Abstract: The bark of Dalbergiatrichocapra Baker is traditionally used in Madagascar as an anti-infective remedy. Beyond the recently known anti-quorum sensing (QS) properties of the D. trichocarpabark n-hexane extract, QS-independent anti-infective activities have been also detected. Indeed, chromatographic fractionation allowed the elution of fraction F1 that affects neither bacterial growth nor the expression of QS-related genes (lasB and rhlA) but significantly reduces the formation of biofilm (55.8 2.3%, as compared to control conditions). Moreover,F1 is able to disrupt the structure of one-day old preformed biofilms, which consequently increases the effectiveness of an antibiotic, levofloxacin, on biofilm-encapsulated bacteria (dead bacteria in presence of levofloxacin-F1 were two-fold higher compared to levofloxacin alone). This F1-triggered disruption of biofilm formation is presumably due to an induced reduction in flagellar-dependent motilities (swimming and swarming) as well as in exopolysaccharides production. The inhibitory effect on biofilm appears reversible as the biofilm formation resumes when F1 is discarded from the culture medium. This interesting non-bactericidal mechanism of action may justify the traditional uses of D. trichocarpa in Malagasy medicine. Further work aims at identifying the compound(s) responsible for this biofilm disruption.

Keywords: Antivirulence, Biofilm, Dalbergiatrichocarpa, Extract, Pseudomonas aeruginosa.