How does H. pylori cause a peptic ulcer?

H. pylori weakens the protective mucous coating of the stomach and duodenum, which allows acid to get through to the sensitive lining beneath. Both the acid and the bacteria irritate the lining and cause a sore, or ulcer.

H. pylori is able to survive in stomach acid because it secretes enzymes that neutralize the acid. This mechanism allows H. pylori to make its way to the "safe" area--the protective mucous lining. Once there, the bacterium's spiral shape helps it burrow through the lining.

source: http://digestive.niddk.nih.gov/ddiseases/pubs/hpylori/index.htm