Escherichia coli K‐12 possesses multiple cryptic but functional chaperone–usher fimbriae with distinct surface specificities

Summary Commensal and pathogenic Escherichia coli adherence to host and environmental surfaces is mediated by a variety of adhesins. Although extensively studied as a model bacterium, 34% of the genes in the E. coli K‐12 genome have no known function. We hypothesized that some of them may correspond to functional adhesins. We characterized E. coli K‐12 ycb , ybg , yfc , yad , yra , sfm and yeh operons, which display sequence and organizational homologies to type 1 fimbriae exported by the chaperone/usher pathway. We showed that, although these operons are poorly expressed under laboratory conditions, six of them are nevertheless functional when expressed, and promote adhesion to abiotic and/or epithelial cell surfaces. While the studied fimbriae display different binding specificities, we obtained evidence of synergy/interference with other adhesins such as Ag43 or type 1 fimbriae. We showed that their expression is under the negative control of H‐NS and, except for yad , subjected to cAMP receptor protein‐mediated activation and carbon catabolite repression. These results therefore demonstrate that ycb , yfc , yad , yra , sfm and yeh operons encode cryptic but functional fimbriae adhesins whose expression following environmental modifications could contribute to E. coli 's ability to adhere to and colonize a wide diversity of surfaces in its various ecological niches.