What Is Your Microbiota, and Why Does It Matter?
Your microbiota is a community of trillions of bacteria, viruses, fungi, and archaea that works as a secondary organ alongside you — here you meet its core concepts and find your own way through the book.
Imagine several trillion living organisms inhabiting your gut right at this very moment. Tiny bacteria, viruses, fungi, and archaea[G] — you've never seen them, yet they shape how you digest breakfast, how you respond to stress, how well you sleep, and sometimes how clearly you think. This is your gut microbiota: a complex ecosystem that develops together with you, adapts to your life, and works so closely with the rest of your body that modern medicine has begun calling it a secondary organ.
Most people live their entire lives without ever thinking about it. This book inspires you not to be one of them.
Your microbiome responds within 24–48 hours to what you eat, how you sleep, how you move, and which drugs you take — and its diversity[G] is one of the most important health indicators, an indicator we cover across chapters 4–7.
What is the microbiota — exactly?
The words "microbiota" and "microbiome" are often used interchangeably, even though they aren't synonymous. The microbiota[G] is the community itself: the trillions of living bacteria, viruses, fungi, and archaea that share a particular site (gut, skin, mouth, vagina). The microbiome[G] is broader: the community plus its collective genetic material, metabolites, and the environment they live in. The Berg et al. 2020 international microbiome consensus formally clarified the distinction, and this book uses that definition. [609]
The numbers usually surprise people. Your gut microbiome contains roughly 38 trillion living microbes — about the same as the number of your own cells. The Sender et al. 2016 PLoS Biology study specifically refined this 1:1 ratio: the older "ten times more microbes than cells" framing was an overestimate. [610] What's genuinely striking, though, is the gene repertoire: it's 100–1000× larger than the human genome's. Your own genome contains about 20,000 protein-coding genes; the gut microbiome's gene catalog runs to 3–5 million. That gene set produces thousands of enzymes and metabolites with which your microbes speak directly to the rest of your body — chapter 2 takes that conversation apart in detail.
In 1984, Australian physician Barry Marshall drank a culture of Helicobacter pylori to prove that stomach ulcers are caused by a bacterium, not merely by stress or excess acid. He developed gastritis within days — and was proven right. The discovery earned a Nobel Prize in 2005 and rewrote the textbooks on ulcer treatment. [2713] The lesson applies to the microbiome too: what we treat today as "settled knowledge" can be refined by the next — often accidental — observation, which is why this book frames everything in evidence tiers.
Diversity, eubiosis, dysbiosis
Three terms that every later chapter relies on.
Diversity refers to how many different microbial species live in you and in what proportions. Higher diversity yields a more resilient ecosystem — much the way a biologically varied forest weathers storms better than a monoculture. One of the most reliable features of a healthy gut microbiome is high diversity.
Eubiosis[G] is the balanced state: beneficial and neutral species dominate, while inflammatory ones are kept in check. This doesn't mean you have no harmful bacteria — it means the community as a whole prevents them from taking over.
Dysbiosis[G] is when that balance breaks down: reduced diversity and/or dominance of inflammatory species. Two important nuances. First, dysbiosis is not a clinical diagnosis, it's a description of state — an observation, not necessarily a cause. Second, and often missed: dysbiosis is not a deviation from some universal ideal microbiome. A healthy adult's microbiome can differ substantially from another's depending on sex, age, diet, lifestyle, climate, and genetic background — and still sit in a stable, functional equilibrium. The "normal microbiome" is therefore not a single target state but an individual-specific stable functional matrix. And in many diseases (see chapter 3's separate category), dysbiosis is the consequence, not the trigger.
The gut as a microbial ecosystem
If you want to picture where your microbes live, imagine a specialized habitat: mildly acidic pH (~6.5–7.0), low oxygen (most gut bacteria are obligate anaerobes), nutrient-rich (carbohydrates, fibers, and proteins not absorbed in the small intestine), and lined with a thick mucus layer. The mucus acts as a buffer: across the gut barrier, microbes don't contact your blood vessels directly — or at least, they shouldn't.
