In the natural environment, bacteria predominantly exist in matrix-enclosed multicellular communities associated with various surfaces, referred to as biofilms. Bacteria in biofilms are extremely resistant to antibacterial agents thus causing serious problems for antimicrobial therapy. In this study, we showed that different plant phenolic compounds, at concentrations that did not or weakly suppressed bacterial growth, increased the capacity of Pseudomonas aeruginosa PAO1 to form biofilms. Biofilm formation of P. aeruginosa PAO1 was enhanced 3- to 7-fold under the action of vanillin and epicatechin, and 2- to 2.5-fold in the presence of 4-hydroxybenzoic, gallic, cinnamic, sinapic, ferulic, and chlorogenic acids. At higher concentrations, these compounds displayed an inhibiting effect. Similar experiments carried out for comparison with Agrobacterium tumefaciens C58 showed the same pattern. Vanillin, 4-hydroxybenzoic, and gallic acids at concentrations within the range of 40 to 400 μg/mL increased the production of N-3-oxo-dodecanoyl-homoserine lactone in P. aeruginosa PAO1 which suggests a possible relationship between stimulation of biofilm formation and Las Quorum Sensing system of this bacterium. Using biosensors to detect N-acyl-homoserine lactones (AHL), we demonstrated that the plant phenolics studied did not mimic AHLs.

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The ability to form biofilms plays an important role in bacteria-host interactions, including plant pathogenicity. In this work, we investigated the action of volatile organic compounds (VOCs) produced by rhizospheric strains of Pseudomonas chlororaphis 449, Pseudomonas fluorescens B-4117, Serratia plymuthica IC1270, as well as Serratia proteamaculans strain 94, isolated from spoiled meat, on biofilms formation by three strains of Agrobacterium tumefaciens which are causative agents of crown-gall disease in a wide range of plants. In dual culture assays, the pool of volatiles emitted by the tested Pseudomonas and Serratia strains suppressed the formation of biofilms of A. tumefaciens strains grown on polycarbonate membrane filters and killed Agrobacterium cells in mature biofilms. The individual VOCs produced by the tested Pseudomonas strains, that is, ketones (2-nonanone, 2-heptanone, 2-undecanone), and dimethyl disulfide (DMDS) produced by Serratia strains, were shown to kill A. tumefaciens cells in mature biofilms and suppress their formation. The data obtained in this study suggest an additional potential of some ketones and DMDS as protectors of plants against A. tumefaciens strains, whose virulence is associated with the formation of biofilms on the infected plants.

Bacteria and fungi emit a huge variety of volatile organic compounds (VOCs) that can provide a valuable arsenal for practical use. However, the biological activities and functions of the VOCs are poorly understood. This work aimed to study the action of individual VOCs on the bacteria Agrobacterium tumefaciens, Arabidopsis thaliana plants, and fruit flies Drosophila melanogaster. VOCs used in the work included ketones, alcohols, and terpenes. The potent inhibitory effect on the growth of A. tumefaciens was shown for 2-octanone and isoamyl alcohol. Terpenes (−)-limonene and (+)-α-pinene practically did not act on bacteria, even at high doses (up to 400 µmol). 2-Butanone and 2-pentanone increased the biomass of A. thaliana at doses of 200–400 μmol by 1.5–2 times; 2-octanone had the same effect at 10 μmol and decreased plant biomass at higher doses. Isoamyl alcohol and 2-phenylethanol suppressed plant biomass several times at doses of 50–100 μmol. Plant seed germination was most strongly suppressed by isoamyl alcohol and 2-phenylethanol. The substantial killing effect (at low doses) on D. melanogaster was exerted by the terpenes and the ketones 2-octanone and 2-pentanone. The obtained data showed new information about the biological activities of VOCs in relation to organisms belonging to different kingdoms.