A clinical isolate of Pseudomonas aeruginosa showed an increase in the minimum inhibitory concentration (MIC) of the antibiotic ciprofloxacin from 2 μg cm−32\ \mu\text{g}\ \text{cm}^{-3}2 μg cm−3 to 128 μg cm−3128\ \mu\text{g}\ \text{cm}^{-3}128 μg cm−3 over a 12-day treatment course. Which of the following statements provides the correct evolutionary explanation for this rapid development of resistance?
The selective pressure of ciprofloxacin induced targeted mutations in the bacterial DNA to actively defend against the drug.
Pre-existing random mutations in the population allowed certain individuals to survive and reproduce in the presence of ciprofloxacin, increasing the frequency of the resistant phenotype.
The individual bacteria acclimated to the increasing doses of ciprofloxacin and passed these acquired physiological modifications to their offspring.
Exposure to ciprofloxacin increased the mutation rate specifically at the locus controlling drug efflux pumps, ensuring survival.
