Difficidin and bacilysin produced by plant-associated Bacillus amyloliquefaciens are efficient in controlling fire blight disease

Representatives of Bacillus amyloliquefaciens were shown to possess biocontrol activity against fire blight, a serious disease of orchard trees caused by Erwinia amylovora. Genome analysis of B. amyloliquefaciens FZB42 identified gene clusters responsible for synthesis of several polyketide compounds with antibacterial action. We show here that the antibacterial polyketides difficidin and to a minor extent bacillaene act efficiently against E. amylovora. Surprisingly, a mutant strain blocked in the production of difficidin (CH8 Δdfn) inhibited growth of E. amylovora and suppressed fire blight disease nearly in the same range as the wild type. In addition, a sfp mutant (CH3 Δsfp) unable to synthesize non-ribosomally lipopeptides and polyketides did still suppress growth of E. amylovora, suggesting that besides action of polyketides another antagonistic principle exist. A double mutant (RS06 Δsfp Δbac) devoid in polyketide and bacilysin synthesis was unable to suppress growth of E. amylovora indicating that the additional inhibitory effect is due to production of bacilysin, a dipeptide whose synthesis does not depend on Sfp. We propose to use B. amyloliquefaciens strains with enhanced synthesis of difficidin and/or bacilysin for development of biocontrol agents efficient against fire blight disease.