PS exposure and membrane blebbing were observed during H pylori infection. Using flow cytometry, platelet surface exposure of PS was examined in response to H pylori stimulation. PS expression was observed by binding with annexin V-FITC, and platelet was labeled with PE-conjugated mouse anti-CD41 IgG. (A) All strains of H pylori can induce PS expression on platelets; however, a higher level of PS expression was found in pro-aggregatory Hp 49503 compared with that of nonaggregatory Hp 43504. (B) Using a scanning electron microscope (Quanta 400F; FEI), the apoptotic sign of membrane blebbing was also observed on platelet aggregates induced by Hp 49503.