There's an entire ecosystem living inside your digestive tract.
Your gut is home to trillions of microorganisms, including bacteria, fungi, viruses and other microbes. Together, they form a complex community known as the gut microbiome.
Scientists are still discovering just how complex this microbial world is and how it interacts with our diet, digestive system and wider health.
So, what is the gut microbiome, what do gut bacteria actually do and can what we eat influence them?
Let's take a closer look.
What Is the Gut Microbiome?
The term gut microbiome is commonly used to describe the enormous community of microorganisms and their genetic material found within the gastrointestinal tract.
Microorganisms can be found throughout the digestive system, but the largest and most diverse populations are found in the large intestine or colon.
These include:
- Bacteria
- Fungi and yeasts
- Viruses
- Archaea
- Other microorganisms
You may also hear the terms gut microbiota and gut flora.
Strictly speaking, microbiota refers to the microorganisms themselves, while microbiome can refer to the microorganisms together with their genetic material and surrounding environment. "Gut flora" is an older term that's still widely used.
Whatever terminology you encounter, the important point is that your digestive tract contains a complex microbial ecosystem rather than simply a collection of "good" and "bad" bacteria.
Does Everyone Have the Same Gut Microbiome?
No.
There can be considerable variation in the microorganisms found in different people's digestive systems.
The gut microbiome can also change throughout life and can be influenced by factors including:
- Age
- Diet
- Environment
- Medication
- Lifestyle
- Geography
- Early-life exposures
This is one reason why it's misleading to talk about one universal "perfect" balance of gut bacteria.
Research into what constitutes a healthy gut microbiome continues to develop, and scientists haven't identified a single microbial composition that everyone should be trying to achieve.
What Do Gut Bacteria Do?
Gut microorganisms aren't simply passive passengers within the digestive tract.
They interact with components of the food we eat and with the environment within the intestine.
Some of their most interesting activities involve dietary fibre.
Gut Bacteria and Fibre
Not all of the carbohydrates we eat are fully broken down and absorbed in the small intestine.
Certain fibres continue into the large intestine, where they can be fermented by microorganisms.
Different microorganisms can utilise different substrates, which is one reason researchers are particularly interested in how the types of fibre we eat may influence the gut microbiome.
This fermentation process produces various compounds, including short-chain fatty acids (SCFAs).
What Are Short-Chain Fatty Acids?
Short-chain fatty acids are produced when certain gut microorganisms ferment dietary components, particularly some types of fibre.
The main SCFAs include:
- Acetate
- Propionate
- Butyrate
These compounds are an important area of microbiome research.
Butyrate, for example, can be used as an energy source by cells lining the colon.
Rather than trying to obtain short-chain fatty acids through a particular "gut health" food, a sensible approach is to eat a varied diet containing different sources of dietary fibre.
Gut Bacteria and Digestion
Your digestive system and gut microbiota interact closely.
Microorganisms within the large intestine can ferment food components that haven't been absorbed earlier in digestion.
As part of this process, microorganisms produce numerous metabolic compounds.
They can also participate in the metabolism of certain nutrients and compounds that enter the colon.
The relationship works both ways: what you eat influences the environment available to your gut microorganisms, while those microorganisms influence how some dietary components are metabolised.
Why Does Fibre Matter to the Gut Microbiome?
Fibre is particularly interesting when we're discussing the microbiome because certain types can be fermented by microorganisms in the large intestine.
Fibre is found naturally in:
Different plant foods contain different fibres and plant compounds.
Rather than concentrating on one supposedly perfect source, aim for variety across your diet. Adults should aim for around 30g of fibre per day. If your current fibre intake is relatively low, increase it gradually and make sure you're drinking sufficient fluids.
Our guide to fibre and gut health explores the subject in more detail.
What Are Prebiotics?
Prebiotics are frequently mentioned when discussing gut bacteria, but they're often misunderstood.
A prebiotic is a substrate that is selectively utilised by host microorganisms and confers a health benefit.
Certain types of dietary fibre can have prebiotic properties.
