Introduction
It is a common scenario in GP surgeries across the UK: a patient arrives feeling persistently "under the weather." They might describe a heavy, dragging fatigue that sleep doesn’t touch, or perhaps a stubborn bloating that makes their favourite jeans feel uncomfortable by mid-afternoon. Often, these individuals have already tried cutting out various food groups or scrolling through endless forums looking for a "miracle" probiotic. When standard tests come back as "normal," many begin to wonder if the answer lies deep within their digestive system—specifically within the vast, invisible world of their gut microbiome, which is why a conversation about targeted thyroid blood testing can sometimes be a useful next step when symptoms overlap.
The sheer scale of this internal ecosystem is staggering. We often think of ourselves as entirely human, but from a microbial perspective, we are more like a walking, talking coral reef. Within your gastrointestinal tract lives a community of trillions of microorganisms, including viruses, fungi, and archaea, but by far the most numerous and well-studied are the bacteria.
Understanding how many types of gut bacteria there are is more than just a scientific curiosity; it is a gateway to understanding how your body processes nutrients, regulates your immune system, and even influences your mood. However, at Blue Horizon, we believe that "data without context is just noise." Identifying that you have a certain type of bacteria is only one piece of a much larger puzzle that includes your symptoms, your lifestyle, and your clinical history.
In this article, we will explore the diverse landscape of the human gut, categorising the hundreds of species that call your body home. We will look at the three main "enterotypes" that act like microbial blood groups, the difference between beneficial and opportunistic bacteria, and how you can take a structured, clinical approach to managing your gut health.
Our approach—the Blue Horizon Method—always begins with a consultation with your GP to rule out serious underlying conditions. From there, we advocate for a structured self-check of your lifestyle and symptoms. Only then, if you are still seeking clarity, do we suggest using targeted blood markers to see the "big picture" of your health and help guide a more productive conversation with your doctor.
The Scale Of The Microbial World
To answer the question of how many types of gut bacteria there are, we must first look at the sheer numbers involved. Current scientific estimates suggest that the human body is home to between 10 trillion and 100 trillion microbial cells. To put that into perspective, your own human cells are outnumbered. In terms of weight, the bacteria in your gut weigh roughly 200 grams—about the same as a medium-sized hamster.
While these microbes are found all over the body, including on the skin and in the mouth, the vast majority reside in the large intestine (the colon). The environment here is "anaerobic," meaning there is very little oxygen, which allows specific types of bacteria to thrive that could not survive elsewhere.
The Genetic Powerhouse
The complexity of the gut microbiome is even more apparent when we look at genetics. The human genome contains roughly 20,000 to 25,000 genes. In contrast, the collective genome of your gut bacteria (the metagenome) contains millions of genes. This means your gut bacteria have roughly 100 times more genetic material than you do. These bacterial genes allow the microbes to perform chemical tasks that the human body cannot do on its own, such as breaking down complex plant fibres and synthesising certain vitamins like B12 and Vitamin K.
The Four Major Phyla: The Building Blocks
In the world of biological classification, a "phylum" is a very broad group. When scientists look at the gut, they find that while there are hundreds of individual species, about 90% to 99% of all gut bacteria belong to just four main phyla. You can think of these as the four main "families" of the gut.
1. Bacillota (Formerly Firmicutes)
This is typically the largest group in the human gut. It contains hundreds of different species, including the well-known Lactobacillus family. These bacteria are heavily involved in energy resorption and the breakdown of fats. Some research suggests that the ratio of Bacillota to other groups may play a role in how efficiently we store energy from our food, though this is a complex area of ongoing study.
2. Bacteroidota (Formerly Bacteroidetes)
This group is composed of "specialist" bacteria that are excellent at breaking down complex carbohydrates and plant fibres that our own digestive enzymes can’t touch. Members of this phylum, such as the Bacteroides genus, are crucial for producing short-chain fatty acids (SCFAs), which provide energy for the cells lining the colon and help maintain a healthy gut barrier.
