Microorganisms

How small is small?

A microorganism is a living thing too small to see without a microscope. Most are a single cell, though a few tiny animals made of many cells count too. Your eye can just make out something about 100 micrometers across, about the width of a thick hair. Most microbes are far smaller than that.

Scientists measure them in tiny units. A micrometer (µm) is one thousandth of a millimeter. A nanometer (nm) is one thousandth of a micrometer. Sizes vary a lot from one kind to the next, so the numbers below are typical ranges, not exact rules.

Biologists sort living things into three big groups called domains. Bacteria and archaea are prokaryotes: their cells have no nucleus. Protists and fungi are eukaryotes, like plants and animals: their cells keep their DNA inside a nucleus. Viruses don't fit in any domain, because they aren't made of cells.

Bacteria

Bacteria are single-celled prokaryotes. Each cell has a cell membrane, a tough cell wall around it, and usually one loop of DNA floating in an area called the nucleoid. Some swim with a whip-like tail called a flagellum.

Bacteria are often described by their shape. A sphere is a coccus (plural: cocci), a rod is a bacillus (plural: bacilli), and a spiral or curve is a spirillum (plural: spirilla). Cocci often stick together in pairs, chains, or bunches.

Most bacteria reproduce by binary fission. The cell copies its DNA, grows longer, and pinches in at the middle until it splits into two cells, each with a full copy. In a warm lab dish full of food, the gut bacterium E. coli can double its numbers in as little as 20 minutes. The bacterium that causes tuberculosis is much slower and takes about 15 to 20 hours.

Viruses

A virus is not a cell. It is a set of genes, made of DNA or RNA but never both, wrapped in a protein coat. It has no way to make energy or proteins on its own. To make copies of itself, a virus has to get inside a living host cell and use that cell's machinery. That is why viruses are called obligate intracellular parasites: they can't reproduce anywhere else.

Almost every kind of life has viruses that infect it, from bacteria to plants, fungi, and people. Most viruses are too small to see with an ordinary light microscope. Scientists study them with electron microscopes.

Are viruses alive? Scientists don't agree, and the answer depends on how you define life. On one side, a virus can't grow or reproduce outside a host cell, so some biologists call viruses non-living. On the other side, viruses have genes, they evolve, and they share a long history with the cells they infect, so other biologists say they belong among living things. Most textbooks avoid putting them in the tree of life because they aren't made of cells. It is fine to say "it depends."

Protists

Protists are eukaryotes that aren't plants, animals, or fungi. Most are a single cell, but that one cell can be surprisingly complex. Some hunt, some make their own food, and some do both.

• An amoeba moves by pushing out bulges of its cell called pseudopods, or false feet, and letting the rest of the cell flow after them. It wraps those same bulges around its food.
• A paramecium is covered in rows of tiny hairs called cilia. They beat together in waves, which sweeps the cell through the water and pushes food into a groove on its side.
• Algae make their own food by photosynthesis. Some are single cells, such as diatoms, which live in glassy shells. Others, such as kelp, grow into huge seaweeds. Algae make about 70 percent of the oxygen and food in water habitats.

A few protists cause disease. The best known is Plasmodium, which causes malaria and is carried by mosquitoes.

Fungi

Fungi (singular: fungus) are eukaryotes with tough cell walls made of chitin, the same material in an insect's outer skeleton. They can't make their own food. Instead they release chemicals that break food down outside their bodies, then soak up the small pieces.

• Yeasts are single-celled fungi. Many reproduce by budding, growing a small new cell off the side of the old one. Baker's yeast feeds on sugar in dough and gives off carbon dioxide gas, and the bubbles make bread rise.
• Molds grow as long, thin threads called hyphae. A tangle of hyphae is a mycelium. The fuzz on old bread is a mold's mycelium, and the dark dots are cases full of spores.
• Mushrooms are the multicellular cousins. A mushroom is the fruiting body of a much bigger fungus whose mycelium spreads through soil or wood, mostly out of sight. The mushroom's job is to make and release spores.

Helpful microbes

The word "germ" makes microbes sound like enemies, but only a small share of them cause disease. Life on Earth would stall without the rest.

