Probiotic bacteria health boon

Gut-friendly bugs don't have to be alive to boost immune system.

The average human gut contains around 100 different species of bacteria. Credit: © Alamy

Television advertisements are filled with advice about the benefit of foods containing 'friendly' bacteria, such as probiotic yoghurt. But a US-led research team now says that you don't need live bacteria to boost your digestive system ? just their DNA.

If true, that should allow the health effects of these bacteria to be incorporated into a range of different types of foods, or even pills and injections. Live bacteria are generally restricted to products such as dairy foods, because cooking or heating the cultures kills them.

The average human gut contains about 100 different species of bacteria, including 'good' bacteria, such as Lactobacillus and Bifidobacterium, which aid digestion, bolster the immune system and battle for space with 'bad' bacteria, such as those that cause food poisoning. Some of these beneficial bacteria are added to products such as live-culture yoghurt. Probiotic foods and supplements are big business ? the worldwide market is worth around US$6 billion a year.

Eyal Raz of the University of California, San Diego, and his colleagues looked at the effect of these bacteria on mice with colitis, a condition similar to inflammatory bowel disease in humans. Surprisingly, they found that the bacteria were just as effective when inactivated with gamma-ray radiation as when live cultures were used1.

A similar health effect occurred when Raz and colleagues treated the mice with a synthetic DNA molecule known as an immunostimulatory oligonucleotide, which mimics the effect of the bacteria on the immune system. "This shows that it's the bacterial DNA that gives the benefit," says Raz.

If the oligonucleotide can be added to food, it may expand the range of probiotic products, says Raz. "Live probiotics are not the best way to deliver the DNA," he adds. "The food starts to ferment. If you add live bacteria to orange juice, you get a different taste every hour."

“It's the bacterial DNA that gives the benefit Eyal Raz, , University of California”

DNA that boosts the immune system could even be given by pill or injection, Raz suggests. "You can't do this with bacteria," he points out.

It's an interesting proposal, says Glenn Gibson, who studies gut bacteria at the University of Reading, UK. But he says that probiotic foods have a raft of other benefits that rely on using live bacteria. They need to reproduce if they are to crowd out harmful strains and combat food poisoning, for example.

"I'm sure probiotics manufacturers would be delighted if they thought they could put it all in a pill," says Gibson. "It would be cheaper and the shelf-life would be a lot longer. But they usually need to be alive ? immune stimulation is the exception."

Gibson suggests that food researchers searching for healthy food additives should turn to 'prebiotics' ? compounds that alter the gut's balance by boosting the growth of friendly bacteria that are already present inside the body.


  1. 1

    Rachmilewitz, D. et al. Toll-like receptor 9 signaling mediates the anti-inflammatory effects of probiotics in murine experimental colitis. Gastroenterology, 126,520 - 528(2004).

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Hopkin, M. Probiotic bacteria health boon. Nature (2004).

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