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The “Hygiene Hypothesis” and Why You Should Care

“Soap and water and common sense are the best disinfectants.”

- William Osler, Canadian physician, founding professor of Johns Hopkins Hospital


“[T]he vast majority of the thousands of species that inhabit us do not cause any problems, and, in fact, seem to come with serious benefits."

- B. Brett Finlay, Canadian microbiologist, author Let Them Eat Dirt: Saving Our Children from an Oversanitized World



As an infectious disease physician, I tried to convince my mother that some germs are our friends. She would have none of it. Instead she adhered to the view espoused by the 18th century English theologian John Wesley that “Cleanliness is, indeed, next to godliness.” She was also aware that there was a “germ theory of disease,” firmly believing that the only good germ was a dead germ. She was mistaken. My mother knew little or nothing about what I’ve come to refer to as the “germ theory of health.” This idea was first postulated in the 1980s by British epidemiologist David Strachan and dubbed “the hygiene hypothesis” by the media. In this Germ Gems post, I’ll discuss the hygiene hypothesis and the related concept that germs are an essential contributor to human health.

What is the hygiene hypothesis? The epidemiologist David Strachan first referred to “the hygiene hypothesis” in an article in the British Medical Journal in 1989. Drawing upon an earlier idea of others that there is a link between infection with parasites and immune disorders, Strachan proposed that a lower incidence of infections in early childhood could be the explanation for a rise of allergic diseases such as asthma and hay fever. As such, the hygiene hypothesis contends that a child’s environment can be “too clean,” and the lack of exposure to germs doesn’t give the immune system a chance to develop resistance to certain diseases. This concept was broadened later beyond allergies and asthma to include other conditions involving the immune system, particularly inflammatory diseases, including type 1 diabetes and multiple sclerosis, and to depression and some types of cancer.


Enter: the “Human Microbiome.” In the early 21st century, interest in the hygiene hypothesis was subsumed by emergence of a new field of research on the human microbiome. Simply put, our microbiome refers to the germs that share our body surfaces. And there are a lot of them. For example, you have an estimated 40 trillion bacteria and about ten times as many viruses in your gastrointestinal tract alone.


Research in the field of the human microbiome was fueled by development in the late 20th century by the science called metagenomics pioneered by the microbiologist Carl Woese. Metagenomics has allowed investigators to probe every conceivable environment for the genetic material of microbes, more than 99% of which cannot be cultured in a laboratory. Funded initially by the National Institutes of Health’s “Human Microbiome Project” (HMP), researchers characterized the microbes (bacteria, archaeans, viruses, fungi, and parasites) that inhabit five human ecosystems (the gut, skin, vagina, mouth, and lungs).


The goal of the HMP is to discover germs that are associated both with health as well as with diseases. The HMP and related research projects have shown that the profile of microbial species found in the gut of people with allergies and asthma differs from healthy controls. This is also the case when the microbial species found in the gut of healthy people are compared to those with autoimmune diseases, such as multiple sclerosis and systemic lupus erythematosus, as well as with obesity, type 2 diabetes, inflammatory bowel diseases (Crohn’s disease and ulcerative colitis), irritable bowel syndrome, cardiovascular disease, and colon cancer.


It is important to remember, however, that correlation does not mean causation. It has not as yet been established that the composition of the microbial species found in the gut of young children plays a role in protecting against diseases that captured the early attention of researchers of the hygiene hypothesis such as asthma and allergies. But we do know that gut bacteria play an important role in training cells of the immune system early in life and that a lower diversity of bacteria in the gut microbiome seems to foster immune system reactivity to harmless compounds in the environment that can trigger allergies and asthma. Recent studies also suggest that indications of susceptibility to these immune-mediated diseases can be found in the very first stool sample (meconium) produced by newborns. As reported by journalist Clare Wilson, “A baby’s first poo reveals if they are at risk of allergies and asthma.”

Appreciate your microbial friends. In a recent National Geographic article entitled “Are we raising little germaphobes?” journalist Connie Chang suggests that our responses to the COVID-19 pandemic, like rigorous handwashing, wearing a mask, and social distancing to avoid SARS-CoV-2, the cause of the pandemic, may lead to an unhealthy fear of germs. Hence, it’s important to remember that a vast majority of the germs we harbor in our body’s ecosystems are either harmless or perform vital functions. In the case of the gut, germs help digest food and train the immune system.


In addition, more and more evidence is emerging to suggest that bacteria in the gut can signal the brain thereby influencing how we feel, think, and act. (Discussed in these previous Germ Gems posts “Does Your Microbiome Shape Happiness?” [January 1, 2021] and “What’s your Pyschobiome Telling You?” [January 13, 2021].) Indeed, the more that we learn about the gut microbiome the more astonished we are. Recent studies from Finland suggest that the types of microbes in your gut can even predict whether you’re more likely to die in the next 15 years.


Our gut microbiome should not be disturbed unnecessarily. One way to profoundly disrupt the gut microbiome is by administering an antibiotic. And the use of antibiotics in the first year of life has been linked to asthma and other allergic diseases. Also by depleting the gut of beneficial bacteria, antibiotics can create a niche for Clostridioides immitis, a bacterial pathogen that causes a severe form of colitis that kills an estimated 15,000 Americans per year. (Discussed previously in Germ Gem post, “Why C.diff Should be a Household Name” [September 22, 2019]).


Preserving the ecosystem of the skin microbiome by bathing less is also garnering some attention. James Hamblin, physician and author, has championed this notion thereby challenging some cultural norms about hygiene. Hamblin, who reportedly hasn’t showered for the past five years, is trying to make people think about germs in a new light. He was recently featured in an NPR article by Emily Vaughn titled “In the Era of Hygiene, ‘Clean’ Author Makes the Case for Showering Less.” It is too early to tell if Hamblin’s method for preserving the ecology of the skin’s microbiome works or even if it does, if he will be successful in changing any of society’s norms regarding bathing. But, if nothing else, he’s saved a lot water as, according to the Water Research Foundation, the average shower uses roughly 17 gallons of water.


During the course of COVID-19 pandemic, we were warned of the possible danger posed by surfaces contaminated with the SARS-CoV-2 virus. We responded by rigorously wiping down and sanitizing surfaces in an attempt to control the virus’s spread. Recently, the New York Times published an article, “Has the Era of Overzealous Cleaning Finally Come to an End?” In it, science journalist Emily Anthes reported on the recent change in the recommendations of the Center for Disease Control and Prevention (CDC) regarding the obsessive cleaning of surfaces. While earlier studies showed that SARS-CoV-2 could survive for several days on plastic and stainless steel, it has not been shown that anyone has acquired the virus from a surface. Moreover, the CDC noted the risk of contracting the virus from touching a contaminated surface is less than 1 in 10,000. It thus appears that the era of “hygiene theater,” the obsessive and excessive cleaning of surfaces, has indeed come to an end.

Germs are here to stay. Modern evolutionary biology suggests that bacteria emerged as the first life form on our planet some 3.8 billion years ago. They and other microscopic life forms can be found in every nook and cranny on Earth. And they are here to stay. The British-American author Bill Bryson put our relationship with germs in proper perspective when he wrote: “Because we humans are big and clever enough to produce and utilize antibiotics and disinfectants, it is easy to convince ourselves that we have banished bacteria to the fringes of existence. Don’t you believe it. Bacteria may not build cities or have interesting social lives, but they will be here when the Sun explodes. This is their planet, and we are on it only because they allow us to be.”

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Main Page images courtesy of Shuxian Hu, MD. Dr. Hu is a scientist in the Neuroimmunology Research Laboratory at the University of Minnesota.

 

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