There are trillions of microbes in your gut, including household names like bacteria, fungi, and viruses. One constituent of your gut microbiome that receives less attention is archaea.
While scientists know that they’re there, compared with gut bacteria, they know much less about their role in health and disease.
In this article, we explain what archaea are and explore the latest research into their activities in your gut.
What are archaea?
To explain what archaea are, we’ll need to briefly cover the domains of life. Experts split all life on Earth into three domains:
-
archaea
-
bacteria
-
eukarya
The first two on that list will now be familiar, and the third, eukarya, is a huge and varied group that includes fungi, plants, animals, and more.
To the untrained eye, archaea look very similar to bacteria. So, for a long time, they were lumped in with bacteria. However, in the 1970s, as technology improved, scientists showed that archaea and bacteria were very different.
The current thinking is that the three domains of life branched off from each other early in the history of life — billions of years ago. Surprisingly, archaea seem more closely related to eukarya (like you) than bacteria.
The first archaea that scientists discovered lived in extreme environments, like scorching hot thermal vents in the deep ocean and toxic oil wells.
But by the time the 20th century drew to a close, it was clear that archaea were everywhere, including soil, oceans, and the human gut, lung, skin, and nasal microbiomes.
Unlike bacteria, fungi, and other microbes, scientists haven’t identified any species of archaea that cause disease.
Archaea in your gut
Because archaea are significantly different from bacteria, some genetic techniques used to identify bacteria don’t work for them.
Below, we outline what we know about the archaea in your gut so far.
Methane producers
Many species of archaea in your gut produce the gas methane as they feed — they’re the only known microorganism that pumps out this gas.
Archaea are also responsible for the methane that cows emit, making them a big player in the ongoing climate crisis.
Studies show that some people breathe out much more methane than others. These individuals have around 1,000 times more methane-producing archaea called Methanobrevibacter smithii than those who breathe out very small amounts of methane.
The percentage of methane breathers varies greatly by region. For instance, around 20% of healthy Western people breathe out high levels, compared with 15% in Japan and around 60% in Southern Africa.
We don’t know what this means for health, but studies show that those who emit more methane have more gut bacteria specialized for breaking down plant fiber. The reason for this will become clear in the next section.
Archaea rely on gut bacteria
Rather than archaea competing directly with bacteria, the relationship between the two is often more friendly — archaea rely on compounds produced by bacteria to survive, and archaea make the environment easier for bacteria to thrive in.
As bacteria feed, they produce metabolites, including hydrogen and carbon dioxide. Archaea then convert these into methane, molecules of which consist of one carbon atom and four hydrogen atoms.
Experts believe that by taking up hydrogen in this way, archaea might prevent the gut from being too acidic — the more hydrogen ions are around, the more acidic it is.
If the gut becomes too acidic, gut bacteria can’t ferment as efficiently.
So, the relationship between archaea and bacteria is, at least some of the time, mutually beneficial.
Archaea and health conditions
As we mentioned above, scientists haven’t found any archaea that can make you sick directly. But studies have found that certain archaea are associated with health conditions.
That doesn’t mean they cause disease, but it may mean they are influencing health in some way — for better or worse.
Archaea and gut conditions
Archaea might be involved in the constipation experienced by some people with irritable bowel syndrome (IBS). Research suggests that methane produced by archaea could make it worse by slowing food’s passage through the gut.
The authors of a review on archaea concluded that constipation caused by slow-moving food might encourage the growth of archaea. This overgrowth then further worsens constipation.
There’s also evidence that a methane-producing archaea called Methanosphaera stadtmanae is more common in people with inflammatory bowel disease (IBD) than those without IBD.
Archaea and infections
Archaea often seem to be associated with “bad,” infection-causing bacteria. This might be because disease-causing bacteria are more likely to produce hydrogen as they feed, which archaea can then use to make energy.
Because of this, most of the research to date has focused on infections. Here are some examples:
Urinary tract infections (UTIs): The authors of one study tested the urine of 383 people; of these, 34 had a bacterial infection. Nearly every sample from someone with a UTI contained the archeae M. Smithii, but none of the uninfected urine samples did.
The study’s authors wonder whether M. Smithii might encourage the growth of pathogens. They suggest that M. Smithii hang around with infectious bacteria to feed on the hydrogen they produce.
As the archaea take in hydrogen from the infection site, it becomes less acidic, which helps the infectious bacteria survive and thrive, thereby encouraging the infection.
Join our mailing list
Opt in to receive ongoing science and nutrition emails, news and offers from ZOE. You can unsubscribe at any time.
Gum disease: One study showed that methane-producing archaea were more abundant in parts of the mouth with bacterial infections. In agreement with the authors of the UTI study, these scientists suggest the archaea may be there to take advantage of the metabolites produced by “bad” bacteria.
Researchers have also connected archaea to long-term sinus inflammation and lung and brain infections.
Archaea and heart health
According to some experts, certain species of archaea might influence heart health. To understand the link, we need to meet trimethylamine (TMA).
As some species of gut bacteria break down certain foods in your gut, they produce TMA, which is absorbed into your blood and travels to your liver.
Your liver then converts TMA into trimethylamine-N-oxide (TMAO). Studies suggest that high levels of TMAO are linked to an increased risk of cardiovascular disease.
So, how do archaea fit in? Scientists have shown that some Methanomassiliicoccales species — a group of archaea — have enzymes that allow them to break down TMA to use as fuel.
In theory, this would mean that people who have more Methanomassiliicoccales would have lower levels of TMAO and, therefore, reduced cardiovascular risk.
A study conducted by ZOE’s Scientific Co-Founder, Prof. Tim Spector, and colleagues adds another line of evidence. Their paper explains how people with higher levels of TMA-producing bacteria have a larger population of Methanomassiliicoccales.
Researchers still need to carry out further research to demonstrate whether greater populations of Methanomassiliicoccales really do reduce cardiovascular risk over a lifespan.
But Tim and his colleagues conclude that one day, these archaea “may form the basis of novel therapies for cardiovascular disease.”
Archaea and colon cancer
There’s some evidence that colon cancer is linked to changes in populations of archaea.
For instance, studies suggest that compared with people without colon cancer, those with the disease have reduced numbers of methane-producing archaea but higher levels of halophiles — species that prefer salty environments.
Research on this topic is ongoing, and scientists don’t know whether changes in archaea populations cause cancer to develop or whether the presence of cancer produces shifts in archaea populations.
Either way, experts hope that this phenomenon might one day help clinicians improve the diagnosis and/or treatment of colon cancer.
What does it all mean?
How archaea in your gut affect health is still to be determined.
Although we’ve seen links between methane-producing archaea and poor health, that certainly doesn’t mean they are all “bad” — there are many species of archaea that live in us and on us that we haven’t even named yet, let alone figured out what they’re up to.
It’s likely that, as with gut bacteria, some species benefit our health and some play a negative role. Some might indirectly benefit “good” or “bad” bacteria, and some may make little difference to health at all.
The complexity grows even deeper when you consider that archaea probably interact with each other, fungi, and the other microbes in your microbiome.
They’re also likely to interact with your cells, tissues, hormones, and immune system — it’s a tangled web.
The path to understanding the benefits and harms of particular species of archaea is long. We’re only just starting on this journey, but it’s sure to be fascinating.