DNA in most bacterial cells is maintained in an underwound state. The DNA double helix responds to underwinding by adopting a minimum energy conformation through the supercoiling of the duplex, the formation of local single-stranded bubbles or a combination of both. This Microbiology Primer summarizes the key topological features of DNA and describes the topoisomerase enzymes that manage bacterial DNA topology. The influences of variable DNA topology on transcription and of transcription (and DNA replication) on DNA topology are also discussed. Finally, the article considers the impact of changes in bacterial metabolism and physiology on DNA topology and their implications for bacterial pathogenesis.