FIG. 2.
Frequency of mutations to rifampin resistance in 88 natural E. coli isolates classified in a mouse model of extraintestinal virulence (26) as highly virulent (9 or 10 mice killed of a total of 10), intermediate (2 to 8 mice killed of a total of 10) or avirulent (0 or 1 mouse killed of a total of 10). The eight strong mutator strains are above the threshold of 2 × 10−7 for the frequency of mutations to rifampin resistance. The three avirulent strains, which exhibit the highest mutation rates of all, encompass one strain (TIM28) devoid of virulence determinants and two strains (SA1923 and SA1902) with numerous virulence determinants.