Bacteria

Bacteria are tiny living things that are found everywhere, from the air we breathe to the deep parts of the Earth. They are usually made of just one cell and are among the first life forms to have appeared on our planet. These small organisms play important roles in nature, such as helping to recycle nutrients and provide energy for other living things.

Most of the bacteria that live on and inside our bodies are harmless or even helpful, especially in our gut. However, some bacteria can make us sick and cause diseases like cholera, tuberculosis, and bubonic plague. Doctors use special medicines called antibiotics to treat these infections, but some bacteria are becoming harder to treat because they are developing resistance.

Bacteria are also useful in many ways. They help clean up oil spills, make foods like cheese and yogurt, and are used in industries to produce important chemicals and recover valuable metals. Scientists study bacteria in a field called bacteriology, which is part of the larger science of microbiology.

Etymology

The word bacteria is the plural form of the Neo-Latin word bacterium. This word comes from an ancient Greek word meaning 'staff' or 'cane'. The first bacteria discovered had a rod-like shape, which is why they were given this name.

Knowledge of bacteria

Scientists have named about 43,000 types of bacteria, but most of them haven’t been studied much. In fact, just 10 types make up half of all research papers, while 75% of named bacteria have no special studies on them. The most studied type, Escherichia coli, has over 300,000 research papers about it, though many of these papers use it to study other things instead of learning more about E. coli itself. Most research focuses on bacteria that can make people sick.

The ancestors of bacteria were the first living things on Earth, appearing about 4 billion years ago. For a long time, most living things were tiny, and bacteria were the main type of life. Even though we can find old bacterial fossils, it’s hard to learn much from them. But by studying genes, scientists can trace how bacteria evolved. These studies show that bacteria split off from other simple life forms very early in Earth’s history.

Habitat

Bacteria are found everywhere on Earth, from soil to deep underwater, even in very harsh places like hot springs and radioactive areas. There are thought to be about 2×10³⁰ bacteria on our planet, making up more living material than plants. They live in lakes, oceans, arctic ice, and geothermal springs, helping to create energy from substances like hydrogen sulphide and methane. Bacteria also live on and inside plants and animals, and most are helpful rather than harmful. They are very important for keeping soil healthy by breaking down waste and recycling nutrients. Even the air contains bacteria, with about one hundred million bacterial cells in every cubic metre. In the oceans and seas, bacteria help produce up to half of the oxygen we breathe. Only about 2% of all bacterial species have been fully studied.

Morphology

Bacteria come in many shapes and sizes. Most are very tiny, about 0.5 to 5.0 micrometres long, but some can be seen without a microscope. For example, Thiomargarita namibiensis can be up to half a millimetre long, and Thiomargarita magnifica can reach 2 cm, which is very big for a bacteria. Some of the smallest bacteria are in the genus Mycoplasma, measuring just 0.3 micrometres.

Most bacteria are either round, called cocci, or rod-shaped, called bacilli. Others have curved or spiral shapes. These shapes help bacteria move, get food, and stay safe. Some bacteria live alone, while others stick together in pairs, chains, or clusters. They can also form groups called biofilms on surfaces like rocks or plants, which help protect them.

Cellular structure

Bacteria are tiny living things made up of just one cell. They have a cell membrane that keeps everything inside the cell safe. This membrane holds important parts like nutrients and proteins. Unlike our own cells, bacteria do not have many tiny parts inside called organelles, such as a nucleus or mitochondria.

Bacteria do not have a nucleus to hold their DNA. Instead, their DNA floats in a part of the cell called the nucleoid. They also have small machines called ribosomes that help make proteins.

Around the outside of the cell, bacteria have a strong cell wall made of special materials. This wall helps the bacteria stay safe. Some bacteria can even form tough protective shells called endospores, which let them survive very harsh conditions.

Metabolism

Further information: Microbial metabolism

Bacteria have many different ways to get energy and grow. Some bacteria, called phototrophic bacteria, use light for energy through photosynthesis. Others, called chemotrophic bacteria, get energy by breaking down chemicals. These bacteria can be further divided into groups based on what they use for energy. Some use simple substances like hydrogen, while others use more complex organic materials.

Bacteria also differ in how they get carbon, which is needed to build their cells. Some bacteria, known as heterotrophs, get carbon from other organic materials. Others, like cyanobacteria, can make their own carbon from carbon dioxide. These bacteria are called autotrophic. In special cases, some bacteria can even use methane for both energy and carbon.

Bacterial metabolism helps keep nature balanced and is useful for humans. For example, some bacteria can change nitrogen gas into forms plants can use. Others help break down pollution. Some bacteria can make alcohol through a process called fermentation, which is used in making drinks like beer.

Reproduction and growth

Bacteria grow by getting bigger and then making copies of themselves through a process called binary fission. This is a way for them to reproduce without needing a partner. When conditions are good, bacteria can grow and divide very quickly, sometimes doubling in number every 17 minutes. In this process, one cell splits into two identical cells.

In labs, scientists grow bacteria using special foods called growth media. They can use solid foods like agar plates to keep one type of bacteria separate, or liquid foods when they need lots of bacteria. In nature, bacteria face limits on food, so they have developed different ways to survive and grow, sometimes working together in groups.

