The Layout Of The Human Microbiota


The human body is inhabited by a plethora of microorganisms like bacteria, archaea, single-celled eukaryotes, and viruses which together constitute the human microbiota. So much so that, every part of the human body which is exposed to the external environment is colonized by the microbiota. The microbial communities that occupy different regions of the human gut have an influence on varied aspects of human health.

In a healthy system, the human microbiota contributes nutrients and energy by fermenting nondigestible dietary components in the large intestine. And a balance is achieved between the microbiota and its host’s (us) metabolism and immune system. And when this balance is disrupted, human microbiota protests by acting as agents of infection and inflammation, leading to gastrointestinal disorders and might have a role to play in diabetes mellitus and obesity.

70% of the microbes observed in the human body are found in the gastrointestinal tract (digestive system). The majority of microbial community found there are Firmicutes, Bacteroidetes, and Proteobacteria while Actinobacteria contribute less to the total bacterial population and mainly are Bifidobacterium. Bacteroidetes include the genus Prevotella and Bacteroides. Firmicutes include members of Eubacterium, Faecalibacterium, Roseburia, and Ruminococcus and clusters of Clostridium.

This article makes an attempt to understand the various microbial populations present in the various parts of the gastrointestinal tract and hence get to know our microbial guests on a first name basis. It’s about time you said hello to them, don’t you think? But, do keep in mind, the names of these microorganisms and their families can be a little tricky but names are usually hard to remember right?

The layout of the human gut microbiota

Before we hail this microscopic taxi to the different locations of our microbial guests, let us make an attempt to understand the nature of the microorganisms present inside our body. So, most of the human microbial community consists of anaerobes (bacteria which cannot utilize oxygen, where the presence of oxygen can be harmful to them) outnumbering the aerobes (bacteria which survive on oxygen) and the facultative anaerobes (bacteria which can utilize oxygen in its presence but can survive without it as well).

More than 50 bacterial phyla (a group of closely related bacteria) have been identified as occupants of the human gut. Out of which, Bacteroidetes and Firmicutes are the most dominant microbial groups followed by Actinobacteria, Fusobacteria, Proteobacteria, Verrucomicrobia, and Cyanobacteria which is found in lesser proportions. Quantitative and qualitative variations of the above mentioned microbial groups were observed based on the respective regions of the gastrointestinal tract studied.

The microbial communities present in our throat and the distal oesophagus are mostly similar. They comprise of  Streptococcus, Prevotella, Actinomyces, Gemella, Rothia, Granulicatella, Haemophilus, and Veillonella as the predominant microbial communities. In both these locations, Streptococcus is the dominant genus (a subgroup of phyla) followed by Prevotella. The remaining microbial diversity consists of Veillonella.

The microbial diversity of the stomach has been found to be dependent on the presence or absence of a microorganism called Helicobacter pylori. Stomach lacking Helicobacter pylori  (H. pylori) contains Streptococcus, Actinomyces, Gemella, and Prevotella. These bacterial groups are mostly found in the throat and assumed to have been temporary residents swallowed from the throat. In stomach where H. pylori are present, they predominate over the members of other bacterial phyla.

Most of the microbes present in the jejunum and ileum (refer to figure) are aerobes and facultative anaerobes. Enterococci, E.coli, Klebsiella, Staphylococci, Streptococci, and Lactobacilli comprise the majority of microbes present in jejunum and ileum.

Small intestine contains aerobic Enterococcus group, Streptococci, Lactobacilli, and Gammaproteobacteria while anaerobes constitute the majority of microbial populations residing in the large intestine. Species belonging to Gammaproteobacteria has been found to be present in the jejunum, ileum, and caecum.

The ceacal (in ceacum) microbial community is mainly comprised of Lactobacillus, Enterococcus, and E.coli along with small populations of fecal microbes such as C.leptum, Bacteroides, and C.coccoides. And the predominant populations of microbial communities in our digestive system (GI Tract) is seen to vary from an individual to another and hence the variation in the gut microbial diversity at an individual level can essentially provide us with a snapshot of the gut health and provides an opportunity to restore balance in our gut ecosystem whenever there is a deviation from a healthy condition.

Given that there are a lot of scientific names which has been presented in this article in an all you can eat platter, we do not have to strain ourselves to remember each of them but this is an attempt to understand our microbial guests better and get familiar with their places of residence.

Before we wrap this up, let us quickly look at some of the key findings in the sphere of human microbial communities so as to understand the relevance of our microbial guests :

■ Molecular surveys have discovered remarkable diversity within the human gut microbiota, but some of the dominant microbial species are reported in faecal samples from most healthy adults.

■ The food we consume has the ability to alter the species composition of our gut microbiota both in the short term and in the long term.

■ The gut microbiota has the potential to influence the host’s (humans)  energy balance through multiple mechanisms, including supplying energy from nondigestible dietary components and also by influencing gut transit, energy intake, and energy expenditure.

Disclaimer: The above article is sponsored by Thyrve, the world’s first Gut Health Program that incorporates microbiome testing and personalized probiotics to ensure a healthier gut, happier life, and a brighter future.