Microbes and viruses

A bacterium is a complete living thing made of one cell, and a very small one. Five hundred of them, end to end, would measure about a millimetre. It is also simpler inside than any cell of your body. Your cells keep their DNA inside a compartment called the nucleus. A bacterium has none, and its DNA lies loose in the cell. Around the outside it has a stiff cell wall that holds its shape.

Most bacteria do people no harm, but some are dangerous pathogens. Bacteria cause cholera, plague, typhoid and tuberculosis, the lung disease that used to be called consumption. In the nineteenth century tuberculosis killed about one person in seven in Europe and the United States.

How does one bacterium become an infection? By dividing. A bacterium grows a little, copies its DNA and splits down the middle into two, each a complete new bacterium. This way of multiplying is called binary fission, and in good conditions, warm and well fed, some bacteria can do it every twenty minutes.

Follow the arithmetic. After twenty minutes there are two, after forty minutes four, after an hour eight. After two hours there are 64, and after seven hours more than two million, all from a single cell. Growth by repeated doubling starts slowly and then runs away. This is why food left in a warm kitchen spoils in an afternoon and keeps for days in a refrigerator, where the cold slows every division. It is also why a small infection can become a serious one overnight.

For most of history physicians blamed epidemics on bad air rising from swamps and rot. What to do in one was a separate matter, and for plague Muslims had a rule. A hadith recorded by al-Bukhārī says: if you hear of it in a land, do not enter it, and if it breaks out where you are, do not leave. Around 639 the caliph ʿUmar, on his way to Syria, learned at its border that plague had broken out. A companion recited these words to him, and he turned back.

When the Black Death swept Granada in 1348 and 1349, the scholar Ibn al-Khaṭīb set down what had been seen. The plague followed garments, vessels, even an earring from a stricken house. It reached healthy ports with travellers from infected ones, and people kept in isolation escaped. He concluded that it passes by contact from the sick to the healthy.

Proof came in the nineteenth century, and it began with clean hands. In 1847 the Hungarian doctor Ignaz Semmelweis noticed that mothers on a Vienna ward were dying of fever after being examined by doctors who had come straight from dissecting corpses. He made the doctors wash their hands in a chlorine solution first. Deaths on the ward fell from 18 per cent to about 2 per cent within months.

Why it worked became clear in the 1860s, when the French chemist Louis Pasteur showed that microbes make broth and milk go bad and that they come only from other microbes. Heating a liquid gently to kill them is still called pasteurisation. Then Robert Koch tied particular bacteria to particular illnesses. The idea that specific microbes cause specific diseases is the germ theory of disease.

A virus is far smaller than a bacterium and far simpler. A typical bacterium is about 2,000 nanometres long, a nanometre being a millionth of a millimetre. A typical virus is about 100, too small for any ordinary microscope. It is not a cell. It has no machinery for feeding, growing or making anything. A virus is only a set of genes, the instructions for building more of itself, wrapped in a coat of protein.

On its own a virus does nothing at all, which is why biologists hesitate to call it alive. Inside the right cell it becomes very active indeed. Viruses cause the common cold, influenza, measles, polio and COVID-19. Because they have none of the working parts of a bacterium, the drugs that kill bacteria do nothing to them.

A virus cannot copy itself. To multiply it has to get inside a living cell, called its host, and use that cell’s machinery.

The steps are always the same. First the virus attaches to the outside of a cell whose surface it fits. Next its genes pass inside. The cell cannot tell these instructions from its own, so it obeys them. It stops its normal work and begins to copy the virus’s genes and build new protein coats, hundreds or thousands of times over. Finally the new viruses burst out, often killing the cell, and each goes on to another.

This explains why a viral illness is hard to treat. The virus spends most of its time inside your own cells, using your own machinery, so a drug that stopped it would often harm the cells as well.

Pathogens are trying to get into you all day, and nearly always they fail. The skin keeps most of them out. Those that get past it meet the immune system, the body’s network of cells and organs that recognises invaders and destroys them.

