The Universe Inside You: Meeting Your Microbial Partners

Have you ever wondered what's actually happening inside your body right now, at this very moment, as you read these words? No, we're not talking about your heart beating or your lungs breathing—though those are pretty amazing too. We're asking something much more interesting: What else is living inside you?

The answer might surprise you. In fact, it might absolutely blow your mind! Right now, as you sit there reading this book, you are not alone. You have never been alone, not since the moment you were born. Hidden away in places you can't see—deep inside your intestines, on the surface of your skin, even in your mouth—there is an entire universe. A universe made up of millions and millions of tiny, tiny living things. These living things are so small that you would need a microscope just to see them. And here's the most incredible part: these little creatures outnumber your own human cells.

That's right—you are more bacteria, virus, and fungus than you are human! This might sound like something from a science fiction movie, but it's absolutely true. Scientists who study these tiny life forms have discovered that the average human body contains about 30 trillion human cells, but it also contains about 38 trillion microbial cells. If you do the math—and scientists definitely did the math—you'll find that the ratio is roughly 1.3 microbial cells for every single human cell in your body.

Let that sink in for a moment. You are walking around with more foreign organisms inside you than the number of cells that actually carry your own DNA. Your body is like a planet, and these microorganisms are like the billions of citizens who call your body home. They eat when you eat. They grow when you grow. Some of them even help determine how you feel each day. You are, in the most literal sense, a world teeming with life.

What Exactly Is This Hidden Universe?

Scientists have a special name for this community of microorganisms living in and on your body. They call it your microbiome (MY-kro-BY-om). But before we dive deeper into the microbiome, let's make sure we understand some important words that will appear throughout this book. Words are like keys to a treasure chest, and once you know them, a whole new world of understanding opens up.

The first important word is microorganism (MY-kro-OR-gan-iz-em). This is simply a fancy way of saying "a living thing that is so small you need a microscope to see it." The word "micro" means small, "organism" means a living thing, and when you put them together, you get a tiny living thing. These microorganisms are everywhere—on your desk, in the air you breathe, on the food you eat, and, most importantly for our story, inside you. Microorganisms include bacteria, viruses, fungi, archaea, and even some tiny single-celled creatures called protists. Don't worry if these words are new to you—we'll meet each of them in detail as we explore this book together.

The second word you need to know is microbiota (MY-kro-by-OH-ta). This refers specifically to the collection of different types of microorganisms that live in a particular environment. So when scientists talk about your gut microbiota, they mean all the different kinds of microorganisms living in your digestive system. Your mouth has its own microbiota, your skin has a microbiota, and yes, even your belly button has its own unique microbiota. In fact, scientists have discovered that each person's belly button contains a unique community of bacteria, and some of these bacteria have only ever been found in human belly buttons. It's like having your very own fingerprint made of invisible creatures!

The third word, microbiome, is closely related to microbiota but has a slightly different meaning. While microbiota refers to the actual organisms (the "who"), the microbiome refers to the entire environment where they live, including all their genetic material. Think of it this way: if your microbiota is the collection of tenants living in an apartment building, then your microbiome is both the tenants AND the building itself, plus all the interactions between them. Scientists often use these words interchangeably in everyday conversation, but technically, the microbiome includes the genes of all those microorganisms, which is actually pretty important. The combined genetic material of all the microorganisms in your body is about 150 times larger than all of your own human genes! Your microbiome contains somewhere between 2 and 20 million unique genes, depending on how you count them, while your human genome contains only about 20,000 genes. In genetic terms, you are more your microbiome than you are yourself.

The fourth word is microbial diversity (MY-kro-BY-ul dih-VER-sih-tee). This is a measure of how many different types of microorganisms are present in a particular environment. A microbiome with high microbial diversity is like a healthy rainforest full of many different species of plants and animals. A microbiome with low microbial diversity is more like a cornfield where only one type of crop grows. Generally speaking, scientists have found that higher microbial diversity is associated with better health, while lower diversity has been linked to various diseases. So when it comes to your microbiome, variety is definitely the spice of life!

The Tiny Tenants: A Story of Living Together

Now that we have our vocabulary sorted out, let's imagine your body as a building—a very special building where millions of tiny tenants have made their home. Imagine walking into this building and hearing all the sounds of life: tiny creatures going about their daily business, some busy building, some busy eating, some busy chatting with their neighbors. This is what's happening inside you right now.

Your digestive system, particularly your intestines, is like the penthouse suite of this building. It's warm, it's dark, it's full of nutrients, and it's the perfect place for microorganisms to live. In fact, your gut is home to the largest concentration of microorganisms in your entire body. Scientists estimate that over 1,000 different species of bacteria alone call your gut home, and that's not even counting the viruses, fungi, and other microorganisms. Each of these species plays a different role, kind of like how different people in a city have different jobs. Some help digest your food, some produce vitamins, some train your immune system, and some just seem to be along for the ride.

Your skin is another popular neighborhood for microorganisms. It's like the outside of your apartment building—exposed to the elements and constantly in contact with the outside world. Your skin has different environments too. The dry, sunny areas like your forearms have different microbial communities than the warm, moist areas like your armpits or between your toes. Have you ever wondered why your feet smell after you take off your shoes? That's the work of millions of bacteria happily breaking down the sweat and dead skin cells on your feet. It's not the most glamorous job in the world, but someone—or rather, some microscopic organism—has to do it.

Your mouth is yet another ecosystem within your body. Think of it as the lobby of your internal building, a warm, wet entrance where all kinds of traffic comes and goes. Every time you eat, drink, kiss someone (yes, even that transfers microorganisms!), or simply breathe, you're inviting new microbial visitors into your oral microbiome. Your mouth is home to over 700 species of microorganisms, making it one of the most diverse microbial habitats in your body. These tiny tenants spend their days breaking down food particles, and when you forget to brush your teeth, they can really throw a party and cause cavities. That's why dental hygiene is so important—you're trying to keep the microbial party from getting out of control!

Even your nose isn't safe from microbial invasion. Recent research has shown that the nasal microbiome is