Acinetobacter baumannii is a Gram-negative opportunistic pathogen responsible for a range of nosocomial and community-acquired infections. The rapidly increasing prevalence of multi-drug resistant (MDR) strains of bacteria raises serious concerns about the availability of antimicrobial treatments. Notably, many Gram-negative bacteria utilise a type VI secretion system (T6SS), which structurally resembles an inverted bacteriophage tail, to deliver toxic effector proteins directly into target cells in a contact-dependent manner. The clinical isolate A. baumannii AB307-0294 delivers three antibacterial effectors via the T6SS, including Tse15. This Rhs-type effector has a central Rearrangement Hot Spot (RHS) domain and a toxic C-terminal region (Tse15-CT) which acts via an unknown mechanism. Tse15-CT toxicity was previously shown to be neutralised by the cognate type VI immunity protein (Tsi15), enabling protection against self/sibling-cell intoxication (1). In this study, we aim to further characterise Tse15 and define the precise region required for toxic activity and for the immunity protein Tsi15 to neutralize activity. Analysis of the transcriptional response to Tse15-CT expression in Escherichia coli revealed several biological pathways that were significantly over-represented in these data including methionine synthesis, putrescine catabolism, and the phage shock response. Additionally, AlphaFold2 modelling used to predict the structure of Tse15-CT revealed an 8-stranded anti-parallel β-barrel. Our preliminary data show that Tse15-CT retains toxicity when the N-terminal 45 or C-terminal 13 amino acids are deleted but loses functionality when both regions are absent. Neutralisation of Tse15-CT by the immunity protein Tsi15 was unaffected by deletion of the Tse15-CT N-terminal 45 amino acids, suggesting that this region is not essential for the effector/immunity protein interaction. Importantly, characterisation of Tse15 and other antibacterial T6SS effectors, provides insight into mechanisms of T6SS-mediated killing and may facilitate the use of these effectors and/or the T6SS system as novel antimicrobial strategies.