Identification and detection of three new F17 fimbrial variants in Escherichia coli strains isolated from cattle

Abstract

F17 fimbriae are produced by pathogenic Escherichia coli involved in diarrhea and septicemia outbreaks in calves and lambs. These proteins result from the expression of four different clustered genes, namely f17A, f17D, f17C and f17G, encoding a pilin protein, a periplasmic protein, an anchor protein and an adhesin protein, respectively. Several variants of f17A and f17G genes have been reported and found genetically associated with typical virulence factors of bovine pathogenic E. coli strains. In this study, a new F17e-A variant, closely related to F17b-A, was identified from a collection of 58 E. coli isolates from diarrheic calves in Iran. While highly prevalent in Iranian F17-producing clinical isolates from calves, this variant was rare among E. coli from a French healthy adult bovine population, suggesting a possible association with virulence. The f17Ae gene was also found in the genome of the Shiga-like toxin variant Stx1d-producing bovine E. coli strain MHI813, and belonged to a gene cluster also encoding a new F17-G3 variant, which greatly differed from F17-G1 and F17-G2. This gene cluster was located on a pathogenicity island integrated in the tRNA pheV gene. The gene coding for a third new F17f-A variant corresponding to a combination of F17c-A and F17d-A was also identified on the pVir68 plasmid in the bovine pathogenic E. coli strain 6.0900. In conclusion, we identified three new F17-A and F17-G variants in cattle E. coli, which may also have significant impact on the development of new diagnostics and vaccination tools.

Figure 1

PFGE profiles of the 20 uncharacterized F17-producing isolates of the collection of E . coli isolates from feces of diarrheic calves in Iran. The PFGE profiles were analyzed with Gelcompar II 6.5 software, using Dice similarity coefficient, 0% optimization, 1% tolerance and UPGMA clustering method. 5 clones were identified, named from clone I to clone V.

Figure 2

Map of the E . coli strain MHI813 pathogenicity or fitness island carrying the F17e-A and F17-G3 variants gene cluster. This 96 kb genomic island is inserted into the phenylalanine tRNA encoding gene pheV and carries notably a P4–like integrase gene, close to the insertion site, F17 fimbriae encoding genes, organized in a gene cluster, a type VI secretion system gene cluster and a microcins I47 and H47 gene cluster.

Figure 3

Map of the f17A gene carried on plasmid pVir68. This gene corresponds to a mixture of f17Ac and f17Ad genes. The highly conserved region identified is shared by the both variants genes f17Ac and f17Ad and constitutes a likely recombination region. This region is located between the two matching regions of the forward specific primers for f17Ac and f17Ad.

Figure 4

Phenograms of F17-A variants and F17-A-related genetic sequences (A) and F17-G variants and F17-G-related genetic sequences (B). The phenograms were drawn using the Maximum Likelihood method based on the Tamura-Nei model [32]. The trees with the highest log likelihood (−3962.3380 for F17-A, and −4843.1422 for F17-G) were shown. The trees are drawn to scale, with branch lengths measured in the number of substitutions per site.