Abstract
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 StdOFF cells (99.7%) and a minor subpopulation of StdON 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, StdON and StdOFF subpopulations may differ not only in the presence or absence of Std fimbriae but also in additional phenotypic traits. Separation of StdOFF and StdON lineages by cell sorting confirms the occurrence of lineage-specific features. Formation of StdOFF and StdON lineages may thus be viewed as a rudimentary bacterial differentiation program.
Highlights
Phenotypic differences in isogenic bacterial cells grown in the same environment can be a consequence of noisy gene expression [1,2]
We show that the std fimbrial operon of Salmonella enterica undergoes bistable expression, a trait far from exceptional among loci that encode components of the bacterial envelope
An unsuspected trait of the std operon is the presence of two genes that encode pleiotropic regulators of gene expression
Summary
Phenotypic differences in isogenic bacterial cells grown in the same environment can be a consequence of noisy gene expression [1,2]. The mechanisms that cause bistability include genetic rearrangement, expansion and contraction of DNA sequence repeats, epigenetic control of gene expression by DNA methylation, and formation of regulatory feedback loops transmissible to daughter cells [5]. This variety, obviously indicative of independent evolution, may suggest that the ability of a given bacterial species to diversify into phenotypic lineages is a product of natural selection. Game theory shows that lineage formation can have selective value either as a division of labour or as a bet hedging [3,6,7,8] In both kinds of strategies, the key biological entity is not the individual cell but the population [9]. The fitness of each cell type is higher under different circumstances, and differentiation into distinct lineages preadapts the population to environmental changes
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