23 November 2020; updated 23 October 2023
H.G. Wells opened his 1898 science fiction classic The War of the Worlds with these immortal lines:
“No one would have believed in the last years of the nineteenth century that this world was being watched keenly and closely by intelligences greater than man’s and yet as mortal as his own; that as men busied themselves about their various concerns they were scrutinised and studied, perhaps almost as narrowly as a man with a microscope might scrutinise the transient creatures that swarm and multiply in a drop of water. With infinite complacency men went to and fro over this globe about their little affairs, serene in their assurance of their empire over matter. It is possible that the infusoria under the microscope do the same.”
H.G. Wells, The War of the Worlds
Wells made bacteria – first observed by the Dutch self-taught scientist Antonie van Leeuwenhoek under his home-made microscope in 1683 – the unlikely heroes of his futuristic novel.
From the moment the vastly technologically superior Martians landed on Earth to commence their invasion, they were colonised by bacteria to which humans, through “natural selection of our kind… [had] developed resisting power.”
The Martians, having eradicated bacteria from their own planet in the distant past, lacked this resistance, and hence “our microscopic allies began to work their overthrow”.
Where human technology proved impotent, “the transient creatures that swarm and multiply in a drop of water” succeeded in bringing about the destruction of the invading Martians in a matter of weeks.
Little did Wells know just how prescient his description of bacteria as “our microscopic allies” would turn out to be.
For a century, researchers slowly but steadily gathered knowledge of the bacteria that inhabit the human gut, their progress hindered by having to rely on culturing bacteria recovered from stool in a petri dish.
Gut microbiota research as we now know it only really got underway in the late 1990s, with the development of technology that allowed scientists to sequence the DNA of our gut bacteria.
At this point it was discovered that the vast majority of the teeming hordes of tiny critters that inhabit our insides can’t be cultured at all, as they die when exposed to air.
And no one – not even Wells, had he lived long enough – would have believed in the last years of the twentieth century, just how great an influence these microscopic life forms would be discovered to exert on every aspect of human health and well-being.
The field of gut microbiota research has mushroomed so dramatically, that a scientific paper published in 2018 calculated that over four-fifths of the total number of scientific publications focusing on the gut microbiota over the previous 40 years were published in just four years – 2013-2017.
And now in 2023, so many scientific articles on the topic are published every day that it’s impossible to keep up with them all.
In just a few decades, researchers have come to understand that the communities of bacteria, archaea, protists, fungi and viruses that live inside our gastrointestinal tract (our gut microbiota), and their collective genetic material (our gut microbiome) are so vital to healthy function that they constitute a distinct organ of the human body.
Here are just some of the roles played by the 100 trillion microorganisms that populate our gut:
- Immune functions: Formation of the gut-associated lymphoid tissue, or GALT (a key component of the immune system in the gut) and ‘training’ of our immune cells to distinguish self from non-self, and friend from foe.
- Gut functions: Maintaining the intestinal barrier (i.e. preventing and repairing leaky gut); digesting complex carbohydrates found in human breast milk and in plants; producing short chain fatty acids which feed the cells that line our colon; keeping disease-causing bacteria, yeasts and fungi at bay; regulating muscle movement in the intestinal tract (motility); and protecting against colon cancer.
- Metabolic functions: Regulating serum cholesterol, blood glucose levels and appetite.
- Vitamin production: Producing vitamins B1, B2, B12 and K, along with biotin, folate and alpha-lipoic acid.
- Central nervous system functions: Stimulating development of parts of the brain, especially the hippocampus (which plays key roles in motivation, emotion, learning, and memory); and producing chemicals that affect areas of the brain involved in appetite control and food cravings.
- Enteric nervous system (‘gut brain’) functions: Producing neurotransmitters – chemicals that nerve cells use to talk to each other, and to muscles and glands – including GABA, serotonin and dopamine, and influencing the neuroendocrine cells in the gut that also release these neurotransmitters.
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