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

Abstract

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 <i>Acinetobacter baumannii</i> strains produce a T6SS, but little is known about the specific effectors or how they are delivered. In this study, we show that <i>A. baumannii</i> AB307-0294 encodes three <i>vgrG</i> loci, each containing a <i>vgrG</i> gene, a T6SS toxic effector gene, and an antitoxin/immunity gene. Each of the T6SS toxic effectors could kill <i>Escherichia coli</i> when produced in <i>trans</i> unless the cognate immunity protein was coproduced. To determine the role of each VgrG in effector delivery, we performed interbacterial competitive killing assays using <i>A. baumannii</i> AB307-0294 <i>vgrG</i> mutants, together with <i>Acinetobacter baylyi</i> 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 <i>A. baylyi</i> and that each VgrG protein is specific for the carriage of one effector. Finally, we analyzed a number of <i>A. baumannii</i> genomes and identified significant diversity in the range of encoded T6SS VgrG and effector proteins, with correlations between effector types and <i>A. baumannii</i> global clone lineages.