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General interest items edited by Janice Flahiff

Learning to tolerate our microbial self: Bacteria co-opt human immune cells for mutual benefit

The image depicts symbiotic microbes in the process of colonizing the mucosal surface of the mouse colon. Yellow cells are Escherichia coli; red cells are Bacteroides fragilis. Intestinal tissues are labeled in green with blue nuclei.

(Credit: S. Melanie Lee/Caltech)

From the 21 April 2011 Science Daily article

ScienceDaily (Apr. 22, 2011) — The human gut is filled with 100 trillion symbiotic bacteria — ten times more microbial cells than our own cells — representing close to one thousand different species. “And yet, if you were to eat a piece of chicken with just a few Salmonella, your immune system would mount a potent inflammatory response,” says Sarkis K. Mazmanian, assistant professor of biology at the California Institute of Technology (Caltech).

Salmonella and its pathogenic bacterial kin don’t look that much different from the legion of bacteria in our gut that we blissfully ignore, which raises the question: What decides whether we react or don’t? Researchers have pondered this paradox for decades.

In the case of a common “friendly” gut bacterium, Bacteroides fragilis, Mazmanian and his colleagues have figured out the surprising answer: “The decision is not made by us,” he says. “It’s made by the bacteria. Since we are their home, they hold the key to our immune system.”

What’s more, the bacteria enforce their “decision” by hijacking cells of the immune system, say Mazmanian and his colleagues, who have figured out the mechanism by which the bacteria accomplish this feat — and revealed an explanation for how the immune system distinguishes between beneficial and pathogenic organisms….

…bacteria actually live in a unique ecological niche, deep within the crypts of the colon, “and thus in intimate contact with the gut mucosal immune system,” he says.

“The closeness of this association highlights that an active communication is occurring between the bacteria and their host,” says Caltech postdoctoral scholar June L. Round.

From that vantage point, the bacteria are able to orchestrate control over the immune system — and, specifically, over the behavior of immune cells known as regulatory T cells, or Treg cells. …

…”Our immune system arose in the face of commensal colonization and thus likely evolved specialized molecules to recognize good bacteria,” says Round. Mazmanian suspects that genetic mutations in these pathways could be responsible for certain types of immune disorders, including inflammatory bowel disease: “The question is, do patients get sick because they are rejecting bacteria they shouldn’t reject?”

On a more philosophical level, Mazmanian says, the findings suggest that our concept of “self” should be broadened to include our many trillions of microbial residents. “These bacteria live inside us for our entire lives, and they’ve evolved to look and act like us, as part of us,” he says. “As far as our immune system is concerned, the molecules made by gut bacteria should be tolerated similarly to our own molecules. Except in this case, the bacteria ‘teaches’ us to tolerate them, for both our benefit and theirs.”…

Journal Reference:

  1. June L. Round, S. Melanie Lee, Jennifer Li, Gloria Tran, Bana Jabri, Talal A. Chatila, and Sarkis K. Mazmanian.The Toll-Like Receptor 2 Pathway Establishes Colonization by a Commensal of the Human MicrobiotaScience, 21 April 2011 DOI:10.1126/science.1206095

[Abstract only, for suggestions on how to get this article for free or at low cost, click here]

  • Do bacteria control your brain? (boingboing.net)
  • Gut Bacteria Mapping Finds Three Global Varieties (wired.com)
  • Friendly Bacteria Fight the Flu (scientificamerican.com)
  • What’s your gut type? (eurekalert.org)
  • People Fall Into Three Categories Of Gut Microbiota : Implications for Nutrient and Medicine Uptake (jflahiff.wordpress.com)
  • Humans Shown To Have Intestinal Bacteria Groups As Well As Blood Groups

    “The three enterotypes show various categories of bacteria with a different impact of the gut. Enterotype 1 is dominated by the Bacteroides intestinal bacteria, which together with a few other species of bacteria, forms a distinctive cluster of gut flora. The dominant bacteria in enterotype 2 is Prevotella. And in enterotype 3, Ruminococcus is the main bacteria, along with other species such as Staphylococcus, Gordonibacter and a species discovered in Wageningen previously, Akkermansia. Enterotype 3 is the most common.

    Furthermore, every cluster of bacteria has its own way of supplying energy. Enterotype 3, for example, specialises in breaking down mucin, a carbohydrate that enters the gut via our food. This allows the gut to absorb these fragments asnutrition for the body. All three enterotypes also produce vitamins, albeit in varying amounts. Enterotype 1 produces the most vitamin B7 (biotin), B2 (riboflavin) and C (ascorbic acid), and enterotype 2 produces mainly vitamin B1 (thiamin) and folic acid. Every enterotype, with its distinctive clusters of bacteria and functional differences, reflects a distinctive way of generating energy that is closely compatible with its host. It is also possible that the enterotypes may interact with their host on various levels, having an impact on the individual’s health.

    In March of last year, the MetaHIT consortium published the first catalogue of genes of human intestinal bacteria (also known as the second genome). These bacteria populations encode 150 times more genes than our own genome. It was shown that from a range of more than a thousand species of bacteria that live in the human gut, every individual is host to several hundred types of bacteria.

    The discovery of the enterotypes will influence the fields of biology, medicine and nutrition, making it much easier to analyse an individual’s needs. The research team sees future opportunities for personal and preventive dietary and medicinal advice.”

  • Learning to tolerate our microbial self: Bacteria co-opt human immune cells for mutual benefit (jflahiff.wordpress.com)
  • Deepak Chopra: Weekly Health Tip: You Are Home to Millions of Microbes! (huffingtonpost.com)
  • Friendly bacteria fight the flu (nature.com)

April 22, 2011 - Posted by | Consumer Health, Medical and Health Research News | , , ,

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