Formation of phenotypic lineages in Salmonella enterica by a pleiotropic fimbrial switch

The std locus of Salmonella enterica, an operon acquired by horizontal transfer, encodes fimbriae that permit adhesion to epithelial cells in the large intestine. Expression of the std operon is bistable, yielding a major subpopulation of Std(OFF) cells (99.7%) and a minor subpopulation of Std(ON) cells (0.3%). In addition to fimbrial proteins, the std operon encodes two proteins, StdE and StdF, that have DNA binding capacity and control transcription of loci involved in flagellar synthesis, chemotaxis, virulence, conjugal transfer, biofilm formation, and other cellular functions. As a consequence of StdEF pleiotropic transcriptional control, Std(ON) and Std(OFF) subpopulations may differ not only in the presence or absence of Std fimbriae but also in additional phenotypic traits. Separation of Std(OFF) and Std(ON) lineages by cell sorting confirms the occurrence of lineage-specific features. Formation of Std(OFF) and Std(ON) lineages may thus be viewed as a rudimentary bacterial differentiation program.