Effect of halogenated indoles on biofilm formation, virulence, and root surface colonization by Agrobacterium tumefaciens

ABSTR A C T Agrobacterium tumefaciens is a plant pathogen that causes crown gall disease in several plant species by trans-ferring its T-DNA to the host genome. Its chemotactic response to a range of chemical compounds released by hosts facilitates its colonization to host surfaces, and thus, novel anti-agrobacterium compounds are needed to prevent its biofilm formation. Here, we investigated 83 indole derivatives against A. tumefaciens, and based on the screening, 4-chloroindole, 6-iodoindole, and 5-chloro-2-methyl indole were selected as candidates that at 50 mu g mL(-1) significantly inhibited the adherence and biofilm formation of A. tumefaciens to abiotic (nitrocellulose and polystyrene) and biotic (roots of Brassica juncea) surfaces. Furthermore, they reduced bacterial growth in a time and concentration-dependent manner and significantly reduced log CFU mL(-1) and survival (%). Changes in biofilm morphologies and biomasses, thicknesses, and substratum coverages were determined, and 2-D and 3-D analyses were performed using a crystal violet assay and bright field, CLSM, and SEM microscopies. Virulence factors such as swimming motility, exopolysaccharide, and exo-protease production, and cell surface hydro-phobicity were markedly inhibited by the three compounds. Transcriptional analysis showed multi-fold down-regulation of biofilm, virulence, motility, and stress-related genes; however, the degrees of these downregulations were variably affected. B. juncea seed germination was only severely affected by 4-chloroindole. This study demonstrates the promising antibiofilm and antivirulence activities of the three indole derivatives tested and their potentials for targeting and curbing A. tumefaciens infections.

Automated summary

This study investigates the potential of three indole derivatives, 4-chloroindole, 6-iodoindole, and 5-chloro-2-methyl indole, as antibiofilm and antivirulence agents against Agrobacterium tumefaciens. The compounds inhibit bacterial adherence, biofilm formation, growth, and the production of virulence factors. They also downregulate biofilm, virulence, motility, and stress-related genes. These findings suggest that the indole derivatives have promising activity against A. tumefaciens infections.