Pseudomonas aeruginosa assembles H1-T6SS in response to physical and chemical damage of the outer membrane.

Abstract

Bacteria respond to environmental stimuli and attacks from competing organisms. Pseudomonas aeruginosa 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 P. aeruginosa 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.