Which bacterium's discovery cost two researchers years of ridicule until one drank a culture?

Helicobacter pylori. Medicine held that ulcers were caused by stress and acid, that nothing could live in the stomach, and that the treatment was antacids for life - a large and profitable market. Robin Warren kept seeing curved bacteria in biopsy samples and Barry Marshall kept being told he was looking at contamination.

The culture succeeded by accident. Plates were normally discarded after two days; over the 1982 Easter holiday a batch was left for five, and the organism grew. Even with that, papers were rejected and conference abstracts turned down. In 1984 Marshall drank a broth of it, developed gastritis within days, biopsied himself to prove the organism was there, and took antibiotics.

They shared the Nobel in 2005. The organism turned out to infect around half the world's population, most of whom have no symptoms, and to be the main cause of stomach cancer - so a disease treated for a century as a consequence of temperament is largely an infection curable with a fortnight of antibiotics. Self-experimentation makes a good story and is bad methodology: one subject, no controls, and if it had killed him it would have proved nothing either.