The Invisible World of Microbes

A single gram of healthy garden soil, about the weight of a paper clip, can contain billions of bacteria. Microbes, the living things too small to see without a microscope, are ubiquitous: they thrive in boiling hot springs, in Antarctic ice, in clouds, in the deepest ocean trenches and all over your skin. They include bacteria, many fungi such as yeasts and molds, and single-celled organisms called protists. For almost all of human history, however, nobody knew that they existed.

The first person known to have seen bacteria and other single-celled creatures was Antonie van Leeuwenhoek, a cloth merchant in Delft, in the Netherlands, who ground his own tiny lenses as a hobby. His single-lens microscopes could magnify objects more than two hundred times, far better than other microscopes of his day. In 1676 he reported to the Royal Society in London that a drop of water was crowded with tiny creatures swimming in every direction, which he called animalcules. Many readers doubted him until other scientists, using his methods, saw the creatures for themselves.

Seeing microbes was one thing; proving what they did was another. Around 1860, the French chemist Louis Pasteur boiled broth in glass flasks with long, curving necks. Air could reach the broth, but dust and microbes settled in the bend of the neck, and the broth stayed clear and fresh. When he tilted a flask so that the broth touched the trapped dust, it soon spoiled. Pasteur concluded that spoilage came from microbes carried in from outside, not from the broth itself. In 1876, the German doctor Robert Koch showed that a particular bacterium caused the deadly animal disease anthrax, and in 1882 he identified the bacterium that causes tuberculosis. Together, their work established the germ theory of disease: the idea that specific microbes cause specific illnesses.

Germ theory has saved millions of lives, but it also gave microbes a bad reputation that most of them do not deserve. Only a small fraction of known microbes cause disease. Bacteria in your intestines break down fiber that your own body cannot digest, produce vitamin K, and help train your immune system to tell friend from enemy. Bacteria living in small lumps on the roots of peas and beans turn nitrogen from the air into a form that plants can use. Tiny ocean organisms, many of them bacteria, produce roughly half of the oxygen made on Earth each year. Even some medicines come from microbes: in 1928, Alexander Fleming noticed that a mold had stopped bacteria from growing in a laboratory dish, a discovery that led to penicillin, the first widely used antibiotic.

Today, scientists can identify microbes by reading their DNA, even the many kinds that refuse to grow in a laboratory dish. This has overturned some long-held beliefs. For decades, textbooks claimed that bacteria in the human body outnumbered human cells ten to one. When researchers recounted carefully in 2016, they found the numbers roughly equal: about 38 trillion bacterial cells to about 30 trillion human cells. The invisible world, it turns out, is still being mapped.