Insights Into an Antibacterial Type VI Membrane-Disrupting Effector Of Adherent Invasive Escherichia coli

Adherent Invasive Escherichia coli (AIEC) strains are associated with ileal Crohn's disease and may contribute to intestinal dysbiosis through antibacterial competition. The AIEC reference strain LF82 encodes two type VI secretion systems (T6SSs), a contractile machine that injects an effector-loaded needle made of Hcp, VgrG and PAAR into neighbouring rivals, favoring niche colonization. LF82 T6SSs harbour several putative antibacterial toxin-immunity modules. Here we characterize the T6SS-1 encoded TmeAIEC, a PAAR-fused T6SS effector carrying a C-terminal Tme domain, hallmark of Tme (Type VI membrane disrupting effector) toxins, and its DUF1240-like immunity protein TmiAIEC. When heterologously produced in E. coli K-12 as well as in E. coli and Citrobacter clinical isolates, TmeAIEC is toxic only upon periplasmic targeting, and its toxicity is neutralized by TmiAIEC. Periplasmically-targeted TmeAIEC inserts into the inner membrane, collapses the proton motive force and increases cell permeability, consistent with a membrane-disrupting activity. Mutational analyses coupled to functional assays identified a predicted hydrophobic transmembrane segment and a conserved aspartate residue within the Tme domain as key determinants for TmeAIEC toxicity. These findings establish TmeAIEC/TmiAIEC as a functional T6SS toxin-immunity pair and reveal conserved determinants underlying the activity of Tme-family membrane-disrupting effectors.