Foods containing naturally occurring prebiotic fibres include foods such as:
- Onions
- Garlic
- Leeks
- Chicory
- Jerusalem artichokes
- Asparagus
- Bananas
- Oats
Prebiotic ingredients such as inulin and fructooligosaccharides (FOS) can also be found in food products and supplements.
It's important to remember that not every type of fibre is automatically a prebiotic.
Read our Prebiotics vs Probiotics guide for a more detailed explanation.
What Are Probiotics?
Probiotics are different again.
Probiotics are live microorganisms that, when administered in adequate amounts, confer a health benefit on the host.
They're often called "good" or "friendly" bacteria, but this description oversimplifies what probiotics are.
Different probiotic species and strains can have different characteristics, so evidence relating to one probiotic shouldn't automatically be applied to another.
This is also why taking the supplement with the highest number of bacteria or the most strains doesn't necessarily make it the best choice.
Read our complete What Are Probiotics? guide to learn about probiotic strains, CFUs, foods and supplements.
What's the Difference Between the Microbiome, Prebiotics and Probiotics?
These terms frequently appear together, so here's the simplest way to separate them.
| Term | What does it mean? |
|---|---|
| Gut microbiome | The microbial ecosystem within your digestive tract |
| Probiotics | Specific live microorganisms that provide a health benefit when administered in adequate amounts |
| Prebiotics | Substrates selectively used by microorganisms that provide a health benefit |
| Fermented foods | Foods produced through controlled microbial activity |
| Dietary fibre | A broad group of carbohydrates that aren't completely digested in the small intestine |
They're related concepts, but they aren't interchangeable.
Are Fermented Foods Good for the Gut Microbiome?
Fermentation has been used to make and preserve food for thousands of years.
Examples of fermented foods include:
- Live yoghurt
- Kefir
- Sauerkraut
- Kimchi
- Miso
- Tempeh
- Kombucha
Fermented foods can be an interesting addition to a varied diet.
However, it's important not to assume that every fermented food is automatically probiotic.
Some undergo heating or processing that means live microorganisms aren't present when the food is consumed. Even where live microorganisms remain, they don't automatically meet the scientific definition of probiotics.
Our guide to fermented foods and gut health explores this distinction further.
How Does Diet Affect the Gut Microbiome?
Diet is one of the factors that can influence the gut microbiome.
This doesn't mean you need to follow a restrictive "microbiome diet".
Instead, one of the most practical approaches is to concentrate on dietary variety.
A plant-rich diet can include:
- Different coloured vegetables
- Fruit
- Whole grains
- Beans and pulses
- Nuts
- Seeds
- Herbs and spices
These foods provide different types of fibre alongside a wide range of plant compounds.
Rather than asking, "What's the single best food for my gut?", it can be more useful to ask:
How much variety am I eating across the week?
Our Eat Your Way to a Healthy Gut guide has plenty of practical ideas for introducing greater plant variety into everyday meals.
What About the '30 Plants a Week' Idea?
You may have heard recommendations to eat 30 different plant foods each week for gut health.
This idea has become popular following microbiome research that found an association between greater plant diversity and greater microbial diversity.
It's an interesting and practical way to encourage dietary variety, but 30 isn't a magic number or an official nutritional target.
You don't need to count every blueberry.
Instead, use the idea as inspiration to vary your choices. Try a different whole grain, add beans to a meal, rotate the vegetables you buy or sprinkle nuts and seeds over breakfast.
Variety matters more than achieving a perfect score.
What Else Can Influence the Gut Microbiome?
Food is important, but it isn't the only factor associated with differences in the gut microbiome.
Researchers are investigating the influence of factors including:
Age
The microbiome changes throughout life, from infancy through adulthood and older age.
Environment and Geography
Where we live and the environments we're exposed to can be associated with differences in our microbiota.
Lifestyle
Physical activity, sleep and other lifestyle factors are being investigated for their relationships with the microbiome.
Medication
Certain medicines can influence microorganisms within the digestive tract.
Antibiotics are an obvious example because they're designed to act against bacteria.
Antibiotics are important medicines and should always be taken as prescribed. If you're considering taking a probiotic alongside antibiotics, ask your pharmacist or healthcare professional for advice rather than changing your medication or supplement routine yourself.
What Is the Gut-Brain Axis?
