A 42-year-old Indian male presents with chronic epigastric pain and a 6-week history of melena. Upper endoscopy reveals a duodenal ulcer with active bleeding. Histopathology of antral biopsies shows chronic active gastritis with lymphoplasmacytic infiltration and neutrophilic exudates. The curved, gram-negative bacilli marked **A** in the diagram are identified on Giemsa staining. Which of the following mechanisms best explains how this organism survives in the acidic gastric environment?