We share our body with up to a thousand varieties of bacteria. They live in our mouth, our skin and mostly in our digestive tract. They total some 100 trillion — 10 times greater than the number of human cells in the body. We have known since the groundbreaking work of Louis Pasteur and Robert Koch in the 19th century that bacteria can cause disease. Koch’s elegant experiment demonstrating that bacteria cultured from a tuberculosis patient were capable of causing the disease in a mouse cemented the bacterial theory of disease and launched the quest for antibiotics. Nobody back then was concerned about disrupting the body’s natural bacterial flora with antibiotics; the primary goal was to treat bacterial diseases. Now we know that the bacteria in our body are an integral part of our internal activities. 

Who could have guessed that whether a baby is delivered vaginally or via caesarean section can affect weight decades later? Yet this appears to be the case. A review of 15 studies examining more than 160,000 births revealed that babies born by C-section were 26 percent more likely to be overweight and 22 percent more likely to be obese as adults. The theory is that babies born by C-section pick up bacteria from the mother’s skin instead of her vaginal tract and that this bacterial population is more efficient at extracting calories from food. That notion is backed by the well-known increase in weight by farm animals treated with antibiotics. The drugs eliminate the bacteria that are less efficient in breaking down food into absorbable components.

It is not only weight that may be affected by bacteria. Some researchers link Crohn’s disease, celiac disease and arthritis with a disturbance of bacterial population in the gut, possibly by antibiotics. The average North American child has three courses of antibiotics in the first two years of life, which may cause a permanent shift in the body’s microbial environment. Exactly how bacteria affect health is not clear but the bacterial digestive processes produce a variety of metabolites that enter the bloodstream and may have an effect on the biochemistry that underlies everything from the control of blood sugar to the control of mood. Even autism may have a connection to the bacteria in our gut. This seems to be the case if we go by some mouse experiments.  

It turns out that injecting pregnant mice with a virus causes immune activity that can lead to autism-like behaviour in their pups. The newborn obsessively self-groom and become disinterested in other mice. What causes this autistic-like behaviour in the pups? It seems they suffer from “leaky gut syndrome,” a condition where molecules produced by gut bacteria can seep into the bloodstream and conceivably reach the brain. It is interesting to note that there is also an increased risk of autism in children born to women who had a severe infection, like the flu, while pregnant and that autistic children commonly suffer from gastrointestinal problems, including leaky gut syndrome.  

Now for the really interesting finding. Not only did the pups suffer from leaky gut syndrome but they had a different bacterial population in their gut than other mice. It seems that maternal infection can alter the microbiome in offspring. Furthermore, 4-ethyphenylsulphate, a chemical that can affect behaviour, was found in higher levels of the autistic mice, possibly generated by a bacterial species that were present in higher concentration than normal. This chemical is similar to para-Cresol CH3C6H4(OH), which has been detected in the urine of people with autism. 

To try to rebalance the bacterial population, the researchers treated their animals with a strain of bacteria known as Bacteroides fragilis that had previously been shown to protect mice from gut inflammation, hoping these bacteria would crowd out the culprits that were causing the leaky gut. That is what appears to have happened. The blood levels of 4-ethylphenylsulphate dropped and the mice stopped some of the autistic-like behaviours, including obsessively burying marbles in their cages. Certainly this study does not mean that autism in people can be treated with Bacteroides fragilis but it does introduce the possibility that treatment with beneficial bacteria — in other words, probiotics — may have a future role to play.  

Joe Schwarcz is the director of McGill­ University­’s Office for Science and Society. Read his blog at www.mcgill.ca/oss.