Introduction: A look in the mirror that changes everything
Imagine for a moment that you're looking in the mirror. You see yourself—an individual, a person. But what if we told you that this image, however familiar, is only a small part of the truth? What if we revealed to you that what you think of as "yourself" is actually a walking, breathing, and thinking ecosystem—a superorganism populated by trillions of invisible cohabitants?
It sounds like the beginning of a science fiction novel, but it's one of the most fascinating realities of modern biology. The scientific community now agrees: purely in terms of cell count, we are more microbes than humans. Estimates suggest that our bodies contain about 10 times more bacterial cells than human cells. So the next time you look in the mirror, remember: you're looking at a bustling metropolis, a complex civilization whose tiny inhabitants work tirelessly to keep you alive.
In this post, we'll take you on a journey into this inner universe. We'll explore who these mysterious passengers are, where they come from, and the crucial roles they play in our health, our mood, and even our thoughts. Buckle up, because you'll see yourself with completely new eyes afterward.
Chapter 1: The Dizzying Arithmetic of Life: More than Just Numbers
The claim that we are 90% bacterial is shocking at first glance. How can that be? We feel human. The answer lies in size. Bacterial cells are many times smaller than human cells. While they don't make up 90% of our body volume or weight (the entire microbiome weighs an estimated 1-2 kilograms, about the same as our brain), when counted individually, they vastly outnumber us.
But the true dimension of this symbiosis only becomes clear at the genetic level. The human genome, the complete set of our DNA, comprises approximately 20,000 to 25,000 genes. These genes determine everything from our eye color to our susceptibility to certain diseases. An impressive number, isn't it? Well, hold on tight: The collective genome of our microbiome—the so-called microbiome genome—contains an estimated 2 to 20 million genes. That's at least 100 times more genetic information than we possess ourselves.
What does this mean? It means that countless biochemical processes in our body, from the digestion of complex carbohydrates to the production of vital vitamins, are controlled not by our own genes, but by those of our bacterial partners. Over the course of evolution, we have essentially outsourced countless tasks to these specialists. We are not a standalone fortress, but the conductor of a vast, genetically diverse orchestra. This realization has revolutionized medicine and forever abolished the rigid separation between "human" and "environment." We are not individuals in the classic sense, but rather walking symbioses—a perfect example of how cooperation is the foundation of life.
Chapter 2: The journey begins: How we get to our unique microbiome
No one is born a sterile, uninhabited island. The colonization of our bodies with microbes is one of the first and most important experiences of our lives. It's a carefully choreographed handover from one generation to the next, an invisible legacy that will shape our health throughout our lives.
The journey begins at birth. During a natural birth, the newborn comes into intensive contact with the mother's vaginal and intestinal flora. This is no coincidence, but an ingenious move by nature. The baby is essentially vaccinated with a "starter kit" of beneficial bacteria, particularly strains such as Lactobacillus and Bifidobacterium . These pioneering bacteria colonize the child's still sterile intestine and lay the foundation for a healthy ecosystem. They train the young immune system and protect against the colonization of harmful germs.
The next crucial phase is breastfeeding. Breast milk is far more than just food. It's a living elixir, rich in antibodies, enzymes, and—you guessed it—bacteria. Even more astonishing, however, is its content of so-called human milk oligosaccharides (HMOs). These are complex sugar molecules that the baby itself can't digest. So what are they for? They're the perfect, tailor-made food for the beneficial bifidobacteria in the infant's gut. Thus, the mother is not only feeding her child, but also specifically feeding its most important microbial helpers.
During the first years of life, the microbiome develops rapidly. Every contact with the environment adds new species: crawling on the floor, a loving lick from the family dog (studies show this increases microbial diversity and can reduce the risk of allergies), contact with siblings, and the introduction of solid food. By around three years of age, a relatively stable, unique microbiome has been established, one as individual as a fingerprint. It tells the story of our origins, our diet, and our lifestyle.
Chapter 3: The invisible architects of our health
What exactly do these trillions of microbes do for us all day? The answer is as complex as it is fascinating. They are not passive passengers, but active participants in almost every aspect of our physiology.
1. The masters of digestion and nutrient production: Our own bodies are surprisingly bad at breaking down certain plant fibers. This is where our gut bacteria come in. They ferment these indigestible fibers and convert them into a goldmine of beneficial substances. These include short-chain fatty acids (SCFAs) like butyrate, which is the main energy source for our intestinal wall cells and has anti-inflammatory properties. Without our microbes, we would miss out on valuable nutrients and energy. They also synthesize vital vitamins that we cannot produce ourselves, or only insufficiently, such as vitamin K (important for blood clotting) and various B vitamins (essential for energy metabolism).
2. The trainers of our immune system: Approximately 70-80% of our immune system is located in the gut. This is no coincidence. The gut is the largest contact area with the outside world. Our microbiome acts here as a kind of training camp for the immune cells. From birth, it teaches them to distinguish between friend (beneficial bacteria, food) and foe (pathogenic germs). A diverse and balanced microbiome ensures a strong yet tolerant immune system. An impoverished microbial community, on the other hand, is associated with an increased risk of allergies, asthma, and autoimmune diseases, in which the immune system overreacts and attacks the body.
3. The "Gut-Brain Axis": Our Second Brain Connection: One of the most exciting discoveries in recent years is the "gut-brain axis," a direct communication link between our gut and our brain. Our gut bacteria can influence the production of neurotransmitters. Amazingly, about 95% of the "happy hormone" serotonin is produced in the gut, significantly influenced by our microbial inhabitants. They communicate with the brain via the vagus nerve, the immune system, and hormonal signals. A healthy gut microbiome is therefore increasingly associated with better mood, lower stress levels, and even better cognitive function. It's scientific proof of the old saying: "Your gut instinct is always right."
Conclusion: The art of being a good host
The realization that we are a superorganism is more than just a biological curiosity. It fundamentally changes how we should think about health, illness, and ourselves. We are not isolated beings battling a hostile world full of germs. We are the guardians of a complex inner garden whose diversity and balance are crucial to our well-being.
Every meal we eat, every step we take in nature, and every lifestyle choice we make is also a choice for or against our trillions of microbial partners. They don't ask for much—a varied, high-fiber diet, less stress, and conscious use of medications. In return, they work tirelessly as our personal pharmacists, coaches, and mood enhancers.
The next time you look in the mirror, you may still see only one person. But perhaps you'll now smile knowing that you're the host of the largest, most important, and most fascinating community in the world. And being a good host is one of the most rewarding jobs there is.