3. Actinomycetota (Formerly Actinobacteria)
While smaller in number than the first two groups, this phylum is incredibly important, particularly in early life. The most famous member of this group is Bifidobacterium. These are often the first "colonisers" of a baby’s gut and play a vital role in training the infant immune system.
4. Pseudomonadota (Formerly Proteobacteria)
This group includes many familiar names, such as Escherichia coli (E. coli). While we often associate these names with illness, most members of this phylum live peacefully in the gut in small numbers. However, an overgrowth of certain species in this group is often seen in cases of "dysbiosis"—the clinical term for an imbalanced gut.
How Many Species Are There?
While the four phyla above represent the broad categories, the number of individual species is much higher. Most researchers agree that a healthy adult colon contains between 300 and 1,000 different species of bacteria.
Why is there such a wide range in that estimate? There are two main reasons:
- The Unculturable Majority: For decades, scientists could only study bacteria that they could grow (culture) in a petri dish. However, most gut bacteria are "obligate anaerobes," meaning they die the moment they are exposed to oxygen. This made them nearly impossible to study until the advent of DNA sequencing. We are still "discovering" new species today simply by identifying their DNA in stool samples.
- Individual Variation: While there is a "core" group of about 30 to 40 species that most humans share, the rest of your microbial makeup is as unique as a fingerprint. Your diet, where you live, your genetics, and even whether you have pets can change the specific species found in your gut.
Key Takeaway: While we all share common "families" of bacteria, the specific mix of the 300 to 1,000 species in your gut is unique to you. Diversity is generally considered a hallmark of a healthy gut, as it means the ecosystem is more resilient to "shocks" like a course of antibiotics or a bout of food poisoning.
The Three Gut "Enterotypes"
In 2011, a landmark study by the European Molecular Biology Laboratory (EMBL) suggested that humans could be categorised into three distinct "enterotypes." Just as we have different blood groups (A, B, AB, O), we may have different "gut types" based on which species of bacteria are most dominant.
Enterotype 1: The Bacteroides Type
This type is dominated by the Bacteroides genus. These individuals tend to have a gut ecosystem that is highly efficient at breaking down proteins and fats. In some studies, this enterotype has been associated with a long-term diet higher in animal fats and proteins. This type is also particularly good at synthesising Vitamin B7 (biotin) and Vitamin C.
Enterotype 2: The Prevotella Type
This type is dominated by Prevotella. These bacteria are specialists in breaking down fibrous plant sugars and mucin (the mucus that lines the gut). This enterotype is often found in people who consume a diet rich in whole grains, fruits, and vegetables. These individuals may be better at synthesising Vitamin B1 (thiamine) and folic acid.
Enterotype 3: The Ruminococcus Type
This is the most common enterotype. It is dominated by Ruminococcus and Staphylococcus. This group is excellent at helping the body absorb sugars and breaking down cellulose.
It is important to note that your enterotype is not a diagnosis. It is simply a way of describing the "landscape" of your microbiome. Researchers are still investigating whether knowing your enterotype can help doctors prescribe more effective medications or recommend more personalised dietary changes in the future.
Beneficial, Pathogenic, and Opportunistic
Another way to answer "how many types of gut bacteria are there" is to look at their behaviour. At Blue Horizon, we prefer to look at bacteria through the lens of their function within the human host.
Beneficial ("Good") Bacteria
These are the "gardeners" of the gut. They perform essential services such as:
- Synthesising Vitamins: Producing Vitamin K, B12, and folate.
- Competitive Exclusion: Occupying space and using up nutrients so that harmful bacteria cannot gain a foothold.
- Immune Training: Communicating with the immune cells that live in the gut lining to prevent them from overreacting to harmless substances.
- Producing SCFAs: Fermenting fibre into butyrate, which fuels the cells of the colon and has anti-inflammatory properties.
Pathogenic ("Bad") Bacteria
These are the "intruders." They are not a normal part of a healthy gut and are usually introduced through contaminated food or water (such as Salmonella or Campylobacter). They produce toxins that damage the gut lining, leading to symptoms like acute diarrhoea, vomiting, and inflammation.
