A clinician treats a mixed bacterial infection with an antibiotic and afterwards the remaining population is resistant. Which explanation best describes what occurred?

• A. The antibiotic directly caused the bacteria to mutate into resistant strains in response to drug pressure in the majority of cells, creating resistance during treatment.
• B. Resistant cells were present before treatment, and the antibiotic preferentially killed susceptible cells so resistant genotypes became more common by selection.
• C. Antibiotics converted nonbacterial nucleic acids in the sample into resistant plasmids that spontaneously integrated into unrelated cells, so resistance was synthetically introduced by the drug chemistry.
• D. The host immune response turned off after antibiotic exposure, allowing previously dormant environmental bacteria to colonize and be mistaken for treatment-induced resistance when in fact they were newly acquired species.

Answer: (B)