetate extract from Bacillus sp. suppressed P. aeruginosa with 12 mm and 17 mm inhibition zones respectively compared to no inhibition zones from hexane extract of B. amyloliquefaciens and B. thuringiensis. Zones of inhibition of 2 mm and 6 mm were observed against P. aeruginosa from ethyl acetate extract of B. amyloliquefaciens and B. thuringiensis respectively. These results suggest that the three isolates are quite rich in the production of bioactive compounds that are also very effective antibacterial agents. These volatile organic compounds are promising compounds for more antibacterial bioactivity development.

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Diffusible and volatile organic compounds produced by avocado rhizobacteria exhibit antifungal effects against Fusarium kuroshium.

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