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The type VI secretion system (T6SS) is a powerful tool deployed by Gram-negative bacteria to antagonize neighboring organisms. Here, we report thatAcinetobacter baumanniiATCC 17978 (Ab17978) secretes D-lysine (D-Lys), increasing the extracellular pH and enhancing the peptidoglycanase activity of the T6SS effector Tse4. This synergistic effect of D-Lys on Tse4 activity enables Ab17978 to outcompete Gram-negative bacterial competitors, demonstrating that bacteria can modify their microenvironment to increase their fitness during bacterial warfare. Remarkably, this lethal combination also results in T6SS-mediated killing of Gram-positive bacteria. Further characterization revealed that Tse4 is a bifunctional enzyme consisting of both lytic transglycosylase and endopeptidase activities, thus representing a novel family of modularly organized T6SS PG degrading effectors with an unprecedented impact in antagonistic bacterial interactions.

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By modulating local environmental pH through D-Lys secretion,Acinetobacter baumanniienhances the activity of a bifunctional cell wall-targeting T6SS effector, increasing its killing activity against Gram-negative and -positive competitors.