Introduction
Have you ever wondered why you feel "off" despite your GP telling you that your standard blood results are within the normal range? Perhaps you struggle with persistent bloating, unexplained fatigue, or a "foggy" brain that makes focusing on your daily tasks feel like wading through treacle. For many in the UK, these "mystery symptoms" lead back to a world existing entirely within us: the gut microbiome. Often referred to as our "second brain," this complex internal ecosystem is far more than just a digestive aid; it is a bustling metropolis of microscopic life that influences everything from our mood and immunity to how we process vitamins and regulate inflammation.
The question of exactly how many species of bacteria live in the human gut is one that scientists have been racing to answer for decades. While we once thought of the gut as a simple tube for processing food, we now understand it to be one of the most densely populated habitats on Earth. Understanding the scale and diversity of this bacterial community is the first step toward understanding how your internal health dictates your external well-being.
At Blue Horizon, we believe that the best health decisions are made when you have the full picture. This article will explore the latest scientific consensus on the number of bacterial species in the human gut, the roles they play in your health, and how you can take a structured, clinically responsible approach to investigating your own well-being. Following the Blue Horizon Method, we always advocate for a "GP-first" approach, ruling out serious clinical concerns before using targeted private testing to gain a more detailed snapshot of your health.
The Magnitude of the Microbiome: A Numbers Game
To understand how many species of bacteria are in the human gut, we must first appreciate the sheer scale of the population. For a long time, a popular scientific "fact" suggested that bacterial cells outnumber human cells by ten to one. More recent and rigorous calculations suggest the ratio is closer to one-to-one, but the numbers remain staggering.
The average human body is home to approximately 30 trillion human cells, but it also hosts an estimated 38 trillion bacterial cells. The vast majority of these—around 95%—reside in the large intestine (the colon). If you were to weigh the bacteria in your gut, they would account for roughly 1 to 2 kilograms of your total body weight, which is roughly the same weight as your brain or your liver.
How Many Species Exist?
When it comes to the number of individual species, the answer depends on whether we are looking at the global human population or a single individual.
Across the entire human race, researchers have identified nearly 2,000 different bacterial species that can inhabit the gut. However, no single person carries all of them. Studies, such as those published in the journal Nature, suggest that while the total "pool" of gut bacteria is vast, an average healthy adult typically carries around 160 different species at any one time.
It is also important to note that while we have identified many species, a significant portion of them are "unculturable." This means they cannot currently be grown in a laboratory setting because they require the highly specific, oxygen-free (anaerobic) environment of the human colon to survive. To identify these "dark matter" microbes, scientists use advanced computational methods and genetic sequencing (metagenomics) to piece together their DNA from stool samples.
The "Core" vs. The "Transient"
While your gut might contain hundreds of species, the distribution is not equal. Approximately 99% of the bacteria in your gut come from just 30 to 40 "core" species. These are the heavy lifters—the species responsible for the bulk of your digestion, vitamin production, and immune regulation. The remaining species are often present in much smaller amounts or may be "transient," meaning they pass through your system based on the food you eat or the environments you visit.
Key Takeaway: While there are nearly 2,000 known species of gut bacteria globally, your individual gut likely hosts about 160 species, with a small "core" group of 30–40 species doing most of the work.
The Major Families of Gut Bacteria
In biology, species are grouped into broader categories called "phyla." Even though there are hundreds of species, the vast majority of gut bacteria belong to just four main families. Understanding these can help you understand the language of gut health.
1. Bacillota (formerly Firmicutes)
This is typically the largest group in the human gut. Many species within this family are responsible for breaking down complex carbohydrates and fibres that our own human enzymes cannot digest. They produce short-chain fatty acids (SCFAs), such as butyrate, which provide energy for the cells lining your colon and help maintain a healthy gut barrier.
2. Bacteroidota (formerly Bacteroidetes)
This family is also highly prevalent and works alongside Bacillota. These bacteria are particularly adept at breaking down plant-based sugars and proteins. The ratio between Bacillota and Bacteroidota is often studied in relation to metabolic health and weight management, although the science is still evolving.
3. Actinomycetota (formerly Actinobacteria)
A key member of this family is Bifidobacterium. These are some of the first bacteria to colonise the gut of a newborn baby and remain crucial throughout life for maintaining the integrity of the gut lining and supporting the immune system.
4. Pseudomonadota (formerly Proteobacteria)
This family includes well-known species like Escherichia coli (E. coli). While some members are necessary for a healthy gut, an overgrowth of certain species in this family is often associated with inflammation or "dysbiosis"—a term used to describe an imbalance in the gut ecosystem.
What Do These Bacteria Actually Do?
The species in your gut are not just "passengers"; they are active participants in your physiology. They function almost like an extra organ, performing tasks that the human body cannot perform on its own.
Digestion and Nutrient Absorption
Human enzymes are limited. We cannot fully digest certain plant fibres, starches, or sugars. The bacteria in our gut ferment these substances, turning them into beneficial compounds like acetate, propionate, and butyrate. These short-chain fatty acids (SCFAs) help regulate our metabolism and even influence our appetite.
