© 2014 Published by Elsevier Ltd. Most bacterial chromosomes and plasmids are covalently closed circular molecules that are maintained in a dynamic supercoiled state. Average supercoil density differs significantly between Escherichia coli and Salmonella. Two related questions are: What protein(s) create supercoil domain boundaries in a bacterial chromosome? and How is supercoil density regulated in different bacterial species? RNA polymerase plays pivotal roles in both of these topological phenomena.