Opportunistic Bacteria
These are the "fair-weather friends." They live in most people’s guts in small, controlled numbers without causing any issues. However, if the beneficial bacteria are wiped out (for example, by a heavy course of antibiotics) or if the host's immune system is weakened, these opportunistic bacteria can "overgrow" and cause problems. A classic example is Clostridium difficile (C. diff), which can cause severe inflammation of the colon when the "good" bacteria aren't there to keep it in check.
The Blue Horizon Method: A Structured Approach to Gut Health
If you are experiencing symptoms like persistent bloating, changes in bowel habits, or unexplained fatigue, it is tempting to want to "test your gut" immediately. However, the science of direct microbiome testing (stool kits) is still in its infancy, and many clinical professionals find the results difficult to translate into practical medical advice.
At Blue Horizon, we recommend a phased journey to help you get to the bottom of your symptoms responsibly, and our how to test thyroid guide follows a similar step-by-step structure.
Step 1: Consult Your GP
Before looking at your bacteria, you must rule out structural or serious medical issues. Your GP can investigate symptoms for conditions like Inflammatory Bowel Disease (IBD), Coeliac disease, or even simple iron-deficiency anaemia. If you experience severe symptoms such as blood in your stool, unexplained weight loss, or intense abdominal pain, you should seek urgent medical attention (NHS 111, your GP, or A&E if necessary).
Step 2: Structured Self-Check
Once serious conditions have been ruled out, look at the patterns in your life. We recommend keeping a diary for at least two weeks:
- Symptom Timing: Does the bloating happen immediately after eating, or hours later?
- The Bristol Stool Scale: Track the consistency of your bowel movements.
- Lifestyle Factors: Note your stress levels, sleep quality, and exercise habits. These all directly influence gut motility (the speed at which waste moves through your system).
- Dietary Fibre: Are you eating a diverse range of plant foods? The UK government recommends 30g of fibre a day, but many of us fall short.
Step 3: Targeted Blood Testing
If you are still "stuck," blood testing can provide a high-trust snapshot of how your gut health is affecting your overall system. Rather than counting specific bacteria, we look at markers that show the impact of your digestive health, and the wider thyroid blood tests collection is often where people start when fatigue is part of the picture.
For example, if your gut microbiome is imbalanced or if you have issues with absorption, it may show up in your nutrient levels.
- Vitamin B12 and Folate: Essential for energy and nervous system health. Low levels can sometimes point to issues in the small intestine.
- Ferritin (Iron Stores): Low iron is a common cause of fatigue and can sometimes be linked to digestive malabsorption.
- CRP (C-Reactive Protein): A marker of systemic inflammation. While it doesn't tell us where inflammation is, a high result can be a prompt for further investigation with your doctor.
- Vitamin D: Crucial for immune function; many people with gut-related concerns are found to be deficient.
Our Thyroid Premium Gold Blood Test is a strong option here. While "Thyroid" is in the name, it is designed as a broader health snapshot and includes the nutrient markers that can help you and your GP join the dots.
These panels also include the full nutrient suite (B12, Folate, Vitamin D, Ferritin) and CRP, giving you a broad health snapshot. By looking at these markers, you can have a much more productive conversation with your GP about why you might be feeling run down.
Factors That Influence Your Microbial "Garden"
If we think of the gut as a garden, the types of bacteria (the plants) are heavily influenced by the environment (the soil). Several factors determine which species will thrive and which will wither away.
Diet and Diversity
The single most important factor you can control is your diet. Different bacteria prefer different "fertilisers." For example, Bifidobacteria love the oligosaccharides found in onions, garlic, and bananas. By eating a wide variety of plant foods (aiming for 30 different types a week), you provide "food" for a diverse range of bacterial species.
Medication
Antibiotics are life-saving medications, but they are like a "forest fire" for the gut. They don't just kill the bad bacteria; they can wipe out large sections of your beneficial microbial community. Other medications, like Proton Pump Inhibitors (PPIs) used for acid reflux, change the pH of the stomach. This can allow bacteria that would normally be killed by stomach acid to survive and travel down into the small intestine, potentially leading to SIBO (Small Intestinal Bacterial Overgrowth).
