SPI1 and infection: Furthermore, in previous work we showed that relevant S. Typhimurium virulence factors required for infection of mammalian host cells and animal models at 37°C, including the type III secretion systems (T3SS) encoded in SPI-1 and SPI-2 pathogenicity islands, are also essential to invade and survive inside D. discoideum cells at 23°C (Riquelme et al., 2016).