Your digestive system and brain communicate with one another through a complex, two-way network commonly known as the gut-brain axis.
This involves several systems and signalling pathways, including the nervous system.
Researchers are also investigating how the gut microbiome may interact with this communication network.
It's a fascinating and rapidly developing area of science, but it has also attracted plenty of exaggerated claims.
The gut isn't a "second brain" in the literal sense, and we shouldn't assume that changing our gut bacteria can directly change our mood or treat mental health conditions.
We'll explore the science in more detail in our guide to the gut-brain connection.
Can You Improve Your Gut Microbiome?
There's no universally agreed "ideal" microbiome, so it's more helpful to think about supporting digestive health than trying to create the perfect collection of bacteria.
Start with the fundamentals:
- Eat plenty of different plant foods.
- Include a variety of fibre sources.
- Aim for around 30g of fibre per day.
- Increase fibre gradually if necessary.
- Drink sufficient fluids.
- Include fermented foods if you enjoy and tolerate them.
- Stay physically active.
- Maintain healthy lifestyle habits.
Most importantly, avoid becoming overly concerned about trying to "cleanse", "detox" or completely rebalance your microbiome.
Your gut microbial ecosystem is complex, individual and constantly changing.
Do You Need Supplements for Gut Health?
Not necessarily.
Food and healthy lifestyle habits should provide the foundation.
Some people may choose supplements such as:
- Probiotics
- Prebiotics
- Fibre
- Digestive enzymes
- Other digestive-health products
These have different purposes and shouldn't simply be grouped together as products that "fix your gut".
If you're considering a supplement, think about what you're actually hoping to support and choose accordingly.
Our guide to the Best Supplements for Digestive Health explains the different options in more detail.
Frequently Asked Questions
What Is the Gut Microbiome in Simple Terms?
The gut microbiome is the complex ecosystem of microorganisms and their genetic material found within the digestive tract. It includes bacteria, fungi, viruses and other microbes.
Are Gut Bacteria Good or Bad?
It's more complicated than dividing every bacterium into "good" and "bad".
Your digestive tract contains a complex community of microorganisms that interact with one another, with your diet and with the gut environment.
Some microorganisms can cause illness under particular circumstances, while others perform functions within the normal gut ecosystem.
How Many Bacteria Live in Your Gut?
The human digestive tract contains trillions of microorganisms. Estimates have changed as scientific methods have improved, so it's better to think in terms of trillions rather than relying on older claims that everyone has exactly 100 trillion bacteria.
What Foods Support the Gut Microbiome?
Rather than one particular food, focus on variety.
Vegetables, fruit, whole grains, beans, pulses, nuts and seeds provide different fibres and plant compounds. Fermented foods can also contribute additional dietary variety.
What Is the Difference Between Gut Microbiome and Gut Flora?
"Gut flora" is an older term traditionally used to describe microorganisms living within the digestive tract.
"Microbiota" and "microbiome" are now more commonly used scientific terms.
Are Probiotics the Same as Gut Bacteria?
No.
Your gut naturally contains an enormous range of microorganisms. Probiotics are particular live microorganisms that provide a demonstrated health benefit when administered in adequate amounts.
Does Everyone Need a Probiotic for Their Microbiome?
No.
A probiotic supplement isn't an essential requirement for everyone. Start with a varied, balanced diet containing plenty of plant foods and fibre.
Understanding Your Gut
We're learning more about the gut microbiome all the time, but there is still a great deal we don't know.
What we do know is that this microbial ecosystem is complex, individual and closely connected with what we eat.
Rather than trying to achieve the perfect balance of "good" and "bad" bacteria, focus on the things we can control: dietary variety, plenty of plant foods, sufficient fibre and fluids, and healthy lifestyle habits.
To continue exploring digestive health, read Eat Your Way to a Healthy Gut, our Prebiotics vs Probiotics guide and What Are Probiotics?, or discover the different options in our Best Supplements for Digestive Health guide.
This article is for informational purposes only and should not replace medical advice. Persistent or unexplained digestive symptoms should be discussed with your GP or another appropriately qualified healthcare professional.
Checked and updated: 19th August 2026
