An infection time course revealed that SPI-1 driven luciferase expression was strongly induced in the intestines of S. Typhimurium invF-lux-infected fed SPF mice, with a maximal signal seen at 15h p.i. (see Fig 6A). Since SPI-1 induction was not synchronized among the whole population of S. Typhimurium, the strong luciferase signal detected from the infected tissues represented a subset of the bacteria, and is not necessarily, directly proportional to the recovered CFU counts. Here, SPI1 is linked to infection.