The Tiny World of Microbes

An Invisible Universe

Right now, you are surrounded by living creatures you cannot see. They cover your skin, swarm inside your mouth, drift through the air you breathe, and live by the trillions in your gut. They cling to every doorknob, float in every drop of pond water, and fill the soil beneath your feet. These creatures are microbes — living things so small that millions of them could fit on the head of a pin.

For almost all of human history, nobody knew they existed. People got sick, food went mouldy, and bread mysteriously rose, but the cause was completely invisible. Then, a few hundred years ago, a curious inventor pointed a homemade lens at a drop of water and discovered an entire hidden universe teeming with life. This is the story of that universe, and of the tiny beings that shaped our planet and keep us alive.

Discovering the Unseen

The microbe-hunting began in the 1670s with a Dutch cloth merchant named Antonie van Leeuwenhoek. He ground tiny glass lenses far better than anyone before him and built simple microscopes that could magnify hundreds of times. When he peered through them at rainwater, scrapings from his teeth and other samples, he was astonished to see tiny creatures swimming and wriggling about. He called them "animalcules" — little animals. He was the first human ever to see bacteria.

For nearly two hundred years, microbes remained a curiosity. Then, in the 1800s, scientists made a world-changing discovery. The French scientist Louis Pasteur and the German doctor Robert Koch proved that certain microbes were the cause of many diseases. This idea is called germ theory, and it transformed medicine forever. Once doctors understood that invisible germs spread illness, they began to wash their hands, sterilise instruments, and keep wounds clean — saving countless millions of lives.

Meet the Microbes

"Microbe" is not one kind of creature. It is a word for any living thing too small to see without a microscope. There are several very different groups, and telling them apart is the first step to understanding the invisible world.

Bacteria are single-celled creatures, each one a complete tiny organism. They come in many shapes — round balls, straight rods and twisting spirals. Bacteria are tough survivors, found almost everywhere on Earth: in boiling hot springs, in frozen ice, deep underground and inside your own body. Most are harmless, many are helpful, and only a few cause disease.

Viruses are stranger still, and far smaller than bacteria. A virus is little more than a packet of genetic instructions wrapped in a protective coat. The puzzling thing about viruses is that they cannot grow, feed or reproduce on their own. A virus can only multiply by invading a living cell and hijacking it, forcing the cell to make copies of the virus. Because of this, scientists still argue about whether viruses are truly alive. Colds, flu, chickenpox and measles are all caused by viruses.

Fungi include the moulds that grow on old bread and the yeasts that make bread rise and turn fruit juice into other drinks. Some fungi are microscopic, while others, like mushrooms, grow large enough to see. Many fungi are nature's recyclers, breaking down dead leaves and wood.

Protists are a mixed group of single-celled creatures that don't fit neatly into the other groups. Some, like the amoeba, ooze along by changing shape. Others swim with tiny whips or hairs. A few protists cause serious diseases, such as malaria, which is spread by mosquitoes.

When Microbes Make Us Ill

When most people think of germs, they think of getting sick — and it is true that some microbes are dangerous. A microbe that causes disease is called a pathogen.

Pathogens make us ill in different ways. Some bacteria release toxins, harmful chemicals that poison our cells. Viruses damage the body by bursting the cells they invade and spreading to new ones. As pathogens multiply inside us, our body fights back, and the battle is what produces the symptoms we feel: a fever, a runny nose, a sore throat or an upset stomach.

Luckily, your body has a powerful defence called the immune system. It is like a tireless army of cells that patrols your body, hunts down invaders, and destroys them. When you recover from an illness, your immune system often "remembers" that particular germ, so it can defeat it much faster if it ever returns. This is why you usually catch chickenpox only once.

We have also invented ways to help. Vaccines are one of humanity's greatest achievements. A vaccine trains your immune system to recognise a dangerous germ without you having to suffer the real disease first, so you are protected in advance. Antibiotics are medicines that kill bacteria or stop them multiplying, and they have saved hundreds of millions of lives. But there is a catch: antibiotics only work against bacteria, not viruses. Taking them for a cold is useless, and overusing them can let bacteria become resistant, which is one of the biggest challenges facing medicine today.

The Microbes That Keep Us Alive

Here is the surprise that overturns everything most people assume about germs: the vast majority of microbes are not enemies at all. Many are absolutely essential to keeping us — and the whole planet — alive.

Inside your gut live trillions of helpful bacteria, together known as your microbiome. These bacteria help you digest food, make certain vitamins your body needs, and crowd out harmful germs so they cannot take hold. In a very real sense, you could not stay healthy without your microbial partners. There are roughly as many bacterial cells living in and on you as there are human cells that make up your body.

Microbes shape the world far beyond our bodies. In the soil, bacteria and fungi break down dead plants and animals, returning their nutrients so new plants can grow. Without these recyclers, the world would be buried under dead material and nothing new could grow. Other microbes pull nitrogen from the air and turn it into a form plants can use as natural fertiliser.

Microbes also make our food and medicine. Yeast makes bread rise and is used to brew and bake. Bacteria turn milk into yoghurt and cheese. Microbes are used to produce life-saving medicines and to clean up pollution. Some scientists even study microbes that eat oil spills or break down plastic waste.

The First Life on Earth

Microbes are not just everywhere today — they were here first. Long before there were dinosaurs, fish, plants or trees, the only life on Earth was microbial. For around the first two billion years of life's history, the entire planet was ruled by single-celled microbes, and nothing larger existed at all.

These early microbes changed the planet in an astonishing way. Tiny ocean bacteria began to capture sunlight and release oxygen as waste, slowly filling the air with the oxygen that every animal — including you — now needs to breathe. Without those ancient microbes, the air would be unbreathable and complex life could never have appeared. Every creature alive today, from the tiniest insect to the largest whale, is descended from those first microbial ancestors.

What We Learned

Microbes are living things too small to see without a microscope, and they were invisible to humanity until the microscope revealed them. They include bacteria, viruses, fungi and protists, each very different from the others. A small number are pathogens that cause disease, fought off by our immune system and by tools like vaccines and antibiotics. But most microbes are harmless or essential — they live inside us and keep us healthy, recycle the planet's nutrients, make our food, and even created the oxygen we breathe. The tiny world of microbes is invisible, but it rules the living world.

To explore how the body fights off these invaders, read How Our Body Works: A First Guide. Or discover the doctors and scientists who first understood germs in A Short History of Medicine.