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Friendly tenants in the human gut: The genome of B. longum

Scientists have sequenced the genome of one of the most important residents in the human gastrointestinal tract, a bacterium that keeps the digestive system running smoothly, blocks the growth of harmful bacteria, and boosts the immune system. The microbe, called Bifidobacterium longum, is often the dominant bacterium found in humans.

Bifidobacterium longum lives in the human gut.

The researchers identified a number of proteins specialized to help B. longum interact with the human host and persist against harmful bacteria. They can now closely look at which genes allow B. longum to live in different environments such as dairy products, vegetables and the human gastrointestinal tract.

Bacteria such as B. longum ferment sugars into lactic acid and have many health benefits. For these reasons, researchers of the food and drug industry have taken a keen interest in using these organisms.

Bifidobacterium longum is among the first colonizers of the sterile digestive tract of newborns and predominate in breast-fed infants, according to the scientists. The research team isolated the bacterium from the feces of an infant. Fabrizio Arigoni, of the Swiss food and infant formula manufacturer Nestlé in Lausanne, led the study.

Formula-fed infants have a different microflora, and this may be related to the higher risk of diarrhea and allergies in these babies. "This progressive colonization is thought to be important for development of immune system tolerance…lack of such tolerance possibly leads to food allergies and chronic inflammation," the researchers write in Proceedings of the National Academy of Sciences.

Recognizing the many benefits of lactic acid bacteria to good health, people have been supplementing their diets with these microbes, which are also called probiotics (meaning 'in favor of life').

Lactic acid bacteria have been included in dairy foods and taken as supplements in powder, liquid extracts, or tablets. Namely live cultures in yogurt have been used as a remedy for hundreds of years to support immune function. Doctors recommend the bacterial supplement to patients who take antibiotics, suffer from bacterial, viral or fungal infections or have various digestive problems.

Other potential uses of B. longum are being investigated in separate studies. Japanese researchers showed that the microbe might be useful as a gene delivery vector for cancer therapy. They injected the bacterium into the tail veins of rats and demonstrated that B. longum is accumulated in the tumor.

The Joint Genome Institute has initiated a Lactic Acid Bacteria Genome Consortium, which also includes a Bifidobacterium longum Genome Project. The consortium has recently sequenced another lactic acid microbe, Oenococcus oeni—used in the secondary fermentation of wine.

Comparative studies of lactic acid bacteria may lead to better understanding the microbes' roles in food fermentation and human health.

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Schell, M.A. et al. The genome sequence of Bifidobacterium longum reflects its adaptation to the human gastrointestinal tract. Proc Natl Acad Sci USA. Published online October 15, 2002.
Yazawa, K. et al. Bifidobacterium longum as a delivery system for gene therapy of chemically induced rat mammary tumors. Breast Cancer Res Treat 66, 165-170 (March 2001).

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