Acinetobacter baumannii Kills Fungi via a Type VI DNase Effector

Many Gram-negative bacteria deploy a type VI secretion system (T6SS) to inject toxins into target cells to promote their survival and replication in complex environments. Here, we report that Acinetobacter baumannii uses its T6SS to kill fungi and that the effector TafE (ACX60_15365) is responsible for such killing. Although ectopically expressed TafE is toxic to both Escherichia coli and Saccharomyces cerevisiae, deletion of tafE only affects the antifungal activity of A. baumannii. We demonstrate that TafE is a DNase capable of targeting the nuclei of yeast cells and that an Ntox15 domain is essential for its ability to degrade DNA. Furthermore, our findings show that A. baumannii is protected from the toxicity of TafE by elaborating the immunity protein TaeI (ACX60_15360), which antagonizes the activity of the effector by direct binding. The discovery of A. baumannii T6SS effectors capable of killing multiple taxonomically distinct microbes has shed light on a mechanism of the high-level fitness of this pathogen in environments characterized by scarce nutrients and the potential presence of diverse microorganisms.