Bistable expression of the Salmonella enterica pathogenicity island 1 (SPI-1) and the flagellar network (Flag) has been described previously. In this study, simultaneous monitoring of OFF and ON states in SPI-1 and in the flagellar regulon reveals independent switching, with concomitant formation of four subpopulations: SPI-1OFF FlagOFF, SPI-1OFF FlagON, SPI-1ON FlagOFF, and SPI-1ON FlagON. Invasion assays upon cell sorting show that none of the four subpopulations is highly invasive, thus raising the possibility that FlagOFF cells might contribute to optimal invasion as previously proposed for SPI-1OFF cells. Time lapse microscopy observation indicates that expression of the flagellar regulon contributes to the growth impairment previously described in SPI-1ON cells. As a consequence, growth resumption in SPI-1ON FlagON cells requires switching to both SPI-1OFF and FlagOFF states.
Single Cell Analysis of Bistable Expression of Pathogenicity Island 1 and the Flagellar Regulon in Salmonella enterica