Technical Comments

Highly Variable Mutation Rates in Commensal and Pathogenic Escherichia coli

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Science  19 Sep 1997:
Vol. 277, Issue 5333, pp. 1833-1834
DOI: 10.1126/science.277.5333.1833


  • Figure 1

    Frequency of mutation to rifampicin resistance among mutator strains. Five hundred and four natural E. coli isolates were screened for forward mutagenesis in the lacI gene. A total of 69 strains (14%) has been found to form dark blue papillae on minimum medium containing limited glucose, X-gal, and P-gal (the latter can only be used as a carbon source by lacI mutants). We monitored the frequency of rifampicin-resistant mutants in three independent cultures of these strains (median value is presented), as well as the frequency of 52 non-papillating strains (data not shown). Non-papillating strains had an average mutation rate to rifampicin resistance of about 1 × 10−8 (a value commonly found for wild-type laboratory strains like E. coli K-12) and a small variance (data not shown). Papillating strains had an average mutation rate of 2.6 × 10−7, ranging from less than 10−8 to more than 10−6, thus validating our initial screen. Strains that do not have increased mutagenesis to rifampicin resistance [which reveals base substitutions (7)] are likely to involve other classes of mutators such as those generating frameshifts, deletions, or insertions. This high polymorphism of mutation rates was observed in all groups [commensal strains isolated from France (□), Mali (○), or Croatia (▵) and in strains involved in diverse pathologies, urinary tract infections (⧫), bacteremia (x), pus (▴), neonatal meningitis (▪), and haemolytic-uremic syndrome or haemorrhagic diarrhea (•)]. Defects in mismatch repair genes were identified by complementation with plasmid-carrying wild-typeMMR genes.

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