Ciprofloxacin is given as an antibiotic to healthy livestock to promote efficient weight gain. Strains of the bacterium Campylobacter jejuni naturally colonize the digestive tracts of livestock, and the C. jejuni can be transferred to people through the handling and consumption of contaminated meat. People infected with C. jejuni also are treated with ciprofloxacin.

Which of the following is the most likely consequence of adding ciprofloxacin to animal feed?

Question

Ciprofloxacin is given as an antibiotic to healthy livestock to promote efficient weight gain. Strains of the bacterium Campylobacter jejuni naturally colonize the digestive tracts of livestock, and the C. jejuni can be transferred to people through the handling and consumption of contaminated meat. People infected with C. jejuni also are treated with ciprofloxacin.

Which of the following is the most likely consequence of adding ciprofloxacin to animal feed?

BlackTom AI · Accepted Answer

Ciprofloxacin targets bacterial DNA gyrase; bacteria with mutations or resistance mechanisms survive exposure and therefore have a selective advantage.

1. Relevant knowledge points and concepts
- Antibiotics exert a selective pressure: susceptible bacteria are killed or inhibited, resistant variants survive and reproduce.
- Resistance can arise by mutation (e.g., gyrA mutations for fluoroquinolones) or by acquiring resistance genes (plasmids).
- Resistant bacteria in livestock can be transmitted to humans via contaminated meat or handling.
- Use of the same antibiotic in animals and humans increases the chance that human infections will be caused by resistant strains, reducing treatment effectiveness.

2. Step-by-step reasoning process
- Start with a heterogeneous C. jejuni population in livestock guts: mostly susceptible bacteria, but some resistant mutants exist at low frequency.
- Adding ciprofloxacin to feed exposes the gut bacteria to the drug continuously.
- Susceptible cells are killed or outcompeted; resistant cells survive and reproduce without competition.
- Over time the bacterial population in livestock becomes dominated by ciprofloxacin-resistant C. jejuni (selective sweep).
- People exposed to contaminated meat or handling these animals can acquire these resistant strains.
- If a person becomes infected and is treated with ciprofloxacin, treatment may fail or be less effective because the infecting strain is resistant, increasing risk of severe or prolonged infection.

3. Calculations/Intermediate steps
- No numeric calculation is required; the logical population-change steps are: initial mixed population → antibiotic exposure → elimination of susceptible variants → proliferation of resistant variants → increased prevalence of resistant strains in meat.

4. Why the correct answer is correct
- Option B correctly describes the evolutionary consequence (selective advantage for resistant strains) and the public-health implication (increased risk of serious infections in people because the same drug is used clinically).

5. Why other options are incorrect
- Option A: Claims increased reproduction of naturally occurring strains and reduced risk — wrong direction; antibiotics do not increase reproduction of susceptibles nor reduce risk.
- Option C: Implies meat will contain a growth-inhibiting chemical that reduces food poisoning; in reality antibiotic use selects resistant bacteria and does not reliably sterilize meat, and residues are controlled separately.
- Option D: Claims bacterial metabolism of meat causes human protein deficiency — biologically implausible and irrelevant.

6. Conclusion
The best answer is: Ciprofloxacin-resistant strains of C. jejuni will have a selective advantage in the digestive tracts of livestock, increasing the risk of serious infections in people. This matches the provided answer.

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