SPI1 and infection: However, on a level of animal-pathogen interaction, SPI-1 mutants can be found easily in mesenterial lymph nodes, liver or spleen in mice, chickens or pigs [2-5] which indicates that in vivo, the invasion of epithelial cells is not strictly required for S. enterica pathogenesis and the SPI-1-encoded T3SS may have additional function(s) during the in vivo infection, as proposed earlier [2].