Type VI secretion phenotypic heterogeneity ensures trade-off between antibacterial activity and resistance in Enteroaggregative E. coli

The type VI secretion system (T6SS) is a nanoweapon deployed by bacteria to deliver effectors into target cells, allowing niche colonization or host invasion. In entero-aggregative Escherichia coli (EAEC), the expression of the T6SS gene cluster is responsive to iron starvation by the Fur regulator, and to Dam-dependent methylation. However, fluorescence microscopy recordings showed that the T6SS displays heterogeneous expression in a clonal population. The population is composed of three types of cells in a stable equilibrium: non producers (OFF), producers, and assemblers (ON). Single-cell analyses further showed that the activation state is transmitted to the progeny, but the overall distribution respects the equilibrium suggesting a genetic control of heterogeneity. Mutagenesis of Fur-binding boxes and GATC sites revealed that specific promoter elements control this heterogeneity. Phenotypic characterization of the ON and OFF subpopulations demonstrated that ON cells eliminate susceptible target cells, whereas OFF cells survive attacks from T6SS-defensive bacteria. The heterogeneity of EAEC T6SS is therefore an attenuation strategy conferring a trade-off between antibacterial efficiency and survival in polymicrobial environments.