A newly identified group of P-like (PL) fimbriae from extra-intestinal pathogenic Escherichia coli (ExPEC) encode distinct adhesin subunits and mediate adherence to host cells

Fimbrial adhesins play a critical role for bacterial adherence and biofilm formation. Sequencing of avian pathogenic Escherichia coli (APEC) strain QT598 identified a fimbrial gene cluster belonging to the {pi} group that we named PL (P-like) fimbriae, since genetic organization and sequence are similar to Pap and related fimbriae. Screening of genomic databases indicated that genes encoding PL fimbriae located on IncF plasmids are present in a diversity of E. coli isolates from poultry, human systemic infections, and other sources. As with P fimbriae, PL fimbriae exhibit sequence divergence in adhesin encoding genes, and strains could be divided into 5 classes based on differences in sequences of the PlfG adhesin protein. The plf genes from two predominant PlfG adhesin classes, PlfG-I and PlfG-II were cloned. PL fimbriae were visualized by electron microscopy, promoted biofilm formation, demonstrated distinct hemagglutination profiles and promoted adherence to human bladder and kidney epithelial cell lines. Hybrid fimbriae comprised of genes from plfQT598 wherein plfG was replaced by papG encoding adhesin genes were also shown to be functional and mediate adherence to epithelial cells, further indicating similarity and functional compatibility between these two types of fimbriae. Although deletion of plf genes did not significantly reduce colonization of the mouse urinary tract, plf gene expression was increased over 40-fold in the bladder compared to during in vitro culture. Overall, PL fimbriae represent a new group of fimbriae demonstrating both functional differences and similarities to P fimbriae and may contribute to adherence to cells and colonization of host tissues. ImportanceFimbriae are important colonization factors in many bacterial species. The identification of a new type of fimbriae encoded on some IncF plasmids in E. coli was investigated. Genomic sequences demonstrated these fimbrial gene clusters have genetic diversity, particularly in the adhesin encoding PlfG gene. Functional studies demonstrated differences in hemagglutination specificity, although both types of Plf adhesin under study mediated adherence to human urinary epithelial cells. Such fimbriae may represent previously unrecognized adhesins that could contribute to host specificity and tissue tropism of some E. coli strains.