Biofilm formation is among the relevant virulence mechanisms of several plant pathogenic bacteria, including Xylella fastidiosa. Mainly composed of exopolysaccharides, biofilm allows bacterial cells to adhere to the surfaces and form communities protected from the hostile xylematic environment. Previous studies have shown that X. fastidiosa biofilm formation is closely related to quorum-sensing regulation. The alteration of this mechanism can have important consequences on the bacterial ability to colonize the host and induce symptoms. Tests have been started in vitro to assess the effects of exogenous substances on the cell growth of the strain “De Donno”, the causal agent of the severe disease denoted “Olive Quick Decline Syndrome”, and its capacity to form a biofilm ring adhering to the surface of glass tubes or microtiter plates. The bacterial response was evaluated in different experimental sets, targeting diverse aspects of biofilm chemistry and regulation. One of the approaches was based on the knowledge that X. fastidiosa rpfF-gene synthesises a diffusible signalling factor (DSF) that controls biofilm formation, by testing the effect of crude extracts of an E. coli engineered to express “De Donno” rpfF gene. The possible action on biofilm synthesis, due to metabolite interference, or as a consequence of an interspecies quorum-sensing modulation, has been tested through competitive assays in presence of the rhizobacterium Paraburkholderia phytofirmans, recently proposed as a candidate for X. fastidiosa biocontrol. In addition, several natural or synthetic compounds (e.g. N-acetylcysteine, thymol, caffeic acid, etc.) have been evaluated for their action on cell growth and biofilm consistency.

Financial support for the present work was from the EU H2020-funded research project XF-ACTORS (GA 727987) and from the project "STIPXYT'' funded by Regione Puglia DGR 1410/2015 .