Which of the following best defines a superinfection in the context of antibiotic therapy?

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?

Before therapy a gut community contains: Commensal A = 1.0 × 10⁹ cells (antibiotic susceptible), Commensal B = 5.0 × 10⁷ cells (partially susceptible, 50% killed by drug), and Pathogen P = 2.0 × 10⁵ cells (resistant). A course of antibiotic is given that kills 99% of susceptible cells but 0% of resistant ones. Calculate the post-therapy abundances for A, B, and P, and determine which taxon dominates numerically after therapy. Show your steps and final numeric answers.

Which immediate infection-control action is most appropriate for a patient newly suspected of having C. difficile in a hospital ward?