Regulatory and functional co‐operation of flagella and type 1 pili in adhesive and invasive abilities of AIEC strain LF82 isolated from a patient with Crohn's disease

Summary Genetic determinants that co‐operate with type 1 pili to mediate invasion were sought for in adherent‐invasive Escherichia coli strain LF82 isolated from a patient with Crohn's disease. Two mutants selected for their impaired ability to invade epithelial cells carried insertions of a TnphoA transposon within genes of the flagellar regulon. An isogenic mutant LF82‐ΔfliC deleted for the flagellin‐encoding gene did not adhere, did not invade and, surprisingly, expressed only a few type 1 pili. Type 1 pili downregulation resulted from a preferential switch towards the off‐position of the invertible DNA element located upstream of the fim operon. This was also correlated with a decrease in the flagellar regulator flhDC mRNA levels, suggesting that the transcriptional regulator FlhD2C2 could control type 1 pili expression directly or indirectly. Transformation with a cloned fim operon allowed bypass of the type 1 pili downexpression in the LF82‐ΔfliC mutant. Thus, we showed that flagella play a direct role in the adhesion process via active motility. In addition to downregulating type 1 pili expression, flagella also play an undefined role in strain LF82 invasion, which is not restricted to motility or flagellar structure, but could be related to co‐ordinate expression of invasive determinants.