A patient receives a broad-spectrum antibiotic for a respiratory infection and three weeks later develops a new gastrointestinal infection by a different organism. Which sequence of events best explains how the antibiotic contributed to this superinfection?

• A. The antibiotic killed many susceptible gut commensals, opening ecological niche space that allowed a resistant organism to colonize and expand, resulting in the secondary infection.
• B. The antibiotic directly converted nonpathogenic commensals into the new gastrointestinal pathogen by horizontal gene transfer occurring exclusively within the treated patient during therapy.
• C. The antibiotic suppressed the patient’s immune system, which alone allowed a latent intracellular pathogen to emerge as a new infection without any change to the gut microbiota composition or colonization ecology.
• D. The respiratory pathogen mutated during therapy into a gastrointestinal species due to systemic exposure to the antibiotic and then migrated to the gut where it caused disease.

Answer: (A)