Identification of Novel Acinetobacter baumannii Type VI Secretion System Antibacterial Effector and Immunity Pairs.

The type VI secretion system (T6SS) is a macromolecular machine that delivers protein effectors into host cells and/or competing bacteria. The effectors may be delivered as noncovalently bound cargo of T6SS needle proteins (VgrG/Hcp/PAAR) or as C-terminal extensions of these proteins. Many strains produce a T6SS, but little is known about the specific effectors or how they are delivered. In this study, we show that AB307-0294 encodes three loci, each containing a gene, a T6SS toxic effector gene, and an antitoxin/immunity gene. Each of the T6SS toxic effectors could kill when produced in unless the cognate immunity protein was coproduced. To determine the role of each VgrG in effector delivery, we performed interbacterial competitive killing assays using AB307-0294 mutants, together with prey cells expressing pairs of immunity genes that protected against two toxic effectors but not a third. Using this approach, we showed that AB307-0294 produces only three T6SS toxic effectors capable of killing and that each VgrG protein is specific for the carriage of one effector. Finally, we analyzed a number of genomes and identified significant diversity in the range of encoded T6SS VgrG and effector proteins, with correlations between effector types and global clone lineages.