**Abstract**

Considering a complex set of interplay with its host, *Salmonella* needs numerous genes for its full virulence. These genes responsible for invasion, survival, and extra intestinal spread are located on pathogenicity islands known as *Salmonella* pathogenicity islands (SPIs) that are thought to be acquired by horizontal gene transfer. A total of 17 SPIs (1–17) are recognized so far. The type III secretion system (T3SS) encoded by SPI-1 is considered as the most important virulence factor for *Salmonella* that delivers effector proteins necessary for invasion and production of enteritis. Among various SPIs, the role in virulence is well proven for SPI1 and SPI2 and further insight into the complex regulatory network of SPIs can contribute to drug investigation and prevention of infection.

**Keywords:** *Salmonella*, virulence genes, *Salmonella* pathogenicity islands, Type III secretion system
