Biophysics

Swimming Through Mucus

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Science  04 Sep 2009:
Vol. 325, Issue 5945, pp. 1182
DOI: 10.1126/science.325_1182c

The stomach is an acidic environment of pH ∼ 2 as anyone who has suffered a bout of emesis can testify. So how does Helicobacter pylori, which is not an acidophilic bacterium, survive? First, it secretes the enzyme urease, which hydrolyzes urea to produce ammonia. This helps to neutralize the ambient hydrochloric acid, at least locally. Another strategem is to burrow beneath the layer of protective mucus that lies between the digestive juices and the epithelial cell lining of the stomach; once there, it is protected in part by locally secreted bicarbonate. The problem for H. pylori is how to penetrate the mucus, which has the remarkable property of transitioning from a viscous solution at neutral pH to a gel-like substance at low pH.

Celli et al. demonstrate that the capacity of this bacterium to neutralize its immediate surroundings, via urease activity, also contributes to reducing the viscoelastic moduli of gastric mucin. Measurements of the time-dependent rotation of the bacterial cell body and its flagellum, when trapped within mucin in its gel-like state, reveal that it possesses a high-torque motor operating at roughly 1 × 10−17 N m, a value that is 3- to 10-fold larger than what has been calculated for other bacteria. Adding urea raises the pH, which reduces the moduli by two orders of magnitude and allows H. pylori to swim through the mucus at 30 µm/s.

Proc. Natl. Acad. Sci. U.S.A. 106, 10.1073/pnas.0903438106 (2009).

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