The human body’s largest population of microorganisms resides in the intestine and is collectively called the gut microbiota. Although initially it was thought that there were more microbial than human cells in the body, recent estimates show microbial and human cells are present in comparable numbers.

In an individual weighing 70kg, the human gut microbiota gathers more than 100 trillion microorganisms and weighs about 200g (equivalent to a medium-sized mango). There are 150/200 times more genes in this individual’s microbiota than in all of their cells put together.

The human genome consists of about 23,000 genes, whereas our microbiome encodes over 3 million genes that produce thousands of metabolites. In other words, in terms of genes, humans are more than 99% microbial.

Although there is a ‘core’ consisting of bacterial groups common to all healthy humans (it has been estimated that one third of the gut microbiota is common to most people), gut microbiota composition is mostly unique to each individual and is influenced by:

Factors over which humans can take action:

feeding methods (breast milk, artificial milk and introduction of solid food);

medication (antibiotics, acid suppressants, anti-diabetic drugs…);

dietary habits and the way food is cooked ;

; environment and lifestyle (rural vs. urban locations, exercise); and

weight gain .

Factors over which humans cannot take direct action:

genetics;

the anatomical part of the intestinal tract (e.g., the large intestine has a higher microbial diversity compared with the small intestine);

gestational age (preterm birth vs. full-term birth);

delivery mode (vaginal delivery vs. C-section); and

aging.

Interestingly, even though each individual has a specific microbiota that acts like an individual identity card, the microbiota in general also brings unique functions to humans (e.g., digesting nutrients that humans cannot digest on their own and producing vitamins, among others).