The GacS/GacA two-component system strongly regulates antimicrobial competition mechanisms of MFE01 strain.

Unlabelled: MFE01 is an environmental bacterium characterized by an hyperactive type 6 secretion system (T6SS) and a strong emission of volatile organic compounds (VOCs). In a previous study, a transposition mutant, 3H5, exhibited an inactive T6SS and altered VOC emission. In 3H5, the interruption of gene by the transposon was insufficient to explain these phenotypes. To determine the actual impact of this insertion, a comparative transcriptomic analysis was performed on the two-component system GacS/GacA, known to influence numerous phenotypes in . The results demonstrated that the gene is less expressed in 3H5 than in MFE01. Phenotypic analysis of a deletion mutant, Δ, confirmed many similarities between Δ and 3H5. Indeed, Δ exhibited an inactive T6SS and an altered VOC emission profile. In-depth analysis of volatilomes and phenotypes correlated with the decrease in transcription, highlighting that the emission of 1-undecene is under the strict control of GacS/GacA. This study confirms that 1-undecene is not the sole volatile molecule responsible for MFE01's inhibition of . Moreover, MFE01 has antimicrobial activity against the phytopathogenic oomycetes via hydrogen cyanide (HCN) emission, which is also controlled by GacS. In MFE01, GacS/GacA is also a partial positive regulator of other VOC emission, whose reduced emission in 3H5 coincides with the decrease in transcription.

Importance: Our model strain MFE01 uses an active type VI secretion system (T6SS) and volatile compounds (VCs) to outcompete other microorganisms in the natural environment. By investigating the cellular mechanism regulating the production of these weapons, we identified the two-component system GacS/GacA. Indeed, GacS cellular membrane sensor plays a crucial role in regulating T6SS activity and VC emission. Among the latter, 1-undecene and hydrogen cyanide are strong aerial inhibitors of the human pathogen and the major plant pest, respectively. The aim is to improve the understanding of the regulation of these volatile molecule emission and the critical role of a global regulator in both plant and human health.