• Decomposers. Bacteria and fungi break down dead plants, animals, and wastes, returning their nutrients to the soil for new plants to use.
• Gut bacteria. Trillions of bacteria live in your intestines, where they help process nutrients from your food and keep harmful microbes in check. When strong antibiotics wipe out many of them, a harmful species can take over. A 2016 estimate put the total at about 38 trillion bacteria in a typical adult, about as many as the roughly 30 trillion human cells. You may have heard that bacteria outnumber your own cells ten to one. That older figure turned out to be too high.
• Food. Bacteria turn milk into yogurt and cheese. Yeast makes bread rise. Molds of the genus Penicillium give blue cheeses their blue veins.
• Oxygen. Scientists estimate that about half of the oxygen made on Earth comes from the ocean, mostly from drifting plankton: algae and some bacteria that photosynthesize. The exact share is hard to pin down because it changes all the time.
• Nitrogen fixation. Plants need nitrogen, and the air is mostly nitrogen gas, but plants can't use it in that form. Among living things, only prokaryotes can turn it into ammonia, a form plants can use. Bacteria called Rhizobium live in bumps, called nodules, on the roots of beans, peas, and other legumes and do this for the plant.

Germs that cause disease

A microbe that causes disease is called a pathogen. Every group has a few.

How germs spread

A pathogen has to get from one host to the next. Scientists group the main routes like this.

Each route has a matching defense. Washing your hands is one of the best ones. The CDC's method is short: wet your hands with clean running water, add soap, lather the backs, between your fingers, and under your nails, scrub for at least 20 seconds (about two rounds of "Happy Birthday"), rinse, and dry. When there's no soap and water, a hand sanitizer with at least 60 percent alcohol helps. Coughing into your elbow, cooking food well, drinking safe water, and avoiding bites from mosquitoes and ticks block the other routes.

Immunity and vaccines

Your skin is the first barrier. It is several layers thick, and its dead outer cells keep flaking off, taking microbes with them. Mucus in your nose and throat traps many more. Germs that get past those barriers meet your white blood cells.

Some white blood cells swallow invaders whole. Others, called B cells, make antibodies: proteins shaped to latch onto one particular germ. Antibodies can block a germ from getting into cells and mark it so other white blood cells destroy it. The first time your body meets a new germ, building the right antibodies takes days, which is why you get sick while it catches up.

When the infection is over, some B cells and some T cells (another kind of white blood cell) stay behind as memory cells. If the same germ shows up again, they respond much faster and more strongly, and often stop it before you feel ill. That is what it means to be immune.

A vaccine trains the immune system ahead of time. It contains an antigen, something the immune system learns to recognize. That might be a weakened or killed germ, a piece of its outer surface, its genetic instructions for making that piece, or a toxin made harmless. The body responds by making antibodies and memory cells, without the person having to catch the disease first. The immune response to a vaccine can make a person feel tired or uncomfortable for a day or two, while the body builds its defenses.

Vaccines also protect people who aren't immune themselves, such as babies too young for a vaccine. When most people in a community are immune, a germ runs out of new hosts and has trouble spreading. This is called herd immunity, or community immunity.

The idea has a long history. In 1796, Edward Jenner used material from a cowpox sore to protect a boy from smallpox. Vaccination later wiped out smallpox: the last natural case was in 1977, and the World Health Assembly declared the world free of it in 1980.

Antibiotics and resistance

An antibiotic is a medicine that kills bacteria or stops them from growing. It works by attacking parts that bacteria have and your cells don't, such as the bacterial cell wall.

Antibiotics don't work on viruses. Viruses don't have those parts, and they hide inside your own cells. So an antibiotic can't cure a cold, the flu, or most sore throats. Some viral infections can be treated with different medicines, called antivirals.

Some bacteria survive a dose of antibiotic because of a lucky difference in their genes. Those survivors multiply and pass the difference on, and soon the antibiotic doesn't work against that strain. This is antibiotic resistance, and it is natural selection happening fast, which you can read more about in the Evolution unit. Using antibiotics when they aren't needed, such as for colds and flu, speeds resistance up. That is why doctors don't prescribe them for viral infections, and why a patient should take an antibiotic exactly as the doctor says.

Who found them?

For most of history, nobody knew microbes existed. These are some of the people who changed that. The dates come from their letters and papers, but when sources disagree by a year, it's noted.

Key words

• Microorganism (microbe)A living thing too small to see without a microscope.
• ProkaryoteA cell with no nucleus. Bacteria and archaea are prokaryotes.
• Binary fissionThe way bacteria reproduce: one cell copies its DNA and splits into two.
• VirusGenes in a protein coat that can only make copies of itself inside a host cell.
• HyphaeThe long, thin threads that make up the body of a mold or other fungus.
• PathogenA microbe that causes disease.
• VectorAn animal, such as a mosquito or tick, that carries a pathogen from one host to another.
• AntibodyA protein made by white blood cells that latches onto one particular germ.
• Memory cellA white blood cell that remembers a germ and responds quickly if it comes back.
• Antibiotic resistanceWhen bacteria evolve so that an antibiotic no longer kills them.

Each card describes one microbe. Sort it into the right group: bacterium, virus, protist, or fungus. The four buttons stay in the same order every time, like four bins.

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