It may be hard to believe, but our bodies are home to about 100 trillion bacteria. In fact, there are so many of these critters that they outnumber our own cells 10-1, arguably making us more microbe than human. Fortunately, they are so tiny that a 200-pound adult human carries only about 5 pounds of bacteria.
Bacteria start to invade our bodies at birth. Although "sterile" in utero, we are immediately infiltrated by these tiny microbes that set up shop on our skin and in the various nooks and crannies of our bodies. One of our first experiences in life is being coated with vaginal microbes that stay with us for life and literally become part of us.
Long thought to be relatively harmless hitchhikers, this population of microbes (referred to collectively as the "microbiome") had been largely ignored by scientists. But in the last few years, two major projects, the Human Microbiome Project (HMP) and the Metagenetics of the Human Intestinal Tract (MetaHIT), were launched by consortia from the US, Europe and China. Their goal was to map the diversity of human bacteria. The scientists examined and identified the bacterial species present in the gut, vagina and mouth in 242 individuals over time. What they found was astounding: The typical human being is home to several thousand distinct species of bacteria. Moreover, the array and number of species differed between anatomical sites, such as skin, nose, gut and vagina, and also between individuals. Another surprise was that we all carry microbes that have the capacity to cause disease. Yet these pathogenic bacteria happily co-exist with all of the other microbes and, under normal circumstances, do not cause us harm.
So what is the significance of all this - is there value in giving these hitchhikers a ride? The answer is yes. Bacteria are not just passive passengers; rather, they are important components of our bodies vital for health and well-being. For example, since humans don't make all the enzymes necessary to digest food, gut microorganisms are essential for breaking down many fats and carbohydrates into a form we can absorb. They also manufacture essential nutrients (such as vitamin K). A particularly interesting finding is that the experimental transfer of microbiomes from fat mice to thin mice induced the recipients to become fat themselves. Some studies have suggested that while everyone's microbiome is unique, they fall into three general classes characterized by specific bacterial populations. Interestingly, an individual's "class" of microbiome may be related to diet, with two of the classes being correlated with either high-meat or vegetarian diets. In support of this idea, it was recently shown that the amount of fat in the diet modifies the bile acids secreted into the gut and thereby alters the distribution of bacterial species present. Interestingly, a high-fat diet promoted increases in bacteria associated with the development of colitis, a disease characterized by inflammation of the colon. The bacteria that predominate in a high-fat diet tend to metabolize sulfur and produce hydrogen sulfide that can irritate the gut lining, a factor in colitis.
To better understand the role of gut bacteria, scientists have bred mice under completely sterile conditions so they lack gut microflora. Surprisingly, these mice developed an asthma-like syndrome, characterized by wheezing, coughing and shortness of breath. On further examination it was found that their intestines and lungs harbored large numbers of a type of immune cell known to elicit inflammatory responses to environmental allergens. Re-housing the mice in normal cages with exposure to environmental and food-borne bacteria did give them a microbiome but did not reduce the number of these cells, nor reduce the asthma. However, their offspring with their normal microbiomes did not become asthmatic and did not harbor large numbers of the immune cells implicated in asthma development. In other words, early exposure to bacteria was key to preventing the disease.
What does this mean for you and me? Well, certainly the concept of probiotics (foods such as yogurts, or tablets that contain live bacteria to repopulate the gut microbiome) is starting to make a lot of sense, although we don't yet have a clear understanding of what really works. Probiotics may be especially important to restore the normal balance of bacteria in the gut after antibiotic treatments for specific infections. They are regularly given to regenerate the intestinal microbiomes of patients infected with hospital-acquired Clostridium difficile, a bacterium that causes diarrhea.
The website www.MicrobeWorld.org notes the amazing fact that there are more microbes on a person's hand than there are people on the entire planet! The saying "no man is an island," originally penned by the poet John Donne, is making more sense now than ever before ...
David L. "Woody" Woodland, Ph.D. is Chief Scientific Officer of Silverthorne-based Keystone Symposia on Molecular and Cellular Biology, a nonprofit dedicated to accelerating life science discovery by convening internationally renowned research conferences in Summit County and worldwide. Woody can be reached at (970) 262-1230 ext. 131 or firstname.lastname@example.org.