Clonal analysis reveals high rate of structural mutations in fimbrial adhesins of extraintestinal pathogenic Escherichia coli - PubMed (original) (raw)

Clonal analysis reveals high rate of structural mutations in fimbrial adhesins of extraintestinal pathogenic Escherichia coli

Scott J Weissman et al. Mol Microbiol. 2006 Feb.

Abstract

Type 1 fimbriae of Escherichia coli mediate mannose-specific adhesion to host epithelial surfaces and consist of a major, antigenically variable pilin subunit, FimA, and a minor, structurally conserved adhesive subunit, FimH, located on the fimbrial tip. We have analysed the variability of fimA and fimH in strains of vaginal and other origin that belong to one of the most prominent clonal groups of extraintestinal pathogenic E. coli, comprised of O1:K1-, O2:K1- and O18:K1-based serotypes. Multiple locus sequence typing (MLST) of this group revealed that the strains have identical (at all but one nucleotide position) eight housekeeping loci around the genome and belong to the ST95 complex defined by the publicly available E. coli MLST database. Multiple highly diverse fimA alleles have been introduced into the ST95 clonal complex via horizontal transfer, at a frequency comparable to that of genes defining the major O- and H-antigens. However, no further significant FimA diversification has occurred via point mutation after the transfers. In contrast, while fimH alleles also move horizontally (along with the fimA loci), they acquire point amino acid replacements at a higher rate than either housekeeping genes or fimA. These FimH mutations enhance binding to monomannose receptors and bacterial tropism for human vaginal epithelium. A similar pattern of rapid within-clonal structural evolution of the adhesive, but not pilin, subunit is also seen, respectively, in papG and papA alleles of the di-galactose-specific P-fimbriae. Thus, while structurally diverse pilin subunits of E. coli fimbriae are under selective pressure for frequent horizontal transfer between clones, the adhesive subunits of extraintestinal E. coli are under strong positive selection (Dn/Ds > 1 for fimH and papG) for functionally adaptive amino acid replacements.

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Figures

Fig. 1

Fig. 1

Unrooted phylogram of MLST loci of E. coli K1 and non-K1 archetypal strains. Based on concatenated 500 bp fragments of MLST housekeeping loci adk, fumC, gyrB, icd, mdh, metG, purA and recA. Branch lengths reflect absolute nucleotide differences between concatenated sequences. Circled numbers indicate number of strains at specified node. Letters in parentheses indicate major phylogenetic group assignment. Grey box indicates serotypes belonging to ST95 and ST390. CFT073, 536 and J96 are archetypal extraintestinal pathogenic E. coli strains. MG1655 and JVEC9 represent out-group strains.

Fig. 2

Fig. 2

Unrooted phylogram of fimA sequences from K1 and non-K1 archetypal strains. Labelling conventions same as for Fig. 1, with the following additions. Non-ST95 strains with sequences identical to complex strains are placed next to, but outside of, grey boxes, without branches. Letters in italics (with and without subscripts) indicate alleles as discussed in text.

Fig. 3

Fig. 3

Unrooted phylogram of fimH alleles from K1 and non-K1 archetypal strains. Figure conventions same as for Figs 1 and 2.

Fig. 4

Fig. 4

CEL-1 digests of homo- and heteroduplexes of fim clusters cloned from three E. coli O18:K1:H7 strains with distinct fimH alleles. A. fim clusters prior to CEL-1 digestion. Lanes 1–3: homoduplexes of F3, RS218 and 89–1449; lanes 4–6: heteroduplexes of 89–1449/F3, RS218/F3 and 89–1449/RS218. B. fim clusters after CEL-I digestion (lane numbering same as for A). White arrowheads indicate fragments generated by CEL-1 cleavage at heteroduplex SNP sites, with similar pattern observed for all three pairs.

Fig. 5

Fig. 5

Monomannose binding properties of distinct fimH alleles expressed in isogenic background. Monomannose binding without inhibitor shown in black bars, with inhibitor in grey. Alleles identified both by letter/subscript terminology used in text and by defining mutations (relative to primary allele in each lineage). SNP-containing allele lineages b 0, c 0 and d 0 demarcated by vertical lines. Error bars indicate standard error of the mean.

Fig. 6

Fig. 6

Primary vaginal epithelial cell binding properties of variant fimH alleles expressed in isogenic background, plotted against monomannose binding strength. _R_2 indicates slope of trend line. Error bars indicate standard error of the mean associated with vaginal cell binding data.

Fig. 7

Fig. 7

Unrooted phylogram of papA alleles. PapA pilin variants, serologic F-types F10 and F11. Other F-types indicated in parentheses. Circled numbers indicate number of strains at specified node. Branch lengths reflect absolute nucleotide differences between sequences. Grey boxes indicate serotypes belonging to ST95. CFT073, 536 and J96 are archetypal extraintestinal pathogenic E. coli strains. Note that two CFT073 alleles are included.

Fig. 8

Fig. 8

Unrooted phylogram of papG alleles. PapG adhesin variants, classes II and III. Figure conventions otherwise same as for Fig. 7.

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