<i>Pseudomonas aeruginosa</i> assembles H1-T6SS in response to physical and chemical damage of the outer membrane

Bacteria respond to environmental stimuli and attacks from competing organisms. <i>Pseudomonas aeruginosa</i> assembles the type VI secretion system (H1-T6SS) to precisely retaliate against aggressive competing bacteria. However, we lack an understanding of how the H1-T6SS assembly dynamically responds to nanomechanical forces. To address this, we analyzed live cells using correlative atomic force microscopy (AFM) and fluorescence microscopy. We show that indentation forces above 7 nanonewtons trigger local, repeated and targeted H1-T6SS assemblies within seconds of impact by the AFM tip. Analysis of the corresponding AFM force curves shows that a breach of a single layer of the cell envelope is necessary and sufficient for triggering H1-T6SS assembly. Accordingly, polymyxin B nonapeptide, which damages the outer membrane, also triggers H1-T6SS assembly. This suggests that <i>P. aeruginosa</i> has evolved a danger-sensing mechanism that enables rapid and precise deployment of its antibacterial H1-T6SS in response to breaches in the outer membrane.