Single Cell Analysis of Bistable Expression of Pathogenicity Island 1 and the Flagellar Regulon in Salmonella enterica

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-1(OFF) Flag(OFF), SPI-1(OFF) Flag(ON), SPI-1(ON) Flag(OFF), and SPI-1(ON) Flag(ON). Invasion assays upon cell sorting show that none of the four subpopulations is highly invasive, thus raising the possibility that Flag(OFF) cells might contribute to optimal invasion as previously proposed for SPI-1(OFF) cells. Time lapse microscopy observation indicates that expression of the flagellar regulon contributes to the growth impairment previously described in SPI-1(ON) cells. As a consequence, growth resumption in SPI-1(ON) Flag(ON) cells requires switching to both SPI-1(OFF) and Flag(OFF) states.