AB5 toxins are comprised of one catalytic A subunit that disrupts distinct essential cellular processes within the cell and receptor binding, pentameric B subunit that enables the toxin to target certain cell types. How these toxins evolved to infect human hosts are poorly understood. The aims of this proposal are to investigate how B pentamer evolved by comparing the structure and function of two related toxins and performing directed evolution on the B-pentamer. The outcomes of this proposal will aid in understanding the mechanism AB5 toxins have evolved to invade human hosts with potential to develop novel antimicrobials