A 47-year-old woman with a lengthy history of heart- burn and dyspepsia experiences sudden onset of abdominal pain On physical examination, she has severe mid epigastric pain with guarding Bowel sounds are reduced. An abdomi- nal 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