How Probiotic Bacteria Protect

Bacterial infections of the gut are one reason children in the developing world are malnourished, and malnourishment can lead to a poorer response to oral vaccines. One way to protect from the effects of such infections is to add probiotic bacteria to food, but scientists are just beginning to learn how such bacteria actually protect. Today, Shinji Fukuda from the RIKEN Institute in Yokohama, Japan, reported that he and his colleagues showed how a certain type of probiotic bacteria called Bifidobacterium that can be found in yoghurt can protect mice from dying from an otherwise lethal Escherichia coli (E. coli) infection (Nature 469, 543, 2011).

The researchers infected germ-free mice, which do not have any bacteria in their gut, with E. coli serotype O157:H7. In humans, this type of E. coli causes illnesses ranging from mild to severe bloody diarrhea. It produces the so-called Shiga toxin which killed the germ-free mice after one week because the toxin entered the blood once the gut became leaky as a result of inflammation and death of gut cells.   

However, Fukuda and colleagues found that the death of mice infected with this pathogenic E. coli strain was prevented in the presence of certain types of Bifidobacterium in their gut. When they compared the entire genomes of the types of Bifidobacterium that prevented the death of the mice with ones that didn’t prevent it, they found that the preventive Bifidobacterium type had a gene that enabled it to import fructose, which the bacterium can turn into acetate. Acetate activates an anti-inflammatory response in colon cells, preventing them from dying and therefore keeping the Shiga toxin from entering the blood. Indeed, the researchers found that the protected mice had less Shiga toxin in the blood than the mice that died.   

Fukuda said food manufacturers could now use this finding to test if the Bifidobacterium types they add to yoghurt have the protective fructose transporter gene. He said acetate production might also be useful to address infection with other harmful gut bacteria.