in Presence of a pH Gradient**

Helicobacter pylori is a bacterium that colonizes the human stomach and can cause gastric ulcers and cancer. While extensive studies have been published on the motility of H. pylori in various media, the majority of the studies were carried out in homogeneous environments. The gastric mucosa where H. pylori lives exhibits a pH gradient varying from 2 to 7 across the mucus layer. We present a live cell tracking study of the motility of H. pylori in Brucella broth and porcine gastric mucin solutions at homogeneous pH levels as well as with a pH gradient created using a microfluidic channel. We found no significant changes in the distribution of swimming speeds in homogeneous environments over pH 3 to 7 and observed that the bacteria either became immotile or died in a pH below 3. We also noted the bacteria body shape became more coccoidal as the pH decreased below pH 4. In response to the presence of a pH gradient, H. pylori travelled in directed trajectories. They can detect the boundary between the regions of neutral and low pH, and were observed to reverse and move away from the low pH region. Analysis of the rotation of the cell body is underway to examine the influence of acidity on the rotational speed of bacteria.