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Scientists unravel probiotics gut defence
Australian and Japanese scientists have uncovered the mechanism by which probiotic bacteria protect the gut from E. coli infection.

Over the last few years, scientists have found that lactic acid bacteria and bifidobacteria have health-promoting effects, but until now exactly how they did this wasn't fully understood.

The human gut is home to a wide variety of microbes that have beneficial effects, including aiding gut function and protecting against illness and infection.

One of these, a probiotic known as Bifidobacterium, modulates host defence responses and protection against infectious diseases.

Lead author, Dr Fukuda of the RIKEN Research Centre for Allergy and Immunobiology in Yokohama, and his team found that a strain of bifidobacterium could protect the mice from a strain of Escherichia coli.

The researchers outline thier findings in a paper published today in Nature.

Enterohaemorrhagic E. coli (EHEC), produces Shiga toxin, which can cause illness ranging from mild diarrhoea, to much more severe diseases, including haemorrhagic colitis (bleeding of the colon).

In the study, the researchers infected mice with EHEC one week after treating them with one of two strains of bifidobacterium. Bifidobacterium convert carbohydrates into a range of short chain fatty acids (SFCA). They found that the strain that was best at converting carbohydrates provided the best protection from EHEC.

The researchers believe acetate improved resistance in the intestinal wall of the mice from Shiga toxin, where it normally transfers into the blood stream.

In the past, it was known that acetate made from carbohydrates fermented by bifidobacteria had health benefits, but the researchers say this is the first time anyone had clearly proved it.

The researchers say this paper has helped demonstrate how important these bifidobacteria strains are in maintaining our health, and what is needed to keep them healthy.
More effective probiotic foods

Dr David Topping, a co-author on the Nature paper, is a Chief Research Scientist with CSIRO Food Futures and Preventative Health Flagships, based in Adelaide. He says that there is a growing understanding of how the gut bacteria work to deliver health benefits.

"Beneficial bacteria exert a wide range of effects through the products of their metabolism of dietary fibre carbohydrates," he says.

"These products are called short chain fatty acids (SCFA) and this study looked at how one of these acids, acetic acid (acetate), could confer protection against the highly pathogenic E. coli. This bacterium poses a major threat of food-borne infection."

Topping says a modified starch that the CSIRO is also developing to deliver specific SCFA to the large bowel also proved highly effective in the study.

"The data confirm that acetic acid was critical for survival of infected animals," he says.

Topping says the study offer promise for the development of more effective prebiotic and probiotic foods to assist in infection control.

But Associate Professor Eiichi Sato, of the Tokyo University of Agriculture, says it may be difficult to apply this experiment directly to humans

"As animals ourselves, we have more than 1000 different kinds of microorganisms living in our guts which produce intestinal biota (bacterial flora) that are largely different from the intestinal biota of a mouse," he says.

"But bifidobacteria are a well-known strain of probiotics so I think it would be safe to expect that someday someone will be able to clarify our defence mechanism against diseases."

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