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This two-part series highlights the emerging relevance of our microbiome in human health. In this first part, we discuss the establishment of the microbiome from infancy to adulthood and the highly responsive nature of these trillions of cells to the environment. The geographical diversity of microbiomes in various cultures, and the mandatory nature of the acquired living situations of immigrants are of immediate relevance. The second part, which will be published in June, will focus on the effects of the microbiome on human health and disease.
Traditionally, our interaction with microbes has been focused on how deleterious they can be. Viruses, bacteria, and other microorganisms have been responsible for several contagious diseases, including typhoid, cholera, mumps, smallpox, polio, malaria to name a few, some of which we take credit for controlling with medicines and vaccines.
However, emerging knowledge of the human microbiome is informing us that an entire army of microbes including bacteria, fungi, viruses, and other fauna coexist in symbiotic (meaning mutually beneficial) and commensal (meaning neutral coexistence) relationships with our bodies, and are beneficial in that they play an important role in maintaining homeostasis and optimal functioning of the body. Comprising of at least an equal number of cells as the human body itself, the microbiome constitutes what is termed a newly discovered organ in our bodies – one that is dynamic, diffuse, and very different from what we conventionally think of as a discrete human organ (liver, heart, brain or lung).
Microbiota colonize the entire surface of our bodies and the specific niches within. Several studies have tried to describe the species that are found in various locations, and these descriptive studies paved the way to deeper ones aimed at understanding how they are established, maintained, and function. We are learning that the establishment of these populations occurs from the earliest days of one’s existence, and their effective maintenance throughout one’s life is as important as having a healthy heart.
Infants are exposed to the maternal microbiome during the process of birth, and differences in the complexity of their microbiomes are seen in normal birthing situations versus cesarean sections. During their early days out of the womb, colonization of microbes within their bodies and also on their skin is associated with good overall health, in the immediate and long term. Taken a step further, this intergenerational transmission can be extended to envision a co-evolution of humans with their microbiomes as a discrete ecological unit. Members of a family, or people who occupy the same household, are known to share similarities in their associated microbiomes. A linear study of 6 large Indian joint families also indicated changes in microbiome with age.
Further afield, one can well imagine that geographical and cultural differences will lead to variations in associated microbiomes. Most of us are aware that microbes exist in the intestines and mouth, and there is an immediate and important influence of diet on these populations. While different species of bacteria have been found to be the dominant population in different geographies, one study reported that the African diet was associated with the most gut microbial diversity. In general, a lack of microbial diversity is associated with urbanization and developed countries, and some factors thought to be related to this outcome are increased consumption of processed foods, increased use of antibiotics, reduced sleep, and loss of natural habitat. This lack of microbiome diversity is directly related to human disease. We are learning that diet can lead to changes in the microbiome in a matter of hours, not days, and can regulate the secretions of the bacteria in addition to the populations of the bacteria themselves.
Even within the same geography, different lifestyles and ethnic groups showed differing gut microbiomes. In a cross-sectional study in Malaysia, the gut microbiome of Malaysians of Malay, Chinese, Indian and Jakan descent varied principally by ethnicity of the subjects, who were of the same socioeconomic status and geographic location. Perhaps more relevant to this discussion, studies conducted to compare gut microbiomes in rural and urban groups in Thailand and India suggested that urban populations have reduced numbers of bacteria that can produce beneficial anti-inflammatory molecules. In most cases, researchers associate these reductions and changes in gut microbial diversity with dietary preferences of urban and rural settings- urbanites tending to include less natural foods, vegetables, and whole-grain in their diets.
Pioneering studies in immigrant communities were conducted in Amsterdam and Minnesota. The Amsterdam study analyzed six immigrant ethnic groups including Moroccan, Turkish, and South Asian Surinamese, and as with the Malaysian (non-immigrant) study the gut microbiome varied principally by ethnicity. Southeast Asian immigrants from Laos, Vietnam, Myanmar, and Thailand were sampled in Minnesota, and it appeared that their gut microbiome assimilated to their new homeland. It took about a decade for this group to substantially transition to the US-associated gut bacterial profile. Although diet would play a big role in this, it is thought that other lifestyle changes, medicines including antibiotics, and other as yet unexplored variables may play a role in this remarkable change. An unfortunate correlation among these immigrant populations is the onset of obesity and diabetes.
Our understanding of this complex and dynamic organ continues to develop, in tandem with our understanding of its involvement in human health. These aspects will be discussed further in the second installment of this article.
L. Iyengar has lived and worked in India and the USA. A scientist by training, she enjoys experiencing diverse cultures and ideas and writing. Her short story will be included in an anthology showcasing a group of international women writers, to be published in 2021 by The Nasiona. She can be found on Twitter at @l_iyengar. www.liyengar.com
Featured image license here.