Introduction The Lactobacillus genus has evolved over thousands of years together with humans through its' natural role in milk and dairy products. However, recent attention of the gastro-intestinal (GI) probiotic effect has concentrated on intestinal function rather than on gastric function. The antibacterial properties of Lactobacilli in controlling gastric colonisation by Helicobacter pylori are well recognised1 but little is known about the effects of these acidophilic bacteria on gastric physiology. Recent evidence that Lactobacillus species have been found in the stomachs of animals and humans suggests that they may also modulate acid secretion.

Methods Lactobacillus rhamnosus (LRH) was chosen because its probiotic properties are well documented, it has been isolated from healthy human gastrointestinal mucosa and its genome was completely sequenced recently.2 LRH was grown in liquid medium for 48 h and conditioned medium (CM) was obtained by centrifugation and micro filtration. The effect of LRH CM was studied using a mouse gastric gland preparation with ¹⁴C-aminopirine accumulation as the indirect method for measurement of acid secretion. Culture medium alone was used as control.

Results LRH CM inhibited 0.1 M histamine stimulated acid secretion by 100% at dilution of 1/40 while the control medium showed only marginal non-significant effect at this dilution (n=4). The IC₅₀ (50% of acid inhibition) was ∼1/120 dilution and at this concentration LRH CM did not cause a shift in the histamine dose response curves indicating that histamine H₂-receptor antagonism is not involved although the efficacy of the response was inhibited by 47.7% (n=5). Furthermore, LRH CM inhibited 0.01 M carbachol and 1.0 M cAMP stimulated acid secretion by 100% and 95%, respectively (n=5), the latter also being suggestive of a post receptor effect. This raises the possibility that bioactive bacterial products secreted into the culture medium act directly on the proton pump or other aspects of parietal cell physiology/metabolism.

Conclusion This is the first report showing that Lactobacillus or its products exert a gastric antisecretory effect. This prompts the notion that colonisation of the gastric mucosa by these acidophilic bacteria or ingestion of their products may modulate acid secretion with the possibility of a beneficial impact on acid related diseases. Future in vivo and clinical studies on the role of Lactobacillus sp on gastric physiology are needed to explore the potential role of these probiotics in the stomach.