High frequency of mutator strains among human uropathogenic Escherichia coli isolates

Abstract

By using a panel of 603 commensal and pathogenic Escherichia coli and Shigella isolates, we showed that mutation rates of strains vary considerably among different ecotypes. Uropathogenic strains had the highest frequency of mutators, while strains from patients with bacteremia had the lowest mutation rates. No correlation between the mutation rates and antibiotic resistance was observed among the studied strains.

FIG. 1.

Mutator strains belonging to different human E. coli and Shigella commensal and pathogenic groups. Bacterial strains are grouped according to their origins and the different pathologies in which they have been involved, including bacteremia strains (n = 93), strains isolated from pus (n = 30), commensal strains (n = 217), enteroinvasive strains (n = 86), enterohemorrhagic strains (n = 26), NBM strains (n = 60), and UTI strains (n = 91). In addition, all pathogenic strains are presented as one group in order to facilitate a comparison of that group with commensal strains. Strains were considered mutators when they exhibited frequencies of mutations conferring resistance to rifampin (100 μg/ml) that were 10-fold higher than the median value of mutagenesis (5.04 × 10⁻⁹) observed for all studied strains (n = 603) (10-fold mutators). Strains that displayed a >50-fold increase in mutagenesis were considered strong mutators (50-fold mutators). Percentages of mutator strains were calculated for every group of studied strains. UTI strains had significantly higher (according to the χ² test) fractions (*) of 10- and 50-fold mutators than commensal strains (P = 0.005 and P = 0.001 for 10- and 50-fold mutators, respectively), as well as other pathogens (P = 0.001 and P = 0.001 for 10- and 50-fold mutators, respectively).

FIG. 2.

Variability of mutation rates of human E. coli and Shigella isolates after elimination of mutators. The mean value (± standard error) of mutation frequency, after removal of those for 10- and 50-fold mutators (see legend to Fig. 1), is shown for each group.

FIG. 3.

Capacities of mutator strains to generate mutations conferring resistance to different antibiotics. The results are presented as mean values (± standard errors) for mutator (>10-fold increase in mutagenesis; n = 21) and nonmutator (n = 47) strains.

FIG. 4.

Squatter colonies inside growth inhibition zone. Growth inhibitory zones for nalidixic acid (disk 1), amoxicillin (disk 2), and phosphomycin (disk 3) are presented for nonmutator (A) and mutator (B) strains. Note the presence of squatter colonies for the mutator strain only.