Which bacterium is commonly associated with antibiotic-associated diarrhea and colitis?

• A. Streptococcus pneumoniae
• B. Candida albicans
• C. Listeria monocytogenes
• D. Clostridium difficile

Answer: (D)

Antibiotic use disrupts normal gut flora, allowing Clostridium difficile to overgrow. This bacterium produces toxins that damage the lining of the colon, leading to antibiotic-associated diarrhea and colitis (often with pseudomembranes). The link to recent antibiotic exposure and toxin-mediated inflammation is what makes it the typical cause of this condition. The other organisms listed cause different diseases—Streptococcus pneumoniae mainly pneumonia and meningitis, Candida albicans is a fungus causing candidiasis, and Listeria monocytogenes causes foodborne illness with a risk of meningitis in certain groups—not the classic antibiotic-associated colitis. Recognizing C. difficile guides management, which centers on stopping the inciting antibiotic and starting targeted therapy such as oral vancomycin or fidaxomicin, with fecal microbiota transplantation considered for severe cases.