The Human Microbiome

Microbes. What do you think of when you hear that word? Bacteria? Viruses? Perhaps you think of all the horrible diseases caused by those nearly invisible little bugs – influenza, tuberculosis, syphilis, pneumonia, strep throat, et cetera. But, should we define a whole biological kingdom based on a few misunderstood villains? I think not. The truth is that the large majority of microbes (I’m admittedly mostly speaking of bacteria here) are not the villainous creatures we think them to be. Most are harmless. In fact, they are all around you right now. They’re on your floor, on your furniture, on your clothes, on your skin, in your mouth and throughout your digestive tract. They are, quite literally, everywhere. And most of the time they are completely harmless to you. Don’t get me wrong, most of them would probably eat you if they could, but they can’t, so they’re harmless.

Better yet, a number of these microbes can actually be tremendously beneficial to you – especially the ones found in your digestive tract. These bacteria (and some fungi) form a symbiotic relationship with their human host. They live all throughout the digestive tract where they find plenty of food and in return they provide a number of benefits to the human host. What kind of benefits do they provide, you might be wondering? Beneficial microbes in the digestive tract can enhance digestion, maintain a healthy gut barrier, decrease blood pressure, improve cardiovascular health, help regulate blood sugar, improve mood, relieve anxiety, improve skin health, maintain healthy weight and help prevent kidney stones. But even more important is the effect these microbes can have on the immune system. A proper balance of these microbes can help to modulate the activity of the immune system and, therefore, inflammation. They can help to both strengthen a weak immune system and normalize an overactive immune system.

All of this is just what we’ve learned so far and there is so much more to learn about these little creatures that reside in our gut. We are, in fact, still learning who resides there. While we’ve discovered many of them, we still haven’t identified them all. And of the ones we have discovered, we’re just skimming the surface of all the possible effects they may have on us – especially considering that there are more bacteria in our gut than there are cells in our body!

How does one attain or maintain a balanced population of gut flora? The development of our gut flora starts early on in our life. In fact, it starts at birth. The bacteria found in the gastrointestinal tract of infants birthed vaginally versus cesarean section differ quite remarkably. The most evident difference is a lack of Bifidobacterium species in the infants whose mother had a cesarean section. This tells us that the vaginal flora are, in part, conferred to the infant through the birthing process. This is our first exposure to the bacteria that begin to populate our GI tract. A similar difference is also found between breast fed children and those fed formula as breast milk contains factors that promote the growth of beneficial gut bacteria. There is speculation as to what this might mean for the long-term health of children born to cesarean section and those not breast fed. At this point, we don’t know anything for sure.

So, our gut flora begins to develop at birth and is influenced by breast milk. But, what else can influence our gut flora? What else might cause dysbiosis (i.e., an imbalance of gut flora)? There are actually a number of contributing factors to dysbiosis. One of the top culprits in our modern world is antibiotics.

Antibiotics have undoubtedly saved countless lives in their ability to halt dangerous infections, but there is a downside to this. While antibiotics help our immune system kill off the dangerous bacteria, they also kill off the beneficial bacteria. When we use antibiotics and a large number of beneficial gut bacteria get killed off, we set the stage for the growth of non-beneficial or even harmful bacteria in our intestines. As an example, one of the most dangerous imbalances that can occur following antibiotic use sets off an infection known as pseudomembranous colitis. This occurs when Clostridium difficile, which is normally found in our colon in small numbers, grows out of control. While large numbers of beneficial bacteria have been killed off, C. difficile has survived the antibiotic treatment. Lacking competition for intestinal space and nutrients, C. difficile grows out of control and takes over the intestinal environment. Small numbers of C. difficile are normal and won’t cause a problem, but large numbers can cause infection.

This same scenario can happen in more subtle ways. Antibiotic use doesn’t always lead to such severe consequences, but it can dramatically change the floral balance and significantly decrease bacterial diversity. The long-term health consequences of dysbiosis are still being understood, but it can set the stage for problems with digestion, weight control, immune functioning and may influence the development of leaky-gut syndrome which can in turn lead to problems with allergies and even autoimmune diseases. Another disease which may serve to illustrate a possible severe consequence of dysbiosis is Crohn’s disease. Researchers have found that Crohn’s patients tend to have a significantly lower number of a specific bacterium known as Faecalibacterium prausnitzii. This reduced number of bacteria was also associated with a reduced number of short-chain fatty acids in the colon, such as butyrate, which are known to play a role in colon health. While this is only correlational at this time, it may very well turn out that we have a disease caused by a lack of a bacterium…quite the opposite of our traditional thinking on disease causation!

Antibiotics are not the only possible cause of dysbiosis, however. There are numerous other causes including a poor diet and psychological or physical stress. Our diet can influence our bacterial population by not providing the proper nutrients for healthy bacteria, while also providing exactly what non-beneficial bacteria are looking for. This may be a diet high in sugar and refined carbohydrates, high in protein, high in animal protein and/or high in sulfates/sulfites. Consider this as one more indication that we should be eating a balanced diet, high in fruits and vegetables. How stress effects the bacteria in our gut may not be fully known, but it may occur through a reduction in IgA. IgA is a specific immunoglobulin produced by the immune system and secreted throughout our digestive tract that is known to decrease with stress. One function of IgA is to bind to pathogenic bacteria. If levels of IgA are low and unable to do this, it may set the stage for the adherence of pathogenic bacteria to the intestinal wall where they can compete with our healthy gut bacteria for nutrients and space in turn leading to dysbiosis. It has also been shown that stress hormones such as epinephrine and norephinephrine can contribute to the growth of less friendly bacteria.

The bottom line is that the human microbiome – the symbiotic relationship we have with our gut bacteria – is extremely beneficial to our health, while also being very sensitive to the environment we expose it to. Maintaining healthy gut flora is one way for us to maintain our own health. It is just one example of the interconnectedness of humans with the rest of our environment, even when that environment is found within.



Leave a Reply

Your email address will not be published. Required fields are marked *