Comparison of motility of H. pylori in broth and mucin reveals the interplay of effect of acid on bacterium and the rheology of the medium it swims in.

To colonize on the gastric epithelium Helicobacter pylori bacteria have to swim across a gradient of pH from 2-7 in the mucus layer. Previous studies of H. pylori motility have shown that at pH below 4 do not swim in porcine gastric mucin (PGM) gels. To separately assess the influence of gelation of PGM and that of pH on motors and pH sensitive receptors of H. pylori, we used phase contrast microscopy to compare the translational and rotational motion of H. pylori in PGM versus Brucella broth (BB10) at different pHs. We observed that decreasing pH leads to decreased fraction of motile swimmers with a decrease in the contribution of fast swimmers to the distributions of swimming speeds and length of trajectories. At all pH9s the bacteria swam faster with longer net displacement over the trajectory in BB10 as compared to PGM. While bacteria are stuck in PGM gels at low pH, they swim at low pH in broth, albeit with reduced speed. The body rotation rate and estimated cell body torque are weakly dependent on pH in BB10, whereas in PGM the torque increases with increasing viscosity and bacteria stuck in the low pH gel rotate faster than the motile bacteria. Our results show that H. pylori has optimal swimming under slightly acidic conditions, and exhibits mechanosensing when stuck in low pH mucin gels.