Final answer:
Clostridium difficile produces Toxin A and Toxin B, leading to severe diarrhea and potentially fatal antibiotic-associated colitis.
Step-by-step explanation:
The bacterium that produces Toxin A and Toxin B, leading to severe diarrhea and potentially fatal antibiotic-associated colitis, is Clostridium difficile. When the normal gut microbiota is eradicated by antibiotics, C. difficile can overgrow and cause antibiotic-associated diarrhea. This bacterium produces two toxins, Toxin A and Toxin B, which inactivate small GTP-binding proteins and result in actin condensation, cell rounding, and ultimately cell death.