A 50-year-old male with a 2-year history of cirrhosis underwent esophageal barium meal examination 1 year ago, revealing worm-eaten filling defects in the lower esophagus. Two days ago, he experienced melena, followed by somnolence and nighttime restlessness, prompting emergency admission. Physical examination showed flapping tremors in both hands. Which antimicrobial is the first choice to reduce gut-derived toxic substances in hepatic coma?