A 42-year-old man from rural India presents with recurrent epigastric pain and dyspepsia. Upper endoscopy is performed and a gastric antral biopsy is obtained. Histology reveals the structure marked **A** — curved gram-negative bacilli with characteristic morphology. The organism is identified using Giemsa staining. Which of the following is the MOST important pathophysiological mechanism that allows this organism to survive in the acidic gastric environment?