Polymorphic mutation frequencies in Escherichia coli: emergence of weak mutators in clinical isolates

Abstract

Polymorphisms in the rifampin resistance mutation frequency (f) were studied in 696 Escherichia coli strains from Spain, Sweden, and Denmark. Of the 696 strains, 23% were weakly hypermutable (4 x 10(-8) < or = f < 4 x 10(-7)), and 0.7% were strongly hypermutable (f > or = 4 x 10(-7)). Weak mutators were apparently more frequent in southern Europe and in blood isolates (38%) than in urinary tract isolates (25%) and feces of healthy volunteers (11%).

FIG. 1.

Distribution of rifampin resistance mutation frequencies of 696 E. coli strains isolated from patients and healthy volunteers from Spain, Sweden, and Denmark. The horizontal arrows show the frequency ranges defining hypomutable strains (H), weakly hypermutable strains (W), and strongly hypermutable strains (S). The peak in the distribution corresponds to the normomutable strains. 1.E-9, 1.0 × 10⁻⁹.

FIG. 2.

Distribution of rifampin resistance mutation frequencies of 300 Spanish E. coli isolates according to the isolate origin: feces from healthy volunteers (100 isolates) (white bars), urinary tract infections (100 isolates) (shaded bars), and blood cultures (100 isolates) (black bars). 1-4E-9, 1 × 10⁻⁹ to 4 × 10⁻⁹.

FIG. 3.

(a) Distribution of rifampin resistance mutation frequencies of uropathogenic E. coli isolates from 226 Swedish (shaded) and 100 Spanish (black) patients. (b) Distribution of E. coli mutation frequencies of bacteremic isolates from 170 Danish (shaded) and 100 Spanish (black) patients. 1-4E-9, 1 × 10⁻⁹ to 4 × 10⁻⁹.

FIG. 4.

Distribution of normomutable (white), weakly hypermutable (shaded), and strongly hypermutable (black) E. coli strains among isolates inhibited by different ciprofloxacin concentrations. (a) Distribution of the 226 uropathogenic E. coli strains isolated from Swedish patients. (b) Distribution of 75 E. coli strains with an enriched proportion of hypermutable strains isolated from Spanish patients.