Calcium in milk and fermentation by yoghurt bacteria increase the resistance of rats to salmonella infection

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 milli 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.