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What is the human microbiome?

Learn more about the trillions of microorganisms on your own body

Wherever you go in life, trillions of microorganisms go with you. The human microbiome is a community of microorganisms that can be found all over our body surfaces, especially in the mouth, gut and vagina, as well as the skin and eyes.1

The human microbiome includes fungi, yeasts, Archaea and viruses,2 but is mostly made up of bacteria. An average human harbors 38 trillion bacterial cells,3 and 97% of these bacteria inhabit the large intestine.3

Your human microbiome is as unique as a fingerprint

Different people harbor different collections of microorganisms, almost like a fingerprint.An adult human harbors more bacterial cells than there are human cells. Together, the bacteria you carry around with you contain many more genes than you do.5


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Probiotics and the gut microbiome

In particular, the gut microbiome is very complex, with more than 5000 bacterial species present.6 Probiotics supplement the important bacteria lactobacilli7 and bifidobacteria,8, 9 which are an integral part of the human microbiome and are associated with health.10–13

How the gut microbiome is key to health

The gut microbiome is important for overall health as it supports gut14, 15 and immune health4, 16, metabolism19, 20 and may also influence the gut-brain axis.21-23

The gut microbiome helps you develop and maintain a balanced immune system24, 25 and helps support integrity of the intestinal barrier.15, 26


Boy and man splashing water by a lake

The human microbiome keeps changing

Your microbiome starts establishing at birth,27 and is at first dominated by bifidobacteria.28 When solid food is gradually introduced the microbiome starts developing towards the more adult type. 

The adult microbiome is characterized by a vast diversity of microorganisms,29 but as we age, the diversity decreases and lactobacilli and bifidobacteria become scarce.30, 31

How lifestyle may influence the human microbiome

Throughout life the microbiome is dynamic and influenced by lifestyle factors. In healthy individuals the microbiome is considered balanced and bodily health.2 During challenges the microbiome may become unbalanced and probiotic supplementation may offer support. 

The gut microbiome at different stages of life

Infant gut microbiome
Shaping of our microbiome starts at birth where the newborn inherits bacteria from the mother, particularly during a natural childbirth.27, 32 During the first months of life bifidobacteria tend to dominate the microbiota, but many fluctuations can be seen.28 The early microbiome has important functions for the infant’s immune,33, 34 metabolic35 and neurological36 development, which affects health and well-being of the child, but also has long-term health effects.37

Adult gut microbiome
In adults, the microbiome is fully established and is characterized by a great richness and diversity of the bacteria present.29 Each individual has their own microbiota profile.4 The adult microbiome is important for maintaining health, by supporting immune system2, 16 and helping to maintain a healthy immune system.24, 25 The adult microbiome is relatively stable, but is sensitive to lifestyle factors. 

The gut microbiome during pregnancy
During pregnancy, progesterone levels increase and this hormone reduces gut bacterial richness. It also stimulates bifidobacteria and other certain types of bacteria.38, 39  Close immunological interactions between the mother and her offspring mean these beneficial bacteria influence the immune development of the fetus.40 Bacteria in the mother also help create the infant microbiome during vaginal birth and breastfeeding.32, 41

The gut microbiome in the elderly
The gut microbiota of elderly subjects shows reduced bacterial diversity,42 shifts in the dominant species43 and a decline in beneficial microorganisms such as lactobacilli and bifidobacteria.44 These changes are the result of a gradual process associated with physiological changes in the gut,30 as well as dietary patterns.45 


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