Cleanliness is harmful to health

Is it possible to eat from the floor?

While working on his new book, scientific writer and biologist from North Carolina State University Rob Dunn changed its working title "Clean Living is Bad for You" to a much more complex version of "The inhabitants of our body: predators, parasites and partners that shape the appearance of modern people" (The Wild Life of Our Bodies, predators, parasites, and partners that shape who we are today). The reason for this was the creeping into the fidelity of the hypothesis initially unconditionally accepted by him, according to which life in excessively clean conditions of modern cities greatly undermines human health.

The prerequisite for this doubt was the blatant lack of reliable information revealed during the work on the book about the complex interactions between the human body and bacteria, a huge number and variety of which inhabit the world around us, as well as the consequences caused by the violation of these interactions.

Bacteria are an integral part of the human body and the complete destruction of microorganisms inhabiting the skin and mucous membranes of the digestive tract will undoubtedly lead to death. So, roughly speaking, living in dirt is much better for health than living in cleanliness. However, there is a somewhat more complex view of this pattern, known as the "theory of (excessive) hygiene" and described roughly as follows. The lifestyle of most people has changed from rural, conducted primarily in the open air, to urban, which is characterized by spending time in closed rooms with central ventilation, sealed windows and surfaces that are disinfected at every opportunity. This transition has significantly reduced the length of time we spend "outside", and, accordingly, our chances of "getting dirty" with real life. This combination caused a violation of the normal development of the human immune system. As a result, the immune system of many citizens sooner or later begins to behave inappropriately and rebels against the body of its host in the form of asthma, allergies, Crohn's disease, inflamed bowel syndrome, as well as, according to some experts, multiple sclerosis and autism. It turns out that living in purity in one form or another can be the cause of most of the most common chronic diseases of our time.

This hypothesis is simultaneously elegant, destructive, important, vague and poorly tested. We know very little about how changes in the quality and composition of the bacterial population surrounding a person can have a negative impact on the immune system (however, the situation is changing rapidly, as more and more researchers are studying this problem). Even less is known about the variations of the bacterial environment associated with different human lifestyles. In the absence of information, many assumptions can look plausible, and incorrect conclusions are very easily accepted as the truth.

However, some things have already been proven. Researchers have demonstrated that the absence of helminths in the digestive tract can affect the functioning of the immune system. The destruction of helminths in the intestine can increase a person's predisposition to autoimmune diseases. Conversely, their return can significantly reduce this risk. How helminths have this effect is still unclear, but the fact that getting rid of them had negative consequences, at least for some people, has already become obvious. However, the water supply and sewerage that deprived us of worms helped to rid humanity of other, much more dangerous pathogens that are also transmitted by fecal-oral route, including the causative agent of cholera. At the same time, helminths are only one part of the story.

If the hygiene hypothesis is correct, it can be assumed that the composition of populations of bacteria and other microscopic creatures inhabiting our organisms and homes varies depending on lifestyle. Accordingly, the state of human health, in turn, should vary depending on the characteristics of the vital activity of these microorganisms. Fortunately, this assumption is easy enough to verify.

One of the possible approaches to such testing is to collect samples of microflora of rural and urban houses and then interview the inhabitants of these houses about their well-being and health status, especially about diseases of the immune system. The hygiene hypothesis does not specify which of the parameters describing microscopic life – diversity (number of species), composition (which species) or abundance (total number) – is the most significant. However, all three parameters are quite simple (although not cheap) to evaluate.

With a high degree of probability, regardless of the results obtained, the significance of these parameters depends on other factors. It is unlikely that the urban life of Rio de Janeiro is similar to the urban life of, say, New York in terms of the impact of the bacterial environment, the composition of which differs both qualitatively and quantitatively. Different regions are also characterized by the presence of different species of birds, rodents, pets and insects. Buildings of the same city may also differ functionally from each other due to the peculiarities of architecture, design and building materials. For example, pigeons prefer to nest on vertical structures. Houses with attics attract bats. However, everything we know in this regard concerns animals, and mainly representatives of vertebrates. As for microbes, it is necessary to find out how the parameters of their populations vary depending on the place and lifestyle of a person. The first steps in this direction have already been taken.

