One of the biggest barriers to understanding bacteria, regardless of the niche they occupy or their pathogenicity, is the ability to genetically modify them, and to construct targeted mutations in genes of interest. From the early days of molecular biology, the study of plasmids has facilitated the development of powerful genetic tools which has enabled major breakthroughs in our understanding of bacterial biology and function. In this presentation, I will discuss how fundamental investigations of plasmid biology were exploited to understand core functions in gut bacteria, including antimicrobial resistance, lifestyle adaptations and virulence.