DNA topology and adaptation of Salmonella typhimurium to an intracellular environment

The expression of genes coding for determinants of DNA topology in the facu ltative intracellular pathogen Salmonella typhimurium was studied during ad aptation by the bacteria to the intracellular environment of J774A.1 macrop hage-like cells. A reporter plasmid was used to monitor changes in DNA supe rcoiling during intracellular growth. Induction of the dps and spv genes, p reviously shown to he induced in the macrophage, was detected, as was expre ssion of genes coding for DNA gyrase, integration host factor and the nucle oid-associated protein H-NS. The topA gene, coding for the DNA relaxing enz yme topoisomerase I, was not induced. Reporter plasmid data showed that bac terial DNA became relaxed following uptake of S. typhimurium cells by the m acrophage. These data indicate that DNA topology in S. typhimurium undergoe s significant changes during adaptation to the intracellular environment. A model describing how this process may operate is discussed.