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J Med Microbiol 55 (2006), 1265-1270; DOI: 10.1099/jmm.0.46611-0
© 2006 Society for General Microbiology
ISSN 1473-5644

Bacterial killing in gastric juice – effect of pH and pepsin on Escherichia coli and Helicobacter pylori

H. Zhu1, C. A. Hart2, D. Sales2 and N. B. Roberts1

Department of Clinical Biochemistry1 and Department of Medical Microbiology2 , Royal Liverpool University Hospital, Duncan Building, Prescot Street, Liverpool L7 8XP, UK

Correspondence
N. B. Roberts
n.b.roberts{at}liverpool.ac.uk

Received 6 March 2006
Accepted 25 May 2006

The susceptibility of Escherichia coli and Helicobacter pylori to pH and the effect of pepsin-mediated proteolysis were investigated. This was to establish the relative importance of their bacterial killing properties in gastric juice. Solutions in the pH range 1.5–7.4 with or without pig pepsin A were used, together with seven gastric juice samples obtained from patients undergoing routine gastric collection. Escherichia coli C690 (a capsulate strain), E. coli K-12 (a rough mutant) and Helicobacter pylori E5 were selected as the test organisms. Suspensions of bacteria (1x106 E. coli ml–1 and 1x108 H. pylori ml–1) were pre-incubated with test solutions at 37 °C for up to 2 h, and then cultured to establish the effect on subsequent growth. Survival of bacteria was diminished at pHs of less than 3.5, whereas killing required a pH of less than 2.5. Pre-incubation with pig pepsin at 0.5, 1.0 and 2.0 mg ml–1 at pH 3.5 reduced viable counts by 100 % for E. coli 690 and E. coli K-12 after 100 min incubation. With H. pylori, the viable counts decreased to 50 % of the control after 20 min incubation in 1 mg pepsin ml–1 at pH 2.5, 3.0 and 3.5. The gastric juices showed bactericidal activity at pH 3.5, and the rate of killing was juice dependent, with complete death of E. coli 690 occurring between 5 and 40 min post-incubation. Thus, killing of E. coli and H. pylori occurs optimally at pHs of less than 2.5. At pH 3.5, little effect is observed, whereas addition of pepsin alone or in gastric juice causes a marked increase in bacterial susceptibility, suggesting an important role for proteolysis in the killing of bacteria.




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