Quorum-quenching endophytic bacteria inhibit disease caused by Pseudomonas syringae pv. syringae in Citrus cultivars.

Two strains of 64 endophytic bacteria, Bacillus cereus Si-Ps1 and Pseudomonas azotoformans La-Pot3-3, isolated from Citrus sinensis and C. sinensis var. Thomson's leaves, respectively, reduced N-acyl homoserine-based quorum sensing in bioindicator strain Pseudomonas syringae pv. syringae (Pss) B728a and the biofilm production and swarming motility of field isolate Pss 3289. A homolog of aiiA gene encoding an AHL-lactonase was found in B. cereus (Si-Ps1), suggesting that this isolate can degrade the quorum-sensing signal molecules of Pss 3289. The crude extract of endophytic bacterium, B. cereus (Si-Ps1), inhibited Pss 3289 biofilm formation after 48 and 96 h by 55% and 58%, respectively. Similar reductions in biofilm formation were conferred by crude extracts of P. azotoformans (La-Pot3-3). Correspondingly, the number of planktonic cells in cultures treated with these extracts was higher than in control cultures, indicating a direct effect on biofilm formation and not on cell growth. In greenhouse assays, the virulence of Pss 3289 to different citrus cultivars was decreased when coinoculated with these endophytic bacteria.