The type VI secretion system and associated effector proteins.

Type VI secretion systems in Gram-negative bacteria deliver contact-dependent antibacterial toxins, and recent work shows toxic effector arsenals and immunity strategies are far more diverse and variable than previously anticipated.

• Why it matters: Understanding how T6SS effector repertoires diversify matters because it reflects an ongoing evolutionary arms race, but the drivers and principles shaping this diversity were not fully appreciated.
• What they did: This Review synthesizes current knowledge on T6SSs, emphasizing how effector diversification is organized and how interbacterial antagonism is discussed as an evolutionary pressure shaping repertoires.
• The result: The review concludes that interbacterial antagonism is increasingly recognized as a key driver of effector diversification, helping frame how T6SS repertoires evolve under competitive pressure.

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