Fim switch

These pili are virulence factors involved in adhesion, especially important in uropathogenic Escherichia coli.

The gene undergoes phase variation mediated via two recombinases and is a model example of site specific inversion.

This equilibrium, shifted towards maintaining the "off" position, due to higher fim E activity,[3] serves as a mode of expressing pili only when adhesion is needed.

Another level of transcriptional control in E. coli is mediated by the sensitivity of the recombinases to pH and osmolarity,[4] further ensuring appropriate expression levels of type-I pili, given the stark differences in osmolarity inside and outside an animal's body.

The transcriptional control can differ widely between species,[5] in Salmonella typhimurium, for example much influence is exerted by a leucine-responsive regulatory protein and there is no fim S element.

English: Phase and Antigenic Variation in Bacteria. pA is the promoter for FimA, pB is the promoter for FimB and pE is the promoter for FimE. IRR is inverted repeat right and IRL is inverted repeat left. FimB and FimE are recombinases that can change the orientation of the FimA promoter by inverting the IRR and IRL.