Microbial toxins are a molecular weapon involved in pathogenesis, immune evasion, and bacterial competition. A prime example of such microbial toolkits are polymorphic toxin systems, which consist of multi-domain proteins and are widespread in all major bacterial lineages. A polymorphic toxin system called MuF has been newly identified and is the first to be discovered in temperate phages and their bacterial hosts. Though it is highly abundant in the human gut microbiome, its biological role has not been defined. To better understand the toxin system, our team is studying a model species Enterococcus faecalis, a commensal bacterium encoding a two-domain MuF toxin protein on one of its phages, consisting of an N-terminal MuF domain and a C-terminal toxin domain. The toxin domain is predicted to be an ADP-ribosyltransferase (ART), which post-translationally attaches ADP-ribose moieties to its target molecules and can profoundly impair cell processes, leading up to cell death. Using genetic approaches to generate phages with malfunctional ART activity, I have found that the mutations change phage infectivity and the morphology of the plaques formed (clear zones in a cell layer formed due to lysis by phage). Moreover, heterologous expression of the toxin domain in E. faecalis results in cell aggregation. From this, I hypothesize that the MuF toxin is delivered by phages to help infection and ensure phage DNA incorporation into host genomes. To further dissect the mechanism by which the MuF toxin system operates, our team is currently developing a fluorescent protein reporter system to investigate and track the detailed process of phage infection. In addition, by applying X-ray crystallography and electron microscopy, I aim to uncover structural information on the toxin, which may lend insight into the mechanism of MuF toxicity and its larger role in the human microbiome.