In December 2011, researchers from the University of Oregon, working under the leadership of Steven Kembel, published the results of a study in which they compared variations in the composition of microbial populations inhabiting hospital wards, depending on the ventilation method used in them (standard ventilation and heating system or open window).

If the hygiene hypothesis is at least partially correct, then it can be assumed that a ventilation system that at least slightly brings a hospital ward closer to a rural house, that is, an open window, should contribute to the appearance of a variety of so-called "good" microbes. Conversely, the "sterility" of modern hospital wards should contribute to the proliferation of pathogens.

For any microbiologist, unlike marketing specialists of advertising companies, it is obvious that it is impossible to destroy bacteria. Every centimeter of the surface of the objects surrounding a person is covered with living cells, which are contained in the air around us. It is unrealistic to completely get rid of them, since there are even microorganisms that can feed on triclosan, the active ingredient of antibacterial soap. Therefore, the task of Kembel and his colleagues was not to get an answer to the question "Is there life in the sterility of hospital wards?", but to find out the features of this life.

As expected, the researchers found that both the diversity and the abundance of bacteria varied depending on the ventilation system of the wards. However, this is not all.

The results of the analysis of the collected data showed that the composition of bacterial communities inhabiting wards ventilated with open windows is intermediate between the compositions of communities characteristic of outdoor air and mechanically ventilated wards. Moreover, at a higher temperature and lower humidity, the composition of bacterial populations of wards ventilated with windows quantitatively and qualitatively approached the composition of bacterial populations of mechanically ventilated rooms. This indicates that the reasons for the difference in the bacterial population of automatically ventilated rooms include the temperature and humidity levels maintained in them.

The data obtained confirm the assumption that the drier, warmer and more airtight a room is, the less diverse its bacterial inhabitants are. According to this hypothesis, the "clean" life we have chosen behind closed windows and with central ventilation turned on demonstrates not the victory of hygiene, but its complete failure. A small variety of bacteria does not allow the developing immune system to fully experience the world in which it will have to exist. Undoubtedly, much remains to be verified, but the results suggest serious thoughts.

While conducting this study, another interesting pattern surfaced. It turned out that the most significant parameter reflecting the number of potentially dangerous bacteria for humans is not the type of ventilation of the room, but the heterogeneity of the population of microorganisms inhabiting it. In other words, the higher the diversity of the bacterial population in a room, the fewer representatives of potentially dangerous species in it.

Can the diversity of neutral and "good" bacteria be the reason for the decrease in the concentration of "bad" microorganisms? A similar pattern is observed in wildlife. A huge amount of scientific data indicates that the more diverse the flora of grasslands, the less susceptible they are to the introduction of alien species. In such ecosystems, various representatives of the plant world effectively use the resources necessary for the life of the invaders. Based on all this, it is obvious that the diversity of bacteria in the habitat not only contributes to the normal development of the immune system, but also gives it the ability to fight enemies. However, this can be said for sure only if the hygiene hypothesis is unconditionally confirmed.

However, as with everything, it is important to remember the rule of the "golden mean". Scientists can prove that living in cleanliness is harmful, but living in dirt in the truest sense of the word can lead to outbreaks of cholera and other dangerous diseases. Scientists cannot disagree with this.

In more detail, the evolutionary processes that resulted in our dependence on some and ignoring other species, including cows, chickens, hamsters, various bacteria, pigeons, swallows, predators, pathogens, bed mites, basement dwellers, fleas, field mice, viruses, yeast, penicillin-producing mold, bed bugs, flies and many others are described in the book "The wild life of the human body, predators, parasites and partners that shape the appearance of modern people."

Based on the article Rob Dunn Eating off the Floor: How Clean Living Is Bad for You, published in the Scientific American Blogging Network.

Evgeniya Ryabtseva
Portal "Eternal youth" http://vechnayamolodost.ru11.03.2012