SPI1 and infection: In addition, they observed that at 2 and 4 days post infection almost all infected piglets had detectable levels of Salmonella in their colon upon necropsy and tissue culture, but gross lesions suggestive of inflammation were absent in S. Derby-infected pigs as opposed to the pigs infected with S. Typhimurium and S. 4,[5],12:i:. These data could be indicative that S. Derby does not invade the intestinal epithelium to a sufficient extent to cause an observable inflammation, which is usually the result of the induction of several SPI-1 effectors.