SPI1 and infection: While little is known about the expression of SPI-1 factors upon oxidative stress, several SPI-1 factors SipA, SopA, SopB, SopD, and SopE2 of S. Typhimurium were found to be expressed in the spleen of infected animals at the late stages of infection when Salmonella is believed to replicate in splenic macrophages [15,16].