The type VI secretion system and associated effector proteins

Bacteria have evolved a myriad of antibacterial weapons to engage in interbacterial conflicts. Among these, many Gram-negative bacteria use type VI secretion systems (T6SSs), which are contractile toxin delivery systems, to eliminate or suppress competitors in a contact-dependent manner. Recent studies have revealed that both the toxic effector arsenals and the corresponding immunity strategies that defend against incoming T6SS attacks are far more diverse and variable than previously anticipated. This diversity is increasingly recognized as the outcome of an ongoing evolutionary arms race, driven by antagonistic interactions between bacteria. In this Review, we provide an overview of our current understanding of T6SSs, with a particular focus on the diversity of antibacterial T6SS repertoires and the evolutionary pressures shaping them. We outline the key principles underlying effector diversification and discuss the emerging role of interbacterial antagonism as a key driver of this process.