The majority of peptic ulcer disease is caused by which type of gram-negative bacteria?

• A. Helicobacter pylori
• B. Escherichia coli
• C. Streptococcus pneumoniae
• D. Pseudomonas aeruginosa

Answer: (A)

Peptic ulcer disease most often stems from infection with a Gram-negative bacterium that colonizes the stomach. The organism is Helicobacter pylori, a curved, motile Gram-negative rod that lives in the gastric mucosa. Its survival in the acidic stomach is enabled by urease, which converts urea to ammonia and CO2, locally raising pH and allowing the bacterium to persist. This persistent infection triggers chronic gastritis and an inflammatory response that damages the protective mucous layer and underlying epithelium, leading to ulcer formation. The other bacteria listed are not typical causes of peptic ulcers; they either do not inhabit the stomach (like Streptococcus pneumoniae and Pseudomonas aeruginosa) or are not associated with ulcer disease (Escherichia coli). NSAID use is another common cause of ulcers, but the question focuses on the Gram-negative bacterium responsible for the majority of cases: Helicobacter pylori.