Salmonella Typhimurium undergoes distinct genetic adaption during chronic infections of mice

Typhoid fever caused by Salmonella enterica serovar Typhi (S. Typhi) is a severe systemic human disease and endemic in regions of the world with poor drinking water quality and sewage treatment facilities. A significant number of patients become asymptomatic life-long carriers of S. Typhi and serve as the reservoir for the disease. The specific mechanisms and adaptive strategies enabling S. Typhi to survive inside the host for extended periods are incompletely understood. Yet, elucidation of these processes is of major importance for improvement of therapeutic strategies. In the current study genetic adaptation during experimental chronic S. Typhimurium infections of mice, an established model of chronic typhoid fever, was probed as an approach for studying the molecular mechanisms of host-adaptation during long-term host-association.

Individually sequence-tagged wild type strains of S. Typhimurium 4/74 were used to establish chronic infections of 129X1/SvJ mice. Over the course of infections, S. Typhimurium bacteria were isolated from feces and from livers and spleens upon termination of the experiment. In all samples dominant clones were identified and select clones were subjected to whole genome sequencing. Dominant clones isolated from either systemic organs or fecal samples exhibited distinct single nucleotide polymorphisms (SNPs). One mouse appeared to have distinct adapted clones in the spleen and liver, respectively. Three mice were colonized in the intestines by the same clone containing the same non-synonymous SNP in a transcriptional regulator, kdgR, of metabolic genes. This likely indicates transmission of this clone between mice. The mutation was tracked to have occurred prior to 2 weeks post infection in one of the three mice and had subsequently been transmitted to the other two mice. Re-infection with this clone confirmed that it is superior to the wild type for intestinal colonization.

During 4 to 6 weeks of chronic infections, S. Typhimurium acquired distinct SNPs in known regulators of metabolic and virulence genes. One SNP, the kdgR-SNP was confirmed to confer selective advantage during chronic infections and constitute a true patho-adaptive mutation. Together, the results provide evidence for rapid genetic adaptation to the host of S. Typhimurium and validate experimental evolution in the context of host infection as a strategy for elucidating pathogen host interactions at the molecular level.