In collaboration with Dr. Stefan Pukatzki (University of Colorado Denver), Dr. Daniele Provenzano (University of Texas Rio Grande Valley), and Dr. Daniel Unterweger (University of Oxford), Paul and Yan conducted an extensive study on the role of horizontal gene transfer in shaping the diversity of type VI secretion system (T6SS) loci in V. cholerae and closely related species. They discovered the existence of complex arrays of effector and immunity genes in these loci. These arrays differ even between closely related sympatric strains and appear to be formed by successive horizontal gene transfer events. The resulting accumulation of large numbers of immunity genes potentially enhance the recipient's fitness in T6SS-mediated bactericidal interactions. Additionally, they show how the accumulation of T6SS elements through horizontal gene transfer could have contributed to the evolution of some V. cholerae strains from harmless environmental bacteria to pandemic pathogens.