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Researchers at the Stanford University School of Medicine have identified a molecule that serves as natural protection against salmonella, an intestinal pathogen. Propionate is a byproduct of metabolism by a group of bacteria called the Bacteroides, inhibits the growth of Salmonella in the intestinal tract of mice. Propionate doesn’t trigger the immune response to thwart the pathogen the molecule prolongs the time it takes the pathogen to start dividing by increasing its internal acidity.
Salmonella infections cause diarrhea, fever and abdominal cramps. Some people may recover within four while others may need hospitalization. According to the Centers for Disease Control and Prevention, it causes about 1 million illnesses, 25,000 hospitalizations and 455 deaths yearly, some of the cases are caused by contaminated food. Humans differ in their response to exposure to bacterial infections. Some people get infected and some don’t, some get sick and others stay healthy, and some spread the infection while others clear it.
The differences in Salmonella growth could be attributed to the natural composition of bacteria in the intestines of each mouse strain. They discovered this by performing fecal transplants, which involved giving mice antibiotics to kill off their gut bacteria and then replacing the microbial community with the feces of other mice, some of whom were resistant to Salmonella infection. Then, determined which microbes were responsible for increased resistance to Salmonella infection by using machine-learning tools to identify which groups of bacteria were different between the strains.
They identified a specific group of bacteria-Bacteroides, which was more abundant in mice transplanted with the microbiota that was protective against Salmonella. Bacteroides produce short-chain fatty acids such as formate, acetate, butyrate and propionate during metabolism, and levels of propionate were threefold higher in mice that were protected against Salmonella growth. Then, the researchers sought to figure out whether propionate protected against Salmonella by boosting the immune system like other short-chain fatty acids do.
The scientists examined their Salmonella model for the potential impact of propionate on the immune system but found that the molecule had a more direct effect on the growth of Salmonella. Propionate acts on Salmonella by dramatically decreasing its intracellular pH and thus increasing the time it takes for the bacterium to start dividing and growing, the study found.
The results show that when concentrations of propionate, which is produced by Bacteroides, in the gut are high, Salmonella are unable to raise their internal pH to facilitate cellular functions required for growth. Treating Salmonella infections require the use of antibiotics, which may make Salmonella-induced illness or food poisoning worse since they also kill off the “good” bacteria that keep the intestine healthy. Using propionate reduces the use of antibiotics is a good benefit because overuse of antibiotics leads to increased incidence of antibiotic-resistant microbes.