Until very recently, it was thought that peptic ulcers were caused by stress, spicy foods, alcohol consumption, and excess stomach acid production. Why was it controversial to suggest that a bacterium could be a root cause for peptic ulcers

Until recently, scientists did not believe that bacteria could survive and reproduce in the harsh, acidic environment of the stomach.

Explanation:

The main fact that was associated with the survival of the bacteria in this harsh environment. Peptic ulcer is caused due to stress, alcohol consumption, and excess acid production from the stomach.

Basically the pH of the stomach is highly acidic due to the production of hydrochloric acid. So the scientist believed that none of the bacterial species could survive in such environment but now after studies it has been proven that there are some disease causing bacteria that can survive in stomach and cause peptic ulcer.