The Circle of Life: Part Human, Part Microbe

The human-microbiome balance
The ‘Circle of Life’ is a concept that all life exists in a delicate balance. It is the idea that life is essentially an ecosystem in which one factor benefits from and provides for another. These essential relationships can have varying time periods, but most have existed for vast lengths of time. An example of this is our very personal and long-standing relationship with our microbiome. 

The microbiome and you
The microbiome encompasses the numerous microorganisms (including bacteria, viruses, fungi and archaea), that live in and on our bodies. These microorganisms have co-evolved with humans over millions of years from one generation to the next, and unsurprisingly have largely been associated with human health. A ‘healthy’ microbiome can be associated with positive health outcomes, whereas a microbiome in dysbiosis is often associated with disease. Several studies have demonstrated, for example, that the composition of our microbiome can be associated with allergies, chronic metabolic diseases (e.g. diabetes and obesity) inflammatory diseases (i.e. irritable bowel diseases, including Chron’s disease and Ulcerative Colitis), and neurodevelopmental diseases. Though these associations are well-described, a logical question that arises is “What are the factors that can affect this sacred human-microbiome relationship, and consequently cause disease?”

DID YOU KNOW?
For every 1 human cell, there are 1.3 bacterial cells living on or in our body.
Side thought: Could this mean we are biologically more microbe than human?

Urbanized and ‘Traditional’ living
While there are numerous factors that can play a role in shaping the human-microbiome relationship and the health outcomes thereof, one factor that is gaining particular interest in the field is the effect of urbanization.

Generally, urbanization refers to “the population shift from rural areas” (in which humans are more often exposed to the natural environment) “to urban areas” (in which humans are less exposed to the natural environment due to improved infrastructure and modern hygiene (see ‘hygiene hypothesis’)). The main effects of urbanization can be observed by differences in diet (i.e. an increase in processed food intake, and high sugar, high fat diets), behaviour and sociality, and disease outcome (communicable versus non-communicable diseases), and an increase in medication use (particularly antibiotics). Ultimately, the underlying root of these factors is the differences in environment and lifestyle.

Indeed, an article published last week by McCall and colleagues (https://doi.org/10.1038/s41564-019-0593-4) noted observable differences in the chemical and microbial characteristics of homes in urbanized communities in comparison to more rural communities from the same country (Brazil). The authors further noted that these differences were also visible when comparing the skin fungal diversity of the house-dwellers. The authors reasoned that these differences were associated with the increased use of cleaning agents and medication use in urban settings. More generally, it has also been demonstrated that people from urban communities tend to have lower microbial diversity, composition and functions.

INTERESTING FACT
In the microbiome field, lower microbial diversity is generally considered ‘less healthy’, except for the vaginal microbiome, where lower diversity is considered ‘healthier’.

Interestingly, these observations are vastly different to populations still living what is considered a traditional ‘hunter-gather’ lifestyle, such as the Hadza tribe in Tanzania. It has been noted that members of the Hadza tribe have higher microbial diversity in comparison to people living more urban lifestyles. These differences have been associated with the Hadza lifestyle and the natural environment that they are in direct contact with. Some contributing factors include exposure to seasonal changes (which in turn affects dietary resources/availability) foraging/hunting behaviour, ‘family structure’ (eg. Hazda babies can be breast fed by several women in the tribe) and sexual division of labour (https://doi.org/10.1038/ncomms4654; https://doi.org/10.1126/science.aan
4834).

So what is the point?
Scientist are now intrigued by and are trying to unravel the effects of urbanization on the microbiome and health outcomes. It is no secret that the prevalence of non-communicable diseases are higher in urbanized settings in comparison to rural environments. Could it be that the burden of non-communicable diseases in rural settings are masked by the enduring burden of communicable diseases (which are associated with environmental and lifestyle differences)? Has the microbiome of urban dwellers co-evolved with their human host in accordance with the urban environment and lifestyle? How beneficial or detrimental are these changes to human health? Can negative health outcomes be reversed with improved knowledge on how urbanization, and our environment and lifestyle at large shapes our microbiome?

A very recent and interesting article by the Sonnenburgs (https://doi.org/10.1126/science.
aaw9255) concluded with some fascinating suggestions for possible future ‘microbiome restoration’ interventions, which “…could be anything from knowing how many minutes a week should be spent outdoors in natural environments to air fresheners that are good for the microbiome.”

Find out more:

– an article by Skelly and colleagues tackles the interesting view-point on how colonialism could impact the microbiome and health outcomes: https://doi.org/10.1002/ajpa.23637

– check out some of the microbiome studies and personal stories of anthropologist Jeff Leach, with the Hadza hunter-gather tribe (http://humanfoodproject.com/author/jeff-leach/), including giving himself a faecal transplant originating from a Hadza man (http://humanfoodproject.com/rebecoming-human-happened-day-replaced-99-genes-body-hunter-gatherer/)

– Jeffrey Gordon and his group’s work on the microbiome and malnutrition in Malawian infants, and how they are trying to use this knowledge to improve the health outcomes of malnourished infants

– some more material from the Sonnenburgs, including their book, “Gut Reactions” (ISBN: 9780552171168)

Image sourced from:
https://www.elsevier.com/connect/forget-paint-bacteria-can-bring-art-to-life-literally: “Peta Clancy’s Visible Human Bodies (VHB), 2007. Australian Centre for Photography, Sydney. Duratran, Perspex, fluorescent lights, 80cm diameter (each image). (Photo: Xain Milke).”