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The afimbrial adhesive sheath encoded by the afa‐3 gene cluster of pathogenic Escherichia coli is composed of two adhesins

The afa‐3 gene cluster determines the formation of an afimbrial adhesive sheath that is expressed by uropathogenic as well as diarrhoea‐associated Escherichia coli strains. It contains six genes (afaA–afaF ), among which the afaE3 gene is known to code for the structural AfaE‐III adhesin (previously designated AFA‐III), whereas no role has yet been identified for the afaD gene product. The afa‐3 gene cluster is closely related to the daa operon that codes for an adhesin, the F1845 adhesin, which is highly related to the AfaE‐III adhesin; however, unlike the AfaE‐III adhesin, F1845 is a fimbrial adhesin. Reported in this work is the construction of chimeras between the afa‐3 and daa operons. Analyses of the phenotypes conferred by these afa‐3/daa chimeric clusters allowed us to conclude that the biogenesis of a fimbrial or an afimbrial adhesin is fully determined by the amino acid sequence of the AfaE‐III and F1845 adhesins. Moreover, the role of the AfaD product in the biosynthesis of the afimbrial sheath was assessed by immunogold and immunofluorescence experiments. The AfaD and the AfaE‐III products were purified and used to raise rabbit and mouse antisera. Similar to AfaE‐III, AfaD was found to be a surface‐exposed protein as well as an adhesin; both AfaD and AfaE‐III are concomittantly expressed by the bacterial cell. These results demonstrate, for the first time, that the afimbrial adhesive sheath expressed by pathogenic E. coli is composed of two adhesins.