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BACKGROUND AND AIMS: Lactulose fermentation by the intestinal microflora acidifies the gut contents, resulting in an increased resistance to colonisation by acid sensitive pathogens. The extent of fermentation should be controlled to prevent acid induced epithelial cell damage. Considering the buffering capacity of calcium phosphate and its intestinal cytoprotective effects, whether supplemental calcium phosphate adds to the increased resistance to intestinal infections by lactulose fermentations was studied. METHODS: In a strictly controlled experiment, rats were fed a purified low calcium control diet, a low calcium/lactulose diet, or a high calcium/lactulose diet, and subsequently infected orally with Salmonella enteritidis. RESULTS: Lactulose fermentation lowered the pH and increased the lactic acid concentration of the intestinal contents, which significantly reduced excretion of this pathogen in faeces; thus it improved the resistance to colonisation. This agreed with the high sensitivity of S enteritidis to lactic acid (main metabolite of lactulose fermentation) in vitro. Calcium phosphate decreased translocation of S enteritidis to the systemic circulation, an effect independent of lactulose. The unfavourable increased cytotoxicity of faecal water caused by lactulose fermentation was more than counteracted by supplemental calcium phosphate. Moreover, calcium phosphate stimulated lactulose fermentation, as judged by the reduced lactulose excretion in faeces and increased lactic acid, ammonia, and faecal nitrogen excretion. CONCLUSION: Extra calcium phosphate added to a lactulose diet improves the resistance to colonisation and translocation of S enteritidis. This is probably mediated by a calcium induced stimulation of lactulose fermentation by the intestinal microflora and reversion of the lactulose mediated increased luminal cytotoxicity, which reduces damage inflicted on the intestinal mucosa.

Calcium in milk products stimulates gastric acid secretion and inhibits the cytolytic activity of intestinal contents. Based on these effects, it was hypothesised that calcium might lessen the severity of food borne intestinal infections. The possible differential effects of a low calcium milk and normal milk products (milk, acidified milk, and pasteurised yoghurt) on the resistance of rats to a salmonella infection was therefore studied. Rats were infected orally with Salmonella enteritidis just after food consumption. The first day after infection, faecal salmonella counts of the yoghurt fed rats were significantly lower than those of the other groups. Thereafter, faecal salmonella excretion declined rapidly in all high calcium groups, whereas rats fed the low calcium milk continued to excrete high numbers of salmonella. The reduced colonisation resistance to salmonella of rats fed low calcium milk might be caused by the high cytolytic activity of faecal water or a high iron concentration in faecal water, already present before infection, or both. The reduced resistance of these rats corresponded with a large infection induced increase in the cytolytic activity of faecal water, an appreciable reduction in apparent iron absorption, and a large increase in faecal mucin and alkaline phosphatase excretion. In yoghurt fed rats, only minor infection induced changes in luminal parameters were noticed. The rats fed milk and acidified milk always showed intermediate reactions. In conclusion, in addition to fermentation by yoghurt bacteria, calcium in milk products strongly enhanced the resistance to salmonella infection by lowering luminal cytolytic activity or diminishing the availability of iron for pathogen growth, or both.