The emergence of antibiotic resistance in bacterial populations is primarily driven by which factor?

• A. Selection pressure from antibiotic exposure
• B. Random genetic drift
• C. Other toxins
• D. Genetic drift plus mating patterns

Answer: (A)

Antibiotic exposure creates the main selective pressure that drives resistance. When antibiotics are present, susceptible bacteria are killed or inhibited, while those with resistance—which can arise by mutation or be acquired through horizontal gene transfer—survive and reproduce. This differential survival increases the frequency of resistance in the population over time, a classic natural selection process driven by the antibiotic environment. Random genetic drift acts randomly and is less influential in large bacterial populations, and bacteria don’t mate in the way multicellular organisms do, though they can exchange genes; the crucial factor pushing resistance up is the selective advantage conferred by resistance in the presence of antibiotics.