We may think of ourselves as just human, but we are really a mass of microorganisms housed in a human shell. Every person alive is host to about 100 trillion bacterial cells. They outnumber human cells 10-to-1 and account for 99.9 percent of the unique genes in the body.
Nature Reviews senior editor Katrina Ray recently suggested that the vast number of microbes in the gut could be considered a “human microbial ‘organ’” and asked, “Are we more microbe than man?”
Our collection of microbiota, known as the microbiome, is the human equivalent of an environmental ecosystem. Although the bacteria together weigh a mere 1.36kg, their composition determines much about how the body functions and, alas, sometimes malfunctions.
Like ecosystems the world over, the human microbiome is losing its diversity, to the potential detriment of the health of those it inhabits.
Martin Blaser, a specialist in infectious diseases at the New York University School of Medicine and the director of the Human Microbiome Program, has studied the role of bacteria in disease for more than three decades. His research extends well beyond infectious diseases to autoimmune conditions and other ailments that have been increasing sharply worldwide.
In his new book, Missing Microbes, Blaser links the declining variety within the microbiome to our increased susceptibility to conditions from allergies and celiac disease to Type 1 diabetes and obesity. He and others primarily blame antibiotics for the connection.
The damaging effect of antibiotics on microbial diversity starts early, Blaser said. The average American child is given nearly three courses of antibiotics in the first two years of life, and eight more during the next eight years. Even a short course of antibiotics like the widely prescribed Z-pack (azithromycin, taken for five days), can result in long-term shifts in the body’s microbial environment.
However, antibiotics are not the only way the balance within us can be disrupted. Caesarean deliveries, which have soared in recent decades, encourage the growth of microbes from the mother’s skin, instead of from the birth canal, in the baby’s gut, Blaser said in an interview.
This change in microbiota can reshape an infant’s metabolism and immune system. A recent review of 15 studies involving 163,796 births found that, compared with babies delivered vaginally, those born by Caesarean section were 26 percent more likely to be overweight and 22 percent more likely to be obese as adults.
Other studies have found major differences in the microorganisms living in the guts of normal-weight and obese individuals. Although such studies cannot tell which came first — the weight problem or the changed microbiota — studies indicate obese mice have gut bacteria that are better able to extract calories from food.
Further evidence of a link to obesity comes from farm animals. About three-quarters of the antibiotics sold in the US are used in livestock. These antibiotics change the animals’ microbiota, hastening their growth.
When mice are given the same antibiotics used on livestock, the metabolism of their liver changes, stimulating an increase in body fat, Blaser said.
Even more serious is the increasing number of serious disorders now linked to a distortion in the microbial balance in the human gut. They include several that are becoming more common in developed countries — gastrointestinal ailments like Crohn’s disease, ulcerative colitis and celiac disease; cardiovascular disease; nonalcoholic fatty liver disease; digestive disorders like chronic reflux; autoimmune diseases like multiple sclerosis and rheumatoid arthritis; and asthma and allergies.