Autism: the intestinal bacteria in question

The increasing number of cases of autism suggests that environmental factors participate in the development of this neurological disorder. Surprising results indicate that disturbances in the composition of the intestinal bacterial flora could be one of these factors.

Autism is a neurodevelopmental disorder that appears before the age of three. Autistic symptoms can vary widely from child to child, but are generally characterized by impairment in social communication, as well as restricted or repetitive interests and behaviors.

Several studies suggest that autism spectrum disorders have significantly increased over the past decades. For example, estimates from the United States Centers for Disease Control and Prevention indicate that while autism affected 7 out of 1,000 children in 2000, that number rose to 15 out of 1,000 children in 2010. Certainly a good portion of this increase can be attributed to better detection of this disorder; however, several scientists fear that this rise is real and that we are witnessing an “autism epidemic” that is caused by the huge lifestyle changes that have taken place over the past few decades.

Heredity involved in half of the cases

There is no doubt that heredity plays an important role in the development of autistic disorders. For example, children with a sibling with autism are 10 times more likely to develop the disease themselves.

However, these genetic causes are not as important as initially thought: on the one hand, even if more than a hundred genes associated with autism have been identified, each of these defective genes is quite rare and, on the other hand, a large study carried out on a very large sample (two million people, including

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14,000 were autistic) clearly showed that heredity is responsible for only half of autism cases, the rest being due to lifestyle factors. The identification of these environmental factors is therefore of enormous importance if we hope to succeed in reducing the incidence of autism in the short and medium term.

Lack of intestinal bacteria

Many children with autism present with gastrointestinal problems and some studies have reported that treating these problems can have a positive impact on their behavior. Since the proper functioning of the intestine calls on the hundreds of billions of bacteria that live there, could these bacteria participate in the development of autism?

To answer this question, scientists analyzed the intestinal flora of 20 children aged 3 to 16 and compared it to that of autistic children of the same age. They observed that the presence of autistic disorders was strongly correlated with a low diversity of flora, that is to say that certain normally abundant bacterial species were present in much lower quantities. These differences are particularly striking for three types of bacteria (Prevotella, Coprococ-

cus and Veillonellaceae), known to play a very important role in the fermentation process of dietary fibers and the production of anti-inflammatory molecules.

Intestinal permeability allows toxins to pass into the blood

It is possible that this imbalance of the bacterial flora makes the intestinal wall more permeable and allows certain toxic molecules released by pathogenic bacteria to reach the blood and reach the brain. These observations are a real revolution, because they suppose the existence of a close link between the intestinal bacteria and the functioning of the brain. And since disturbances of this bacterial flora can influence neurodevelopment and lead to disorders such as autism, the discovery of agents capable of positively influencing the bacterial composition of the intestine could have extraordinary implications for the prevention and the treatment of this disease.

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– Sandin S et al. The familial risk of autism. JAMA; 311:1770-7.

– Kang DW et al. Reduced incidence of Prevotella and other fermenters in intestinal microflora of autistic children. PLoS One, 8:e68322.

– Hsiao EY et al. Microbiota modulate behavioral and physiological abnormalities associated with neurodevelopmental disorders. Cell; 155:1451-63.


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