Complex sentence: Later, several studies have shown that TcdB plays a role major in the disease of PMC and ADD, when study demonstrated that "C. difficile" that did not produces TcdA, yet showed symptom for the disease.
Split into simpler sentences: Later, several studies showed that TcdB plays a major role in the disease of PMC and ADD. The study demonstrated that even though "C. difficile" did not produces TcdA, it still showed symptoms for the disease.