Biocontrol of Bacterial Fruit Blotch by Bacillus subtilis 9407 via Surfactin-Mediated Antibacterial Activity and Colonization

Abstract

In this study, <i>Bacillus subtilis</i> 9407 showed a strong antibacterial activity against <i>Acidovorax citrulli in vitro</i> and 61.7% biocontrol efficacy on melon seedlings 4 days post inoculation under greenhouse conditions. To understand the biocontrol mechanism of <i>B. subtilis</i> 9407, identify the primary antibacterial compound and determine its role in controlling bacterial fruit blotch (BFB), a <i>srfAB</i> deletion mutant (Δ<i>srfAB</i>) was constructed. The Δ<i>srfAB</i> which was deficient in production of surfactin, not only showed almost no ability to inhibit growth of <i>A. citrulli</i> but also decreased biofilm formation and reduced swarming motility. Colonization assay demonstrated that <i>B. subtilis</i> 9407 could conlonize on melon roots and leaves in a large population, while Δ<i>srfAB</i> showed a four- to ten-fold reduction in colonization of melon roots and leaves. Furthermore, a biocontrol assay showed that Δ<i>srfAB</i> lost the biocontrol efficacy. In summary, our results indicated that surfactin, which consists of C13- to C16-surfactin A was the primary antibacterial compound of <i>B. subtilis</i> 9407, and it played a major role in biofilm formation, swarming motility, colonization and suppressing BFB. We propose that the biocontrol activity of <i>B. subtilis</i> 9407 is the results of the coordinated action of surfactin-mediated antibacterial activity and colonization. This study reveals for the first time that the use of a <i>B. subtilis</i> strain as a potential biological control agent could efficiently control BFB by producing surfactin.