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Jason Tetro

Each and every day our bodies – and indeed our trillions of microbes – are exposed to foreign chemicals. Collectively, these are known as xenobiotics, taken from the Greek word for stranger, xenos. Back in the 1960s, when the amount of such chemicals in the environment surged due in part to industrial activity, researchers aimed to determine how these molecules affect normal biological processes and our health.

Xenobiotics are not limited to toxic industrial chemicals. Every molecule not formed by our bodies falls under this category. Many come to us not through manufacturing but through nutrition. Fruits, vegetables, cereals and other foods we tend to consume on a daily basis are rich in xenobiotic compounds such as beneficial antioxidants and anti-inflammatories. On the other hand some lifestyle routines, such as smoking or taking medications, can lead to exposure with potentially harmful side effects.

The exploration into the effects of xenobiotics on the human body continues, and the information gleaned so far only reveals a quarter of their potential impact.

Each of us is a collection of about 37 trillion cells – and about 100 trillion microbes. Most of these are bacteria. Much like human cells, these organisms have the ability to sense and also react to a foreign entity. The reaction can be cordial, when the chemical is thought to be beneficial. But the response can also be hostile, when the bacteria tend to go on the defensive and fight for survival. In either case, the events have a direct impact on our health.

The most studied xenobiotics affecting microbes are antibiotics. These medicinal weapons against disease have been with us for some 70 years and for the most part have helped save millions of lives. But a closer look reveals a rather serious problem. Many bacterial species can resist the effects of exposure to antibiotics and find ways to survive. So the overall result of a prescription isn't a clean slate but an altered diversity in which the usual population of between 500 and 1,000 species of bacteria is reduced to a fraction of this number. When this happens, our bodies tend to suffer as we lose out on several benefits of a healthy microbial population, including proper digestion of foods as well as balance of the gut immune system. This can lead to, in some cases, diarrhea.

An altered diversity can also lead to infection with one of Canada's most important microbial pathogens, Clostridium difficile. When the microbial population in our gut is highly diverse, there is no room for C. difficile to form colonies and cause infection. But when that number drops, there is more than enough room for settlement. As the pathogen grows, it reaches a critical density in which the amount of available nutrients is no longer enough to sustain the population. At this point, it forms a toxin in the hopes of poaching nutrients from the harmed and dead bacterial cells.

Another growing branch of xenobiotic research focuses on foods. Recently, much attention has been given to artificial sweeteners and their effect on our microbes and, by consequence, our bodies. These sugar substitutes are thought to be an important addition to some weight-reduction programs. Yet, for some reason the opposite may be true. The use of chemicals such as saccharin and aspartame may lead to an increase in body fat and make a person more prone to the development of Type 2 diabetes.

The body and its microbes are unable to use such chemicals for food. Bacteria suffer the most as they have a relatively short lifespan. Without real sugar, they simply die off or have to look to other options for nutrients such as fats, proteins and starches. The change in food supply eventually causes a shift in the diversity of the microbes in our guts. Sadly, the bacteria normally useful in keeping our metabolism and immunity balanced are the ones we lose. In their place, other bacteria take over and start to produce chemicals known to trigger imbalance in our bodies. The most prominent is called propionate. An increase in this particular chemical has been associated not only with weight gain and a higher risk of diabetes but also irritable bowel syndrome, mood dysfunction and even possibly autism spectrum disorder.

These two examples are mainly gut-focused, but the reach of xenobiotics isn't limited to this area. Foreign molecules can have effects on our microbial population elsewhere in the body. In the lungs, cigarette smoke tends to turn bacteria more pathogenic, which may lead to an increase in the risk of infection and pneumonia. The use of chemical antiperspirants can change the microbial population on the skin and allow even worse-smelling bacteria to find a home. Even a change in chewing gum can affect the microbes in our mouth – although in this case for the better. The chemical xylitol can help to eliminate bacteria known to cause cavities and gum disease.

The field of xenobiotics, microbes and our health is growing and we can expect to see even more revelations on how the chemicals in our lives affect our well-being. Granted, the majority of results will still be limited to the lab and not apply directly to our lifestyles. Yet the information we gather in the short term will inevitably help us down the road as we make better health decisions to deal with all that foreign contact.

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Jason Tetro is a Toronto-based microbiologist with more than 25 years experience in research. The self-described germs relationship therapist strives to improve humanity's bond with the unseen world. His science bestseller, The Germ Code, is out now. You can follow him on Twitter at @JATetro