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.
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?'"