Vitamin Synthesis
Your gut bacteria are responsible for producing several essential vitamins. These include Vitamin K (crucial for blood clotting) and various B vitamins, such as B12, biotin, and folate. If your bacterial diversity is low, you may find that your levels of these vitamins are suboptimal, even if your diet seems adequate.
Immune System Training
Roughly 70% to 80% of your immune system is located in your gut. Your bacterial species act as "trainers," teaching your immune cells to distinguish between harmless food particles and dangerous pathogens (viruses or "bad" bacteria). A diverse microbiome is linked to a more resilient and balanced immune response.
The Gut-Brain Axis
There is a direct communication line between your gut and your brain, known as the vagus nerve. Gut bacteria produce neurotransmitters, including serotonin and dopamine, which regulate mood and anxiety. This is why "gut feelings" are a biological reality; an imbalanced microbiome can contribute to feelings of brain fog, low mood, or irritability. If you want the wider context behind this connection, our guide on what a gut microbiome means for your health is a helpful companion read.
Signs of an Imbalanced Microbiome (Dysbiosis)
When the diversity of your gut species decreases, or when "pathogenic" (harmful) species begin to outnumber the beneficial ones, we call this dysbiosis. This is not a specific diagnosis in itself, but rather a state of imbalance that can lead to various symptoms.
Typical signs that your gut ecosystem might be struggling include:
- Digestive discomfort: Persistent bloating, excessive gas, or changes in bowel habits (such as constipation or diarrhoea).
- Systemic inflammation: Skin flare-ups, joint aches, or a general feeling of being "inflamed."
- Nutrient deficiencies: Low levels of Iron, Vitamin B12, or Vitamin D, which can manifest as fatigue and weakness.
- Metabolic changes: Difficulty managing weight or fluctuations in blood sugar levels.
If you are trying to understand how these patterns are explored in practice, how gut microbiome testing works is a useful next step.
Safety Note: If you experience sudden, severe symptoms—such as unexplained weight loss, persistent blood in your stools, severe abdominal pain, or difficulty swallowing—you must seek urgent medical attention via your GP, A&E, or by calling 999. These symptoms always warrant immediate clinical investigation to rule out serious underlying conditions.
The Blue Horizon Method: A Step-by-Step Approach
If you suspect that your gut health is impacting your overall well-being, it can be tempting to jump straight into expensive niche testing. However, at Blue Horizon, we advocate for a phased, responsible journey.
Step 1: Consult Your GP
Your first port of call should always be your GP. They can perform standard NHS investigations to rule out clinical conditions like Coeliac disease, Inflammatory Bowel Disease (IBD), or infections. It is essential to ensure that your symptoms are not caused by an underlying medical issue that requires immediate treatment.
Step 2: Structured Self-Check
While working with your GP, start a diary. Track your symptoms alongside your diet, sleep patterns, and stress levels. Notice if certain foods or lifestyle factors (like a stressful week at work) coincide with a flare-up of bloating or fatigue. This data is invaluable for both you and your healthcare professional.
Step 3: Targeted Blood Testing
If your GP has ruled out major illnesses but you still feel "stuck," or if you want a more comprehensive "snapshot" of how your body is functioning, private blood testing can provide the clinical context you need. To understand the practical side of booking and sample collection, our how to get a blood test guide explains the process clearly.
While stool tests can count bacterial species, blood tests tell us how those bacteria are affecting your host health—your systemic inflammation, your nutrient absorption, and your metabolic markers.
How Blood Testing Complements Gut Health
At Blue Horizon, we offer tiered testing options that can help you see the "bigger picture" of your health. While these tests do not diagnose gut conditions, they provide markers that you can discuss with your GP or a nutritionist.
The Gold and Platinum Panels
Our Gold and Platinum tiers are particularly useful for those investigating long-term symptoms that might be linked to gut function. You can explore the full range on our thyroid blood tests collection.
- Vitamin B12 and Folate: Because your gut bacteria help produce and absorb these, low levels can often be a "red flag" for gut issues.
- Ferritin (Iron Stores): Gut inflammation or malabsorption can often lead to low iron, which is a primary cause of fatigue.
- Vitamin D: Low Vitamin D is frequently seen in people with gut health challenges, as it plays a massive role in maintaining the gut barrier.
- C-Reactive Protein (CRP): This is a marker of systemic inflammation. If your gut is "leaky" or imbalanced, your CRP may be slightly elevated, even if you don't have a specific infection.
- HbA1c: This measures your average blood sugar over the last few months. Since gut bacteria influence glucose metabolism, this marker provides insight into your metabolic health.
For readers comparing the higher-tier options, the Thyroid Premium Gold blood test and the Thyroid Premium Platinum blood test are the most relevant pages to review.
The Thyroid Connection
Interestingly, gut health is closely tied to thyroid function. About 20% of the conversion of the inactive thyroid hormone (T4) into the active form (T3) happens in the gut. If your gut species are imbalanced, this conversion can be less efficient.
