_Salmonella enterica_ serovar Typhimurium is a Gram-negative bacterium that is the causative agent of typhoid fever, a potentially fatal systemic infection. The bacterium is typically transmitted through the ingestion of contaminated food or water, and it initially colonizes the gut. In some cases, however, _S. Typhimurium_ can also invade the bloodstream, causing bloodstream infections (BSIs).
BSIs caused by _S. Typhimurium_ can be life-threatening, and they are associated with a high mortality rate. The risk of developing a BSI is highest in individuals with weakened immune systems, such as the elderly, the very young, and those with chronic illnesses.
Evolution of a killer
_S. Typhimurium_ is a relatively recent evolutionary newcomer, having emerged as a distinct serovar of _S. enterica_ within the past few thousand years. This serovar is thought to have evolved from an ancestor that was primarily adapted to living in the gut of animals. However, over time, _S. Typhimurium_ acquired a number of mutations that enabled it to invade the bloodstream and cause disease.
One of the key mutations that enabled _S. Typhimurium_ to become a successful pathogen is the acquisition of the _Salmonella_ pathogenicity island 1 (SPI-1) gene cluster. SPI-1 encodes a type III secretion system (T3SS), which is a molecular syringe that allows _S. Typhimurium_ to inject effector proteins into host cells. These effector proteins disrupt the host cell's normal function, allowing _S. Typhimurium_ to survive and replicate within the cell.
Another important mutation that contributed to the success of _S. Typhimurium_ is the acquisition of the _Salmonella_ virulence plasmid (pSLT). pSLT encodes a number of virulence factors, including the SopE protein, which is essential for _S. Typhimurium_ to invade epithelial cells.
Conclusion
The evolution of _S. Typhimurium_ from a gut commensal to a deadly pathogen is a fascinating example of how bacteria can adapt and change over time. The acquisition of key mutations, such as SPI-1 and pSLT, enabled _S. Typhimurium_ to become one of the most successful pathogens in the world.
