A 47-year-old woman with a lengthy history of heartburn and dyspepsia experiences a sudden onset of abdominal pain. On physical examination, she has severe mid epigastric pain with guarding. Bowel sounds are reduced. An abdominal plain film radiograph shows free air under the left leaf of the diaphragm. She is immediately taken to surgery, and a perforated duodenal ulcer is repaired. Which of the following organisms is most likely to have produced these findings?

Although they are not found in the duodenum, Helicobacter pylori organisms alter the microenvironment of the stomach, causing the stomach and duodenum to be susceptible to peptic ulcer disease. Virtually all duodenal peptic ulcers are associated with H. pylori infection. Ulceration can extend through the muscularis and result in perforation, as in this case. The other organisms listed are not related to peptic ulcer formation, but infectious diarrheal illnesses. Salmonella typhi may produce typhoid fever with more systemic symptoms; the marked ulceration of Peyer patches may lead to perforation.