Which bacterium is MOST commonly associated with duodenal ulcers?

Helicobacter pylori is the primary cause of duodenal (and gastric) peptic ulcers — present in >90% of duodenal ulcer cases. H. pylori is a gram-negative spiral rod that survives in the acidic stomach by producing urease. Treatment: triple therapy (PPI + clarithromycin + amoxicillin). E. coli causes GI infections but not ulcers.