How probiotics protect against infection

It's long been known that good gut bacteria - so-called probiotics - are beneficial to our health, although the mechanisms behind their benevolent action have remained a mystery until now.

Researchers in Japan and Australia have revealed how species of beneficial bacteria of the genus Bifidobacteria are able to affect their host's immune response and aid against invasion by harmful bacteria.

The found that probiotic bacteria metabolism leads to an increase in acetate levels, which aided in preventing harmful E. coli bacteria from moving from the gut and invading blood cells in mice.

The infected mice with the potentially lethal enterohaemorrhagic Escherichia coli O157:H7, which causes illnesses ranging from mild diarrhoea to severe diseases such as haemorrhagic colitis and haemolytic uraemic syndrome.

Those mice infected by the E. coli died within seven days unless they were colonised with a probiotic bacteria, Bifidobacterium longum subsp. longum JCM 1217.

The researchers employed a range of 'omics approaches, including genomics and metabolomics, to investigate the causes of this protective activity.

They found that it was a gene in the Bifidobacterium helped protect the mice against the harmful effects of the E. coli by producing acetate.

Dr David Topping, Chief Research Scientist with the CSIRO Food Futures and Preventative Health Flagships, and co-author on the paper, says that it's only now that we're beginning to understand how these probiotic bacteria have done their good work.

“These beneficial bacteria (probiotics) exert a wide range of effects through the products of their metabolism of dietary fibre carbohydrates. These products are called short chain fatty acids (SCFA) and this study has looked at how one of these acids, acetic acid, could confer protection against a highly pathogenic organism E coli 0157."

While this finding sheds light into the activity of this beneficial bacteria, some scientists urge caution in generalising the results to humans.

"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," says Associate Professor Eiichi Sato, from the Tokyo University of Agriculture.

"Therefore, it would be difficult to apply this experiment the way it is exactly onto humans since these results were gathered using cultured cells and germ-free mice. This is a dilemma many scientists struggle with. But bifidobacteria are a well-known strain of probiotics so I think it would be safe to expect that some day someone will be able to clarify our defence mechanism against diseases.”

The paper was published in the journal Nature.