Everything we discuss in chapters 4–7 (diet, sleep, movement, stress, environment, drugs) modifies this environment in some way: the acidity, the mucus integrity, the nutrient supply, or the barrier function.
The book uses three foundational terms per international consensus definitions. Probiotic (WHO/ISAPP 2014, Hill et al.): "live microorganism that, when administered in adequate amounts, confers a well-defined health benefit" — meaning the generic "probiotic" label is not by itself a clinical product category; only the strain + dose + indication triplet is defensible. [561] Prebiotic (ISAPP 2017, Gibson et al.): a substrate that selectively nourishes beneficial microbes; not every fiber is a prebiotic. Microbiome (Berg et al. 2020 consensus): the microbial community plus its associated genetic-functional space — broader than the older "the genes within the microbes" definition. [609] These definitions form the clinical foundation of every subsequent chapter.
Who is this book for?
We address four reader groups, and the structure provides a different path for each.
The healthy adult who wants to optimize wants to know what's worth doing. For them, chapter 4 (nutrition) and chapter 5 (sleep, movement, stress) offer the most useful entry — these contain the largest levers that measurably improve microbiome diversity within days.
The reader living with symptoms (chronic bloating, IBS-like patterns, alternating bowel) takes a different path. Chapter 3's evidence map and chapter 10's diagnostic guide give the structured answer: what to investigate with a doctor first, and what can be tried via lifestyle modification — without confusing the two.
The diagnosed patient — living with IBD, IBS, diabetes, an autoimmune, neurological, or oncological diagnosis — seeks microbiome-directed steps as an adjunct to primary therapy, never as a replacement. Chapter 3 tells you what to expect realistically for your specific disease (and what not to), chapter 7 covers drug–microbiome interactions, and chapter 11 the therapeutic toolbox.
The clinician (gastroenterologist, GP, dietitian, pharmacist) uses the book in collaboration with their patient. Every chapter contains Clinical deep-dive boxes with RCT-grade evidence, effect sizes, doses, and contraindications — the layer that lets the book speak simultaneously to the reader and to the treating physician.
How to use the book
Three parallel entry points to choose from.
Linear reading — chapter by chapter, 1 through 12. The structure moves from foundational concepts (1–2) through the disease evidence map (3), the lifestyle levers (4–7), life stages and personalization (8–9), diagnostics and therapy (10–11), and finally the action guide (12).
Question-based entry — if you have a specific question ("What should I eat after antibiotics?", "Is 16:8 fasting worth it?", "I have IBS, what should I do?"), the table of contents (`_book_en.md`) includes a question index that tries to answer each question by pointing to the relevant chapter and section.
Disease-based entry — if you live with a diagnosis or specific symptom, start with chapter 3's evidence map. From there, the order of chapters relevant to your profile (chapter 7 drugs, chapter 11 therapy, VII.5 (When to See a Doctor) red flags) follows naturally.
Every chapter follows the same template
For ease of navigation, every chapter is built from the same six elements:
- In one sentence box at the top — TL;DR if you're in a hurry or looking for a refresher.
- Lay text — main content with explanations and examples.
- Clinical deep-dive boxes — RCT references, effect sizes, doses, contraindications. If you're not a clinician and not interested, skip them.
- What does this mean for me? boxes — chapter- or section-level, profile-based interpretation.
- What you can do tomorrow — 3–5 concrete steps.
- ⚠️ When to see a doctor — red-flag symptoms, when a specialist is needed.
What this book does not contain
For transparency:
- Specific drug prescriptions. Always your treating physician's job.
- Daily diet plans. General principles and examples yes; exact daily menus no — those should be built with a dietitian, ideally personalized.
- Pseudo-scientific "detox" programs. Chapter 6 explains why we don't endorse them.
- Microbiome-cure hopes for autism, Parkinson's, or clinical depression. Chapter 3's and tiers give a nuanced picture — when standard therapeutic options are exhausted, microbiome-directed approaches may be worth exploring, but anyone offering them today as primary therapy stands on premature scientific ground.
