Passage II

Some mutations in Escherichia coli allow the bacteria to survive exposure to an antibiotic. These antibioticresistant bacteria may have a different relative fitness (a measure of survival and reproductive success) than E. coli without mutations. Scientists conducted a study to determine the relative fitness of 5 E. coli strains—1 nonmutated (Strain U) and 4 mutated (Strains W, X, Y, and Z)—when the strains were exposed for 24 hr to each of 5 different concentrations of the antibiotic streptomycin (see Table 1). The effect of the mutation in each of Strains W−Z is listed in Table 2.

Table 1 adapted from Viktória Lázár et al., “Bacterial Evolution of Antibiotic Hypersensitivity.” ©2013 by EMBO and Macmillan Publishers Limited.

12. In the study, the relative fitness of a nonmutated strain that was grown for 24 hr in the absence of an antibiotic was set to 1.0. Was this strain more likely Strain U or Strain Z, and was this strain grown for 24 hr at a streptomycin concentration of 0 μg/mL or at a streptomycin concentration of 8 μg/mL ?

Answer and Explanation

Your Answer is

Correct Answer is F

Explanation

The background text information mentions "1 nonmutated (StrainU)", so combined with the "nonmutated strain" in the title stem, we can know that it should be Strain U; ​​combined with Table 1, we can know that when the streptomycin concentration is 0, the relative fitness is 1.0, and when it is 8, it is 0.0 , that is, no bacteria survived (relative fitness) at 8 o'clock, so choose F