Spatiotemporal replication dynamics of Salmonella during systemic disease

Abstract

Salmonella enterica serovar Typhimurium (S. Typhimurium) causes a systemic disease in susceptible mouse strains that is widely used as a model of human typhoid fever. I developed a reporter system based on fluorescence dilution that measures bacterial replication directly at both the population and single cell level. To understand how S. Typhimurium colonises host tissues during disease, I applied fluorescence dilution to study S. Typhimurium replication during acute murine typhoid, following oral inoculation. Bacteria that had not replicated were found in the Peyer’s patches (PP), mesenteric lymph nodes (MLN) and spleen. Hence, replication is not a prerequisite for S. Typhimurium to traverse the intestinal wall and reach deeper tissues. Furthermore, non-replicating bacteria were found to persist for long periods of time within the intestine and, to a smaller extent, in the spleen, suggesting that these bacteria may represent dormant reservoirs that cause chronic infections. Bacteria replicated rapidly upon invasion of the PP, while the MLN represented a more restrictive niche for bacterial replication. In addition, the spleen was seeded preferentially by bacteria that had already replicated elsewhere. Further analysis of bacterial replication in the spleen provided insights into the contribution of the Salmonella-encoded pathogenicity island-2 type III secretion system to replication in this organ. Experiments also showed that S. Typhimurium preferentially colonised and replicated within macrophages within the spleen. Therefore, the use of fluorescence dilution has provided detailed insights into the relationship between the replication of S. Typhimurium and its colonisation of host tissues, and has revealed the existence and localisation of non-replicating bacteria that could persist during chronic infections