Expression and transcriptional control of the Salmonella typhimurium Ipf fimbrial operon by phase variation

The lpf operon mediates adhesion of Salmonella typhimurium to murine Peyer's patches. To investigate expression of this virulence factor, a transcriptional fusion between the lpf operon and the genes lacZYA of Escherichia coli was constructed and introduced into the S. typhimurium chromosome. The resulting strain yielded both Lac+ and Lac- colony phenotypes. Alternation between Lac+ (phase ON) and Lac- (phase OFF) phenotypes occurred by a heritable phase variation mechanism, as inoculation of broth cultures with bacteria picked from a Lac+ colony gave rise to a considerably higher proportion of Lac+ colonies than inoculation with bacteria picked from a Lac- colony. During growth in vitro, phase transition from ON to OFF and from OFF to ON occurred at rates 6.8 x 10(-3) and 2.4 x 10(-4) events per cell per generation respectively. In a murine intestinal organ culture model, selection for the ON expression state occurred when attached bacteria were recovered from Peyer's patches, suggesting that Lac phase variation correlated with expression of lpf mediated adherence. Selection for either the ON or the OFF expression state of the Ipf operon in vivo was studied in mice immunized with either GST or GST-LpfA fusion protein. A strong selection against phase ON cells occurred only in animals immunized with GST-LpfA. The ability of animals immunized with GST-LpfA to distinguish between phase ON and phase OFF bacteria provided evidence for the presence of LpfA fimbrial protein in phase ON cells and for its exposure to the immune system.