The salmochelin receptor IroN itself, but not salmochelin-mediated iron uptake promotes biofilm formation in extraintestinal pathogenic Escherichia coli (ExPEC)

The key to success of extraintestinal pathogenic Escherichia coli (ExPEC) to colonize niches outside the intestinal tract and to establish infection is the coordinated action of numerous virulence and fitness factors. Intense research revealed not only an arsenal of unique virulence determinants with specific action, but also the multi-functionality of single elements. Especially iron uptake systems of ExPEC proved to be of prime importance. Apart from iron acquisition they optimize certain virulence properties. Here we analyzed the contribution of the salmochelin siderophore system to the ability of ExPEC to form biofilms. In the same iron limited environment, ExPEC displayed a distinct transcriptional profile of siderophore systems. During biofilm formation the iroN gene coding for the specific receptors of the siderophore salmochelin was highly upregulated. Almost no induction was observed during planctonic growth. Disruption of iroN resulted in a reduction of almost 50% in biofilm production. Efficient biofilm formation was not affected in a salmochelin synthesis mutant. Thus, the contribution of IroN is independent from the ability to produce salmochelin. Enhanced expression of IroN did not increase significantly the capacity to form biofilms in ExPEC. Interestingly, the additional expression of IroN or even the acquisition of the entire salmochelin system was not able to improve biofilm formation in a poor biofilm producer like a laboratory E. coli K12 strain. However, complementation with only IroN in an ExPEC iroA deletion mutant was able to restore biofilm formation. The contribution of IroN to biofilm formation appears to require a certain background found in ExPEC, but not in E. coli K12. This study identified the contribution of IroN to biofilm formation and highlights the multi-functional role of iron uptake systems in ExPEC.