Bacterial growth has four stages. First is the lag phase, when bacteria adjust to new food sources. Next is the logarithmic phase, where they grow very fast. Then comes the stationary phase, when food runs low and growth slows down. Finally, in the death phase, bacteria die off when they run out of food.

Genetics

Most bacteria have one circular piece of DNA, called a chromosome. This chromosome can be very small or quite large, depending on the type of bacteria. Some bacteria also have small pieces of extra DNA called plasmids, which can help them survive in different environments.

Bacteria usually have only one copy of each of their genes, unlike humans who have two copies. This means if a important gene in a bacterium gets damaged, the bacterium cannot make the needed proteins and may not survive. Bacteria can change their genes through mistakes when copying their DNA or by picking up DNA from other bacteria, which helps them adapt and survive.

Behaviour

Movement

Many bacteria can move on their own using different ways. One common way is with flagella, which are like tiny tails that spin and help the bacteria move in a spinning motion. Bacteria can move forward and then tumble to change direction, making their path look like a random walk.

Bacteria can also move in other ways, such as twitching or gliding, using special structures on their surface. Some bacteria are attracted to or repelled by certain things in their environment, which helps them find food or avoid danger.

Communication

See also: Prokaryote § Sociality

Some bacteria can make light, which might help them attract larger animals. Bacteria often work together in groups called biofilms, sharing signals to help each other. This teamwork lets them protect themselves better and use resources more efficiently. They can also sense when there are enough of them to do certain tasks together, like breaking down food or making light.

Classification and identification

Scientists try to sort and name bacteria by grouping them based on what they look like and how they work. They look at things like the parts inside the bacteria, such as DNA, and how the bacteria get their energy. Sometimes it’s hard to tell if two bacteria are different kinds or just slightly different versions of the same kind, because bacteria can share pieces of their DNA with each other.

To help sort bacteria, scientists now use special tools that look at the tiny building blocks inside the bacteria. This helps them tell the exact name and kind of bacteria, which is very important when doctors need to treat infections caused by bacteria.

Classification by staining

The Gram stain, created long ago, helps scientists see the difference in the walls around bacteria. Some bacteria keep a purple color, while others turn pink. This helps group them into four main types.

Classification by culturing

Scientists grow bacteria in special dishes to see which ones can grow and what they need. They use different dishes for different kinds of samples, like mucus from the lungs or stool from the intestines, to find the bacteria that might be making someone sick.

Molecular classification

New ways to find bacteria use tiny pieces of the bacteria’s DNA. These methods are faster and more exact than growing the bacteria. They even help find bacteria that can’t be grown in a dish, by looking at DNA taken directly from the environment. This has helped discover thousands of new types of bacteria, and scientists think there may be millions more out there.

Phyla

See also: Bacterial phyla

Valid phyla

The following groups of bacteria have been officially recognized. Some of these do not belong to a larger group called a kingdom and are shown in bold:

• Abditibacteriota
• Acidobacteriota
• Actinomycetota
• Aquificota
• Armatimonadota
• Atribacterota
• Bacillota
• Bacteroidota
• Balneolota
• Caldisericota
• Calditrichota
• Chlamydiota
• Chlorobiota
• Chloroflexota
• Chrysiogenota
• Coprothermobacterota
• Cyanobacteriota
• Deferribacterota
• Deinococcota
• Dictyoglomerota
• Elusimicrobiota
• Fibrobacterota
• Fidelibacterota
• Fusobacteriota
• Gemmatimonadota
• Kiritimatiellota
• Lentisphaerota
• Minisyncoccota
• Mycoplasmatota
• Nitrospinota
• Nitrospirota
• Planctomycetota
• Pseudomonadota
• Rhodothermota
• Spirochaetota
• Synergistota
• Thermodesulfobacteriota
• Thermomicrobiota
• Thermotogota
• Verrucomicrobiota
• Vulcanimicrobiota

Candidate phyla

The following groups of bacteria have been suggested but not yet officially recognized. Some of these do not belong to a larger group called a kingdom and are shown in bold:

• "Acetithermota"
• "Aerophobota"
• "Auribacterota"
• "Babelota"
• "Binatota"
• "Bipolaricaulota"
• "Caldipriscota"
• "Calescibacteriota"
• "Canglongiota"
• "Cloacimonadota"
• "Cosmopoliota"
• "Cryosericota"
• "Deferrimicrobiota"
• "Dormiibacterota"
• "Effluvivivacota"
• "Electryoneota"
• "Elulimicrobiota"
• "Fermentibacterota"
• "Fervidibacterota"
• "Goldiibacteriota"
• "Heilongiota"
• "Hinthialibacterota"
• "Hydrogenedentota"
• "Hydrothermota"
• "Kapaibacteriota"
• "Krumholzibacteriota"
• "Kryptoniota"
• "Latescibacterota"
• "Lernaellota"
• "Lithacetigenota"
• "Macinerneyibacteriota"
• "Margulisiibacteriota"
• "Methylomirabilota"
• "Moduliflexota"
• "Muiribacteriota"
• "Nitrosediminicolota"
• "Omnitrophota"
• "Parcunitrobacterota"
• "Peregrinibacteriota"
• "Qinglongiota"
• "Rifleibacteriota"
• "