Differential stability and trade-off effects of pathoadaptive mutations in the Escherichia coli FimH adhesin

Abstract

FimH is the tip adhesin of mannose-specific type 1 fimbriae of Escherichia coli, which are critical to the pathogenesis of urinary tract infections. Point FimH mutations increasing monomannose (1M)-specific uroepithelial adhesion are commonly found in uropathogenic strains of E. coli. Here, we demonstrate the emergence of a mixed population of clonally identical E. coli strains in the urine of a patient with acute cystitis, where half of the isolates carried a glycine-to-arginine substitution at position 66 of the mature FimH. The R66 mutation induced an unusually strong 1M-binding phenotype and a 20-fold advantage in mouse bladder colonization. However, E. coli strains carrying FimH-R66, but not the parental FimH-G66, had disappeared from the patient's rectal and urine samples collected from 29 to 44 days later, demonstrating within-host instability of the R66 mutation. No FimH variants with R66 were identified in a large (>600 strains) sequence database of fimH-positive E. coli strains. However, several strains carrying genes encoding FimH with either S66 or C66 mutations appeared to be relatively stable in the E. coli population. Relative to FimH-R66, the FimH-S66 and FimH-C66 variants mediated only moderate increases in 1M binding but preserved the ability to enhance binding under flow-induced shear conditions. In contrast, FimH-R66 completely lost shear-enhanced binding properties, with bacterial adhesion being inhibited by shear forces and lacking a rolling mode of binding. These functional trade-offs may determine the natural populational instability of this mutation or other pathoadaptive FimH mutations that confer dramatic increases in 1M binding strength.

FIG. 1.

Phylogenetic tree and PFGE profiles. To the left is a phylogenetic tree (dendrogram) built based on MLST haplotypes concatenated from the integral fragments of seven housekeeping genes, with distances indicating absolute numbers of nucleotide differences distributed proportionally among branches. To the right are PFGE profiles obtained using XbaI digestion. ST, sequence type. Abbreviations for TOP17 strains further identified by clinical setting: AC, acute cystitis; R, rectal; ABU, asymptomatic bacteriuria; RC, recurrent cystitis. Abbreviations for site of isolation: U, urine; S, stool; R, respiratory secretions; B, blood. Abbreviations for place of isolation: WA, Washington (Seattle); IA, Iowa; MN, Minnesota (Minneapolis); SE, Sweden; NY, New York; TO, Tonga.

FIG. 2.

Amino acid and DNA sequences for FimH adhesin variants. Consensus residues are indicated below each position number. For sequenced strains, only polymorphisms relative to the consensus are shown, with identical positions indicated by hyphens. Published genomes compared to TOP17 isolates (A) and S66 and C66 FimH variants identified from our clinical fimH sequence database (B).

FIG. 3.

(A) 1M-specific (filled bars) and 3M-specific (open bars) binding of wild-type isolates under static conditions. (B) 1M-specific (filled bars) and 3M-specific (open bars) binding of isogenic strain pairs under static conditions: (i) FimH-G66 versus FimH-R66 variants from the TOP17 isolates. (ii) FimH-C66 and its parent G66 variant. (iii) FimH-S66 and its parent G66 variant. (C) T24 bladder cell binding of the isogenic strain pair expressing the TOP-17 FimH-R66 and parent FimH-G66 variants. (D) Mouse bladder colonization at 24 h of TOP17-AC-R66 and TOP17-AC-G66 strains. (E) Fifty percent inhibitory concentrations (IC₅₀) of α-methylmannoside for the isogenic strain pairs. Results represent mean values with standard errors of the mean.

FIG. 4.

(A) Surface accumulation of isogenic strain pairs featuring FimH position 66 polymorphisms under a range of shear stresses. (B) Percentage of surface-rolling bacteria to total (rolling and firmly bound) bacteria for the isogenic strain pairs under a range of shear stresses. Results represent mean values with standard errors of the mean.