Our Platinum Thyroid panel is the most comprehensive available. It includes the base markers (TSH, Free T4, Free T3) plus antibodies and metabolic markers like HbA1c and Reverse T3. Crucially, it also includes the Blue Horizon Extras: Magnesium and Cortisol. These cofactors are vital because:
- Magnesium is essential for the enzymes that convert thyroid hormones.
- Cortisol (the stress hormone) can be elevated by gut inflammation, which in turn can suppress thyroid function.
If you are exploring the wider relationship between digestion, immunity, and thyroid health, the article on why a healthy gut microbiome matters is a strong companion read.
By looking at these markers together, you and your GP can have a much more productive conversation about why you might still be feeling fatigued despite having "normal" TSH levels.
Sample Collection and Practicalities
We aim to make clinical-grade testing as accessible as possible.
- Bronze, Silver, and Gold Tiers: These can be completed at home with a simple fingerprick sample or using a Tasso device. Alternatively, you can choose a clinic visit or a nurse home visit if you prefer a professional blood draw.
- Platinum Tiers: Because these panels are so comprehensive, they require a larger volume of blood. Therefore, they require a professional venous blood draw at a clinic or via a nurse home visit.
- Timing: We generally recommend taking your sample at 9am. This ensures consistency, as many hormones (like cortisol) and markers fluctuate throughout the day.
If you prefer support with the collection process, our patient stories and FAQs can help set expectations before you order.
How to Improve Your Gut Species Diversity
While testing provides the data, the "cure" for a low-diversity gut often lies in consistent, gentle lifestyle changes. You should always discuss significant dietary changes with a professional, especially if you have existing health conditions like diabetes or a history of disordered eating.
1. Diversify Your Fibre
Different species of bacteria eat different types of fibre. Aim for "30 plant points" a week. This doesn't mean 30 different meals, but 30 different types of vegetables, fruits, nuts, seeds, legumes, and wholegrains. Each new plant introduces a different prebiotic fibre that can feed a different species of bacteria.
2. Embrace Fermented Foods
Natural yoghurt, kefir, sauerkraut, and kimchi contain live cultures (probiotics) that can temporarily join your gut community and support the resident species.
3. Focus on Sleep and Stress
The gut-brain axis works both ways. High stress and poor sleep can physically alter the composition of your gut bacteria, making it harder for "good" species to thrive. Prioritising a consistent sleep routine is as important for your gut as eating your greens.
4. Be Cautious with Antibiotics
Antibiotics are life-saving medications, and you should always take them when prescribed by a doctor. However, they are "broad-spectrum," meaning they can kill off beneficial gut species alongside the harmful ones. If you have recently finished a course of antibiotics, focusing on gut-supporting foods can help your microbiome recover more effectively.
Summary: The Journey to Better Health
Knowing that there are nearly 2,000 species of bacteria in the human gut is fascinating, but the most important number is the one that relates to you and how you feel.
Health is rarely about one isolated marker or one single "bad" bacterium. It is about the harmony of the whole system. If you are experiencing persistent symptoms, remember the phased journey:
- Consult your GP to rule out clinical disease.
- Track your symptoms and lifestyle to find patterns.
- Consider a structured blood test (like a Blue Horizon Gold or Platinum panel) to provide a snapshot of your systemic health and nutrient levels.
- Work with a professional to interpret those results within the context of your life and symptoms.
Our thyroid and wellness panels are designed to give you the data you need to be an active participant in your own healthcare. By looking at cofactors like magnesium and cortisol alongside standard markers, we help you see the "bigger picture"—helping you move from mystery symptoms toward a clearer understanding of your internal ecosystem.
You can view current pricing and explore our full range of tests on our thyroid and general health testing pages.
FAQ
How many species of bacteria live in the human gut?
While researchers have identified nearly 2,000 different species across the global human population, an individual person typically hosts around 160 different species at any given time. The vast majority of your gut population is made up of a "core" group of about 30 to 40 species.
Can a blood test tell me which bacteria are in my gut?
No, a blood test cannot count or identify the specific species of bacteria living in your gut; that requires a stool (fecal) analysis. However, blood tests are essential for measuring the effects those bacteria have on your body. They can reveal inflammation (CRP), nutrient deficiencies (B12, Iron, Vitamin D), and metabolic health (HbA1c), which are often influenced by the state of your gut microbiome.
What is the most common bacterial family in the gut?
Most gut bacteria belong to two main phyla: Bacillota (formerly Firmicutes) and Bacteroidota (formerly Bacteroidetes). Together, these two families usually make up about 90% of the total bacterial population in a healthy adult gut.
How do I know if my gut bacteria are "unbalanced"?
An imbalance, often called dysbiosis, doesn't always have one single symptom. Common signs include persistent bloating, changes in bowel habits, unexplained fatigue, and skin issues. If you have ruled out serious illness with your GP, looking at markers like B12, Iron, and inflammatory markers in a blood test can help provide evidence of how your gut health is impacting your overall systemic function.
