Stomach bacteria may play a role in multiple sclerosis, according to an animal study.
From stomach to brain. I'm Bob Hirshon and this is Science Update.
In multiple sclerosis, or M.S., the immune system attacks the nervous system. But strangely, this attack might be provoked by gut bacteria. Caltech biologist Sarkis Mazmanian and his colleagues experimented with mice that had sterile digestive systems. They tried to induce M.S. in both the sterile mice and comparable, normal mice.
And what we showed was that animals that were sterile developed very little—if any—symptoms of multiple sclerosis.
Then they reintroduced just one kind of bacteria into the sterile mice—a bug previously linked to a type of immune cell involved in M.S. Suddenly, the formerly healthy mice developed M.S. symptoms. While that particular bug can't live in people, Mazmanian says other gut bacteria might have a similar effect on human M.S. I'm Bob Hirshon, for AAAS, the science society.
Making Sense of the Research
Every one of us is a walking bacterial zoo, with trillions of bacteria living inside of us. Our digestive tract can hold a thousand different species of microorganisms. Most of these bacteria are harmless or even beneficial, but scientists are learning that some may have surprising effects. For instance, bacterial infections are now known to cause ulcers—too much acid was wrongly blamed before—and bacteria have been increasingly linked to heart disease.
Still, scientists would have reason to be skeptical that stomach bacteria could play a role in M.S., which affects the central nervous system. That's because nervous tissue itself is sterile. However, Mazmanian says that many clues had been accumulating prior to his study. For example, patients with M.S. have been observed to either increase or decrease their symptoms during episodes of bacterial infection. And some patients have shown improvements in symptoms after being treated with antibiotics (for some other ailment).
To study the issue more directly, Mazmanian and his colleagues worked with lab mice that had been completely cleared of their normal gut bacteria. They tried to induce the mouse model of M.S. in both the sterile mice and in normal mice. As you heard, the sterilized mice were highly resistant to M.S.
Then, the researchers reintroduced just one kind of bacteria into the sterile mice—a bug from a group known as segmented filamentous bacteria, which have been linked to intestinal inflammation and, more intriguingly, to an immune-system cell known as Th17. Th17, in turn, help set off the inflammatory chain reaction that causes M.S. in animals. Suddenly, the formerly healthy mice developed M.S. symptoms. What's more, their Th17 count spiked not only in their stomachs, but in their brain and spinal cord as well.
Although the microbe in this study doesn't colonize people, Mazmanian says other gut bacteria might have a similar influence on Th17, and human M.S. symptoms. He's not saying that bacteria technically cause M.S., though. Rather, the presence of certain bacteria may be part of a complex picture that allows M.S. to develop. This may help explain why M.S. seems to depend heavily on the environment, rather than just genetics; even identical twins, who share the same genetic code, have only a 25 percent chance of sharing the disease. If bacteria do turn out to play a role, then targeted antibiotic therapies may be able to slow or halt the disease's progression.
Now try and answer these questions:
- Why would one NOT suspect that gut bacteria would be linked to M.S.?
- What were some of the reasons the experiment was conducted?
- Why was it important that the researchers added bacteria back into the sterile mice, to see if they developed M.S.?
- Suppose the sterile mice still didn't develop M.S. after receiving the bacteria. What conclusion would you draw?
You may want to check out the August 13, 2010, Science Update Podcast to hear further information about this Science Update and the other programs for that week. This podcast's topics include: re-growing joints and re-constructing faces, the link between gut bacteria and multiple sclerosis, and octopus venoms that could treat pain.
For more information on this topic, check out the related resources below.
The Ecology of Your Skin 1: Bacteria that Live on the Skin
6-12 | Audio
Genes and Geography
6-12 | Audio