A 62-year-old woman with diagnosed type 2 diabetes lived alone and did essentially nothing to manage her illness, including disregarding her physician's instructions. She was taken to her local emergency room (ER) with severe, multiple infected foot lesions, which yielded a variety of opportunistic microbes with a mixture of antibiotic susceptibilities. The physician decided to treat with systemic and topical antimicrobials. Which of the following antimicrobial agents must only be used topically?