High Mutation Frequencies Among Escherichia coli and Salmonella Pathogens

Abstract

Here it is reported that the incidence of mutators among isolates of pathogenic Escherichia coli and Salmonella enterica is high (over 1 percent). These findings counter the theory, founded on studies with laboratory-attenuated strains, that suggests mutators are rare among bacterial populations. Defects in methyl-directed mismatch repair underlie all mutator phenotypes described here. Of nine independently derived hypermutable strains, seven contained a defective mutS allele. Because these mutant alleles increase the mutation rate and enhance recombination among diverse species, these studies may help explain both the rapid emergence of antibiotic resistance and the penetrance of virulence genes within the prokaryotic community.

Insertions, Deletions, and Single-Nucleotide Polymorphisms at Rare Restriction Enzyme Sites Enhance Discriminatory Power of Polymorphic Amplified Typing Sequences, a Novel Strain Typing System for Escherichia coli O157:H7

“…Similar indel-directed differences between the two sequenced O157 strains, EDL 933 and Sakai, can be observed (5,13,33). The probable existence of O157 in hypermutable states, due to defects in its methyl-directed mismatch repair system, may contribute to the large number of horizontal transfer events occurring in the genome, leading to genetic variation in the serotype and its strains (21,22,28). A recent study comparing closely related species and strains from six very distant systematic groups, including apes, sea urchins, bacteria, insects, nematodes, and plants, has shown that indels are responsible for many more unmatched nucleotides between closely related DNAs than are base substitutions (5).…”

Highly Variable Mutation Rates in Commensal and Pathogenic Escherichia coli

“…It suffices to say now that the evolution of a constant, low mutation rate-similar across species-seen in laboratory strains (2) may not pertain as we move into the realm of natural strains and emerging pathogens. With regard to the statistical treatment of results obtained using natural strains, we would like to reinforce the word of caution in note 9 of our report (1). As E. coli is a versatile enteric, it is imperative to draw distinction between a commensal and a true pathogen that may be derived from an asymptomatic subject.…”

“…Escherichia coli in humans is a commensal inhabitant of the gastrointestinal tract as well as one of the most frequently isolated bacterial pathogens (1). In studying foodborne E. coli pathogens, as well as the ECOR collection of natural isolates (2), LeClerc et al found that mutant bacteriadefective in methyl-directed mismatch repair (MMR)-were present in pathogenic strains at an unexpectedly high frequency (over 1%), thus raising the possibility of a link between mutator phenotype and pathogenicity (3).…”

The Isolation And Cloning of Chromosomal Dna Specific for a Clonal Population of Staphylococcus Aureus By Subtractive Hybridisation

“…Studies by Le Clerc and coworkers [35] have revealed that certain subsets of E. coli and Salmonella have a higher than normal mutation rate, and these subsets are common among the pathogenic strains of these bacteria [35]. This suggests that such subsels might be better equipped to adapt in a challenging cnvironment.…”