Extra.Interaction. Autism

Bacteria: A role in autism?

One of the more surprising links between bacteria and behavior has emerged in studies of autism. This brain disorder makes it hard for people to interact and communicate with others. Research in mice by Elaine Hsiao is adding to the theory that changes in gut bacteria may underlie some autistic behaviors.
Hsiao is a neurobiologist at the California Institute of Technology, in Pasadena. She studies how the brain and nervous system function.
Studies had shown that people with autism are more likely to have too many or too few good bacteria in their gut. What’s more, the walls of their intestines also tend to leak. Wastes produced by those bacteria might leak out into the bloodstream. From there, they could quickly flow into the brain, the command center for behavior and mood.

What is autism?

But just because studies have found abnormal levels of gut bacteria in patients with autism doesn't mean that microbes cause the disorder. To probe that, scientists are doing tests with mice. (There is no guarantee that mice and people will react the same way to changes in their gut bacteria. However, the bodies of both animals share many similarities. That makes the small animals a useful model in scientific studies.)
Certain mice actually show symptoms of autism. Like people with this disorder, they have both altered levels of gut bacteria and leaky guts, Hsiao found. So her team fed these mice applesauce laced with a human gut bacterium called Bacteroides fragilis (BAK-teh-ROY-dees FRAA-ji-lis) for three weeks. Afterward, levels of several species of gut bacteria returned to normal. The animals’ guts also leaked less.
More importantly, some autistic behaviors in the treated mice improved. And Hsiao’s team knows that because it measured three types of autistic behavior before that treatment began.
In one test, mice were placed in a box attached to two others. One box contained another mouse; the other a toy. Mice could choose to play with the toy or the mouse. Mice without symptoms of autism showed normal social behavior. They played with the mouse. Autistic mice, by contrast, preferred the toy.
A second test measured communication. Mice typically “speak” in an ultrasonic range, frequencies people can't hear. Hsiao recorded their chatter with a special microphone that can pick up ultrasonic frequencies. Autistic mice “produced fewer calls and the calls were shorter," she reports. In other words, they say less than normal mice do.
Finally, Hsiao placed mice in a bin containing wood shavings and a few marbles. In the wild, mice normally bury things. But the autistic mice kept burying the marbles. Then the mice dug them up and reburied them — over and over.
After eating the applesauce with B. fragilis, the autistic mice stopped compulsively burying marbles. They also communicated like normal mice. What didn’t change: They still preferred to play with toys, not other mice.
Hsiao and her colleagues published their findings Dec. 19, 2013, in the journal Cell.
How bacteria alter behaviors in these mice remains unknown. Hsiao says one theory is that bacterial wastes leaking into the blood might enter the brain, changing its chemistry. And chemical activities in the brain underlie many of our behaviors. Another theory: Gut bacteria somehow communicate with the brain over the vagus nerve. That long nerve runs between the gut lining and brain.
No matter how the change takes place, this study suggests "good bacteria" might help diminish autistic behaviors, says Hsiao.

Bacteria bug us, for better or for worse

There is much still to learn about how bacteria affect animal behavior. That’s one thing each of the researchers who was profiled here wants to emphasize. For example, how many bacteria are good? How many bacteria does it take to alter behavior? And precisely how might bacteria — or the chemicals they produce — trigger behavioral changes?
The research is all very new. As McFall-Ngai points out, scientists began to realize bacteria can affect behavior only in the last four to five years.
"It's like all of a sudden a door has opened up," she says. "Animal researchers are now looking at their studies and asking 'Could what I'm seeing be influenced by bacteria?'"