Any lifestyle or microbiome-directed recommendation in this book is overridden by a red-flag symptom. Immediate medical consult is required if you experience: blood in stool (fresh or dark) / unintentional weight loss (>5% in 6 months) / nocturnal abdominal pain or diarrhea / persistent fever with abdominal symptom / family history of colorectal cancer under 50 or IBD. Detailed red flags: VII.5 When to See a Doctor chapter.
The book's authors
The MicroBiota Handbook is a joint work of the MicroBiome Bank team: Dr. Gabor Patay (Barcelona, FMT therapeutic protocol), Dr. Attila Bezzegh (Budapest, microbiota diagnostics), Dr. Anna Munar (Barcelona, exposome specialist, lifestyle–microbiome interactions). The three disciplines together ensure that neither the patient's nor the clinician's perspective has blind spots.
The book is a living document: the current v2.0 reflects the state of evidence as of May 2026, and updates follow the pace of clinical publications — likely annually.
Where to begin
If you're not yet sure which profile fits you, three suggestions:
- Read chapter 3 at least in overview. The evidence map is one of the book's most important chapters and grounds your own decisions.
- If you're after general "maintenance," jump to chapters 4 and 5 — these hold the fastest-returning lifestyle levers.
- If you have a specific life situation (an antibiotic course, pregnancy, a new diagnosis, caring for an elderly parent), the table of contents' question and disease indexes guide you to the right chapter.
You don't have to read the book in one go. It is also a reference: come back to it when a specific life situation arises.
What's next
Chapter 2 examines how your microbiome actually works — gut barrier, short-chain fatty acids, gut–brain axis, immune modulation, drug metabolism. With the mechanisms in hand, the lifestyle advice in chapters 4–7 stops looking like an arbitrary list and starts revealing visible cause-and-effect chains.
References
[561] Hill C, Guarner F, Reid G et al. The International Scientific Association for Probiotics and Prebiotics consensus statement on the scope and appropriate use of the term probiotic. Nat Rev Gastroenterol Hepatol. 2014. Link
An ISAPP expert panel convened in October 2013 reviewed the probiotic field 13 years after the original FAO/WHO definition. The panel affirmed the FAO/WHO definition — 'live microorganisms which when administered in adequate amounts confer a health benefit on the host' — as still relevant and sufficiently broad to accommodate current and anticipated applications. Inconsistencies between the FAO/WHO Expert Consultation Report and the FAO/WHO Guidelines were clarified in light of advances in science and use. The consensus statement promotes more precise use of the term 'probiotic' to help clinicians and consumers differentiate products on the market.
[609] Berg G, Rybakova D, Fischer D et al. Microbiome definition re-visited: old concepts and new challenges. Microbiome. 2020. Link
An international expert panel within the EU MicrobiomeSupport project (approx. 40 leaders, supported by over 100 online survey respondents) addressed the lack of a commonly agreed definition of the term 'microbiome'. The paper proposes a definition based on the compact, comprehensive description provided by Whipps et al. in 1988, supplemented by new recommendations reflecting the latest technological and research developments. A consensus on best practices and a clearer conceptual framework are intended to harmonize microbiome research across disciplines.
[610] Sender R, Fuchs S, Milo R. Revised estimates for the number of human and bacteria cells in the body. PLoS Biol. 2016. Link
Reported values for cell counts in the body vary by orders of magnitude and are rarely supported by measurements. Integrating up-to-date information, the authors estimate the total bacterial count in a 70 kg 'reference man' at 3,8·10^13. They estimate 3,0·10^13 human cells, with the hematopoietic lineage dominating the count (about 90%). The widely cited 10:1 bacteria-to-human-cell ratio is revised: bacterial and human cell numbers are of the same order, with a total bacterial mass of about 0,2 kg.
[2713] Marshall BJ, Warren JR. Unidentified curved bacilli in the stomach of patients with gastritis and peptic ulceration. The Lancet. 1984. Link
Marshall and Warren's 1984 Lancet paper identified the curved bacterium later named Helicobacter pylori on the gastric mucosa of patients with gastritis and gastric/duodenal ulceration, reinforced by Marshall's famous self-experiment. It rewrote ulcer pathology, replacing the prior "stress and acid" paradigm with an infectious model and enabling antibiotic eradication therapy. The work received the 2005 Nobel Prize in Medicine.