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.

To study the role of Quorum Sensing (QS) regulation in the control of the cellular processes of Burkholderia cenocepacia 370, plasmid pME6863 was transferred into its cells. The plasmid contains a heterologous gene encoding AiiA N-acyl-homoserine lactonase, which degrades the signaling molecules of the QS system of N-acyl-homoserine lactones (AHL). An absence or reduction of AHL in the culture was revealed with the biosensors Chromobacterium violaceum CV026 and Agrobacterium tumefaciens NT1/pZLR4, respectively. The presence of the aiiA gene, which was cloned from Bacillus sp. A24 in the cells of B. cenocepacia 370, resulted in a lack of hemolytic activity, reduced the extracellular proteolytic activity and decreased the cells’ ability to swarming migration on the surface of the agar medium. The introduction of the aiiA gene did not affect lipase activity, fatty acids synthesis, HCN synthesis, or biofilm formation. Hydrogen peroxide was shown to stimulate biofilm formation by B. cenocepacia 370 in concentrations that inhibited or weakly suppressed bacterial growth. The introduction of the aiiA gene into the cells did not eliminate this effect but it did reduce it.

Для изучения роли Quorum Sensing (QS) регуляции в контроле клеточных процессов Burkholderia cenocepacia 370 в клетки этого штамма была передана плазмида pME6863, содержащая гетерологичный ген ацил-гомосеринлактоназы AiiA, деградирующей сигнальные молекулы QS систем N-ацил-гомосеринлактоны (АГЛ). При использовании биосенсоров Chromobacterium violaceum CV026 и Agrobacterium tumefaciens NT1/pZLR4 было показано соответственно отсутствие или уменьшение содержания АГЛ в культуре. Присутствие в клетках B. cenocepacia 370 гена aiiA, клонированного из Bacillus sp. A24, приводило к отсутствию гемолитической активности, снижению внеклеточной протеолитической активности, ослаблению способности клеток мигрировать по поверхности агаризованной среды (сворминг-миграция, swarming). Не наблюдалось влияния введения гена aiiA на липазную активность, синтез жирных кислот, синтез HCN и образование биопленок. Установлено, что пероксид водорода в субингибиторных или слабо подавляющих рост концентрациях вызывал стимуляцию образования биопленок. Введение гена aiiA в клетки не снимало этого эффекта, но уменьшало его величину.

The effect of the natural ketones emitted by bacteria (2-nonanone, 2-heptanone, 2-undecanone) on the functioning of the Quorum Sensing (QS) systems was studied. In this work, three lux-reporter strains containing the components of the LasI/LasR, RhlI/RhlR, LuxI LuxR QS systems were used as biosensors for the N-acyl-homoserine lactones. It was shown that at concentrations of ketones that exhibited little or no bactericidal action the ketones could modulate the QS-response by suppressing the expression of the lux-operon reporter to a greater extent than the cell viability of these strains.

The effect of natural ketones emitted by bacteria (2-nonanone, 2-heptanone, and 2-undecanone) on the functioning of the Quorum Sensing (QS) systems was studied. In this work, three lux-reporter strains containing the components of the LasI/LasR, RhlI/RhlR, and LuxI/LuxR QS systems were used as biosensors for N-acyl-homoserine lactones. It was shown that, at concentrations of ketones that exhibited little or no bacterial action, the ketones could modulate the QS response by suppressing the expression of the lux operon to a greater extent than the cell viability of these strains.

In this work, we have shown that salicylic, indole-3-acetic, gibberellic, and abscisic acids at subinhibitory concentrations or concentrations that have a low inhibitory effect on bacterial growth stimulate biofilm formation by P. aeruginosa PAO1 and A. tumefaciens C58. At higher concentrations, these substances suppress this process

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.

In natural ecosystems, most bacteria exist as specifically organized biofilms attached to various surfaces; the biofilms have a complex architecture and are surrounded by an exopolymeric matrix. The bacteria in the biofilms are extremely resistant to antibacterial agents. The ability of the pathogenic bacteria to produce biofilms causes serious problems in medicine. Therefore, the study of the effect of different compounds with antibacterial activity on the formation of biofilms is of great interest. In this work, we studied the effect of hydrogen peroxide (H2O2) on the formation of biofilms by Pseudomonas aeruginosa PAO1. It was shown that, in concentrations that do not suppress bacterial growth (or suppress it only weakly), H2O2 stimulates formation of biofilms. At higher concentrations, H2O2 inhibits formation of biofilms. To determine whether stimulation of biofilm formation depends on quorum-sensing (QS) regulation, the plasmid pME6863 containing the heterologous gene aiiA encoding the N-acyl-homoserine lactonase AiiA was introduced into P. aeruginosa PAO1. Synthesis by cells of this enzyme degrading N-acyl-homoserine lactones (AHLs), signaling molecules of the QS systems, led to absence of stimulation of biofilm formation by the H2O2. This fact indicates that stimulation of biofilm formation in the presence of H2O2 depends on the functioning of QS systems of regulation of gene expression of P. aeruginosa PAO1.