Figure 1.

Salmonella infection requires invasion of host cells. Following ingestion and passage through the stomach, Salmonella encounters the intestinal epithelial layer, a barrier which must be breached. It is thought that the pathogen preferentially enters M cells, and is trancytosed and passed to underlying macrophages. Here Salmonella can evade killing by causing apoptosis leading to release, or survive and replicate long enough to allow systemic spread through the reticuloendothelial system. Salmonella can also force its own uptake by epithelial cells from the apical side, i.e., the lumen of the intestine. Following uptake, the bacteria can replicate intracellularly, or be transcytosed and released on the basolateral side, from here they can infect further epithelial cells. Additionally, dendritic cells can directly capture Salmonella from the lumen and transport them across the epithelium.