Bacteria and Viruses

Take a pinch of soil and you are holding more living things than there are people on Earth, all of them invisible. Microbes, organisms too small to see, were the only life on the planet for most of its history, and they still dominate it in numbers, in mass and in chemical influence. Two kinds matter most to us: bacteria, which are alive, and viruses, which may not be.

Bacteria

A bacterium is a single cell, usually a few thousandths of a millimetre long, with no nucleus and no internal compartments. Its DNA floats free in a loop; its machinery is simple. Yet bacteria are the most successful organisms that have ever existed. They live in boiling springs, in Antarctic ice, kilometres down in solid rock, in acid, in radioactive waste and in the clouds. There are an estimated five million trillion trillion of them on Earth, and their total weight exceeds that of all animals combined.

They reproduce by splitting in two, some as often as every twenty minutes, so a single cell can become billions in a day. They also swap genes with one another directly, passing useful traits sideways between unrelated strains, which is how resistance to an antibiotic can spread through a hospital within years.

What bacteria do for the world

Without bacteria, life would stop. They break down dead plants and animals and return their nutrients to the soil. They take nitrogen from the air, which plants cannot use, and convert it into forms plants can absorb; every protein molecule in your body contains nitrogen that was fixed by a microbe. Photosynthetic bacteria in the ocean produce a large share of the oxygen you breathe, and their ancestors produced all of it in the first place.

Inside you, about 38 trillion bacteria, roughly equal to your own cell count, live mostly in your gut. This microbiome digests fibre you cannot, makes vitamins, trains your immune system and crowds out harmful invaders. Antibiotics that wipe it out can leave a person vulnerable until it regrows.

When bacteria harm

A small minority of bacteria cause disease, by invading tissue, by releasing toxins, or both. Tuberculosis, cholera, plague, typhoid, tetanus and many kinds of pneumonia and food poisoning are bacterial. For most of history they were the leading cause of death. Then in 1928 Alexander Fleming noticed that a mould contaminating one of his culture dishes had killed the bacteria around it. The substance it made, penicillin, became the first antibiotic, and the drugs that followed have saved hundreds of millions of lives. Bacteria, however, evolve. Strains resistant to most or all antibiotics are now spreading, and infections that were easily cured are becoming dangerous again. Resistance is considered one of the greatest threats to global health.

Viruses

A virus is far smaller and far simpler than a bacterium: a package of genetic material, DNA or RNA, wrapped in a protein coat, sometimes with an oily envelope. It has no cells, no metabolism and no machinery of its own. On its own it does nothing at all. It is, in effect, a set of instructions looking for a factory.

The factory is a living cell. A virus attaches to a cell whose surface it fits, slips its genes inside, and hijacks the cell's machinery to make thousands of copies of itself, which burst out or bud off to infect more cells. Whether this counts as being alive is debated; viruses sit at the border of life and chemistry.

Viruses infect every form of life. The most numerous are those that attack bacteria, the bacteriophages, which kill about a fifth of the ocean's bacteria every day and shape the whole marine ecosystem. Human viral diseases include the common cold, influenza, measles, chickenpox, polio, HIV, Ebola, rabies, smallpox and COVID-19. Because viruses use our own cells' machinery, they are hard to attack with drugs without harming the cells, and antibiotics, which target bacterial structures, have no effect on them at all.

Fighting back

The body's immune system learns to recognise a virus and destroy infected cells, which is why most infections end and why you rarely catch the same one twice. Vaccines exploit this by showing the immune system a harmless piece or copy of the virus so it is ready in advance. Vaccination eradicated smallpox from the planet in 1980, has nearly eliminated polio, and in 2020 was developed against a new coronavirus within a year of its appearance.

Why it matters

Microbes are the foundation of every ecosystem and the oldest and most abundant life on Earth. They also remain our deadliest natural enemies: pandemics are not a thing of the past, and influenza alone kills hundreds of thousands of people a year. Understanding the smallest living things is understanding both the engine of the biosphere and the threat that has shaped human history.