Inactivation of ibeA and ibeT results in decreased expression of type 1 fimbriae in extraintestinal pathogenic Escherichia coli strain BEN2908

IbeA in extraintestinal pathogenic Escherichia coli (ExPEC) strains was previously described for its role in invasion. Here we investigated the role of IbeA and IbeT, encoded by a gene located downstream of ibeA, in the adhesion of the avian ExPEC strain BEN2908 to human brain microvascular endothelial cells (HBMEC). The Delta ibeA mutant was less adhesive to HBMEC than the wild-type strain BEN2908 was. Because strain BEN2908 also expresses type 1 fimbriae, we measured the adhesion specifically due to IbeA by comparing the adhesive properties of a Delta fim derivative of strain BEN2908 to those of a double Delta fim Delta ibeA mutant. No differences were observed, indicating that the reduction of adhesion in BEN2908 Delta ibeA could be due to a decrease in type 1 fimbria expression. We indeed showed that the decreased adhesion of BEN2908 Delta ibeA was correlated with a decrease in type 1 fimbria expression. Accordingly, more bacteria had a fim promoter orientated in the off position in a culture of BEN2908 Delta ibeA than in a culture of BEN2908. Expression of fimB and fimE, two genes encoding recombinases participating in controlling the orientation of the fim promoter, was decreased in BEN2908 Delta ibeA. A reduction of type 1 fimbria expression due to a preferential orientation of the fim promoter in the off position was also seen in an ibeT mutant of strain BEN2908. We finally suggest a role for IbeA and IbeT in modulating the expression of type 1 fimbriae through an as yet unknown mechanism.