Its first fighters are white blood cells that patrol the blood and the tissues. Some of them deal with a bacterium directly: the cell flows around it, takes it inside and digests it. This swallowing is called phagocytosis, from the Greek for cell eating.

The whole body can join in. In a fever the brain raises the body’s temperature a degree or two above its normal 37 °C. Many microbes multiply more slowly in the heat, and the body’s own defences work faster. A fever is unpleasant, but it is usually part of the cure and not the disease itself.

White blood cells have a second, more exact weapon. When a new pathogen appears, some of them start making antibodies: proteins shaped to grip one particular microbe and no other, as a key fits one lock. Antibodies stick to the invader, block it and mark it for destruction. The first time, building the right ones takes a week or more, which is why you are ill for a while.

Afterwards the body does not forget. Some of the cells that made the winning antibody live on for years as memory cells. If the same pathogen returns, they respond within days, usually before you feel anything. That is why most people catch measles only once.

If the body remembers a pathogen it has beaten, can it be taught the lesson without the illness? That is what a vaccine does. It contains a killed or weakened form of the microbe, or a harmless piece of it. The immune system responds as if to the real thing and keeps the memory. A vaccine trains the immune system in advance.

Smallpox shows what this can achieve. Around 900 the Persian physician al-Rāzī wrote a careful description of it, the first to set out how to tell it from measles. For centuries it went on killing about three in ten of those it infected. In 1796 Edward Jenner made the first vaccine against it, and in 1980 it was declared wiped out worldwide.

Vaccines protect the unvaccinated too. When enough people are immune, a pathogen cannot find its next victim and dies out. This is herd immunity.

In 1928 the Scottish scientist Alexander Fleming came back from a holiday to find a mould growing on a dish of bacteria he had left out. Around the mould the bacteria had died. The mould was making a substance that killed them, and he named it penicillin. By the 1940s it was being made in bulk, and infections that had been death sentences could be cured in days.

Penicillin was the first of the antibiotics, medicines that kill bacteria or stop them multiplying. They work by attacking parts that bacteria have and human cells lack. Penicillin, for one, stops a bacterium building its cell wall. That is also their limit. A virus has no cell wall and no machinery of its own to attack, so antibiotics do nothing against a cold or influenza.

Antibiotics have a weakness that grows with use. In any large population of bacteria, a few happen to carry a mutation, a chance change in their DNA, that lets them survive a particular drug. Normally these few are lost in the crowd. Then the antibiotic arrives. It kills the susceptible bacteria and spares the protected ones. With their rivals gone, only the resistant ones survive, and they divide to fill the space. The next infection they cause no longer answers to that drug. This is antibiotic resistance.

The drug does not create the resistant bacteria. It selects them, and every unnecessary course of antibiotics gives it another chance to do so. Resistant infections already kill more than a million people a year. This is why doctors refuse antibiotics for colds, and why a prescribed course should be taken exactly as directed.

Disease is the exception. Most of what microbes do keeps the living world running, and you carry the proof. About 38 trillion bacteria live in and on your body, roughly one for each of your own cells, and most of them are in your gut. This community is called your microbiome. Gut bacteria break down parts of food that you cannot digest, make some vitamins, and take up space and food that harmful bacteria would otherwise use.

Out of doors, microbes do an even larger job. When a leaf falls or an animal dies, bacteria and fungi feed on the remains and break them down into simple substances that plants can take up again. Living things that do this are called decomposers. Without them the soil would have run out of nutrients long ago.

People were putting microbes to work long before anyone had seen one. Stir a little yeast, a single-celled fungus, into bread dough and leave it somewhere warm. The yeast feeds on the sugars in the flour and gives off carbon dioxide gas. The bubbles are trapped in the stretchy dough and swell it, which is what makes a loaf rise and gives bread its holes.

This breaking down of sugars by microbes, without any need for air, is called fermentation, and different microbes give different products. The bacteria that turn milk into yoghurt make an acid, which thickens the milk and gives yoghurt its sour taste. The same acid stops most other microbes from growing, so fermented food keeps far longer than fresh. Cheese and traditional pickles are made in the same way.