The Gut-Brain Axis & Autism

August 14, 2018

Hippocrates, the forefather of western medicine, once said “all disease begins in the gut.” Thousands of years later, we are now beginning to realize the significance of his words. Indeed, there exists an enormous world inside of us that most of us constantly overlook.

 

Within the digestive tracts of humans and other animals is a complex system of microorganisms. Scientists refer to this system of bacteria in our intestines as gut microbiota.The idea that gut bacteria can influence digestion, allergies, and metabolism is becoming more common—as we continually see new books and magazines detailing how external factors such as diet can lead to better bacterial health. Recently, however, research has emerged indicating that these intestinal microbes might have a greater influence on our body than scientists previously considered. 

 

Evidence suggests a strong connection between our gut microbes and our brain. Researchers believe this connection plays a pivotal role in neurodevelopmental disorders.

 

Scientists refer to the connection between the intestinal microbes and the brain as the “gut-brain axis”—the bidirectional communication between the gut microbiota and central nervous system.

 

One reason scientists started to appreciate the gut-brain connection is due to the intense research involving autism. Different groups working to understand the molecular and cellular basis of autism realized the potential of the gut-brain axis and its implications for possible treatments in the future. 

 

How did they make the connection?

 

Correlation studies show that the gut bacteria of individuals with autism differ significantly from the control groups. This result, coupled with the awareness that large percentages of individuals with autism have gastrointestinal abnormalities, such as food sensitivities, have prompted scientists to consider a potential interaction between gut microbes and autism.

 

Dr. Sarkis Mazmanian, a microbiologist from the California Institute of Technology, furthered the research by specifically focusing on species of bacteria that are not as prevalent in the intestines of children with autism. Dr. Mazmanian and his team introduced these species of bacteria to mice that presented abnormal behaviors compared to the control mice in order to examine how these microbes might influence their behavior. 

 

What was the result?

 

By elevating these species of bacteria in mice, their social behavior improved, and overall stress hormone levels were reduced. The re-colonization of these bacteria and the subsequent change in behavior of the mice led to the idea that the presence of certain gut bacteria can influence the central nervous system. If this phenomenon can be recapitulated in humans, it could lead to alternative biotic-based treatments for neurodevelopmental disorders. 

 

Where are we now? 

 

Although the results are intriguing for scientists, the exact mechanism of the interaction between gut and brain remains a mystery. Furthermore, promising results on mice do not guarantee similar results for other species. Humans are far more complex than mice, and therefore, countless variables must be considered before an official clinical trial could be considered. 

 

Well, what do we know?

 

Researchers are aware that certain bacteria can produce metabolites and chemicals that can directly affect the behaviors of the host. Studies have shown that many bacteria are capable of producing active chemicals that could cross the blood brain barrier and influence our brain. However, mapping out a specific mechanism of a chemical’s effect on a complex neurodevelopmental disorder like autism is still highly unlikely at the moment. It is also important to point out that the pathogenesis of autism typically begins early during pregnancy as critical stages of brain development are occurring, but an infant’s gut microbiome is not established until after birth. Thus, differences in gut microbiota may very well have an effect on behavior, but these differences are probably not the underlying cause of autism.

 

Despite the mystery surrounding the gut-brain axis, scientists remain optimistic about the implications of improved understanding of these microbial networks. Treatment for disorders such as autism would be centered on restoring microbial health by elevating certain species of bacteria to alleviate symptoms. Scientists in the field are hopeful that probiotic treatments could help improve behavioral symptoms related to autism in the future. 

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    Department of Biological Sciences

    California State University, Sacramento

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