SPI1 and infection: The changing growth phase-dependent levels and/or activities of RpoS, ppGpp and DksA may also potentially be expected to have a significant impact upon the ‘timing’ of SPI1 and SPI2 expression during infection, since it is established that SPI1 and SPI2 genes tend to be expressed under conditions conducive to either invasion and intracellular replication respectively, although some overlap has been found [5].