Age
Our microbiome changes throughout our lives. Babies have very simple microbiomes dominated by Bifidobacteria. As we move into adulthood and eat a wider diet, diversity increases. In the elderly, diversity often begins to decline again, and we sometimes see an increase in "bad" bacteria like Clostridium perfringens. Supporting gut health becomes even more important as we age to maintain a robust immune system.
Motility: The "Turnover"
How often you have a bowel movement matters. If waste moves too slowly through the colon (constipation), bacteria have more time to ferment waste products, which can lead to gas and bloating. If it moves too quickly (diarrhoea), the "good" bacteria don't have enough time to perform their essential tasks like vitamin synthesis.
Supporting Your Gut Responsibly
Improving the balance of your gut bacteria is a marathon, not a sprint. We advise caution when making significant changes.
- Probiotics: These are "live" beneficial bacteria found in supplements or fermented foods like kefir and sauerkraut. They can be helpful, but they don't usually "colonise" the gut permanently; they are more like "tourists" that provide a benefit as they pass through. If you have a complex medical history or are pregnant, always check with your GP before starting new supplements.
- Prebiotics: These are the fibres that feed your existing "good" bacteria. Introducing them too quickly (e.g., suddenly eating huge amounts of beans or lentils) can cause temporary gas and discomfort. It is best to increase your fibre intake slowly and drink plenty of water.
- Stress Management: The "gut-brain axis" is a two-way street. High levels of stress can physically change the composition of your gut bacteria. Incorporating relaxation techniques isn't just good for your mind; it's good for your microbes.
Summary: Seeing the Bigger Picture
The question "how many types of gut bacteria are there?" reveals a world of incredible complexity. With up to 1,000 species and 100 trillion individual cells, your microbiome is a major organ in its own right. While we can categorise these bacteria into phyla and enterotypes, the most important thing is how this ecosystem functions for you.
At Blue Horizon, we believe in a balanced, clinical approach to gut health.
- Rule out the serious: Work with your GP to rule out IBD, Coeliac disease, or infections.
- Track the lifestyle: Use a diary to find patterns in your diet, stress, and symptoms.
- Check the impact: Use targeted blood testing, such as our what blood test checks for thyroid guide, to see if your digestive health is impacting your nutrient levels or inflammatory markers.
By following this phased journey, you move away from "guessing" and towards a structured understanding of your body. Your gut bacteria are a vital part of your health, but they are just one chapter in the story of your overall wellbeing.
FAQ
How can I tell if my gut bacteria are "unbalanced"?
Symptoms of an imbalance, often called dysbiosis, can include persistent bloating, excessive gas, diarrhoea or constipation, and "brain fog." However, because these symptoms can also be caused by many other medical conditions, it is essential to consult your GP first to rule out more serious causes before assuming the issue is purely microbial.
Does a "gut microbiome test" give a diagnosis?
Currently, most commercial stool kits that sequence gut DNA are used for wellness insights rather than medical diagnosis. Doctors generally do not use these tests to diagnose disease because we don't yet have a single definition of what a "perfect" microbiome looks like. They can be interesting "snapshots," but they should be used alongside clinical blood tests and GP advice.
Can antibiotics permanently change the types of bacteria in my gut?
Antibiotics can significantly reduce the diversity of your gut bacteria. While many people’s microbiomes recover within a few weeks or months, some research suggests that certain species may take longer to return, or may not return at all without dietary support. Eating a wide variety of fibre-rich foods following a course of antibiotics is the best way to help your "garden" regrow.
Which Blue Horizon test is best if I have gut issues and fatigue?
If you are struggling with fatigue alongside digestive "mystery symptoms," our Thyroid Premium Gold Blood Test is a popular starting point. It checks your TSH and thyroid hormones, but crucially, it also includes Ferritin, Vitamin B12, Folate, Vitamin D, and CRP. These markers help you see if your gut health is affecting your nutrient absorption or causing low-level systemic inflammation. For an even deeper look, the Thyroid Premium Platinum Blood Test adds HbA1c (blood sugar) and a full iron panel. If you want to compare the wider range, you can also review the full other thyroid-related tests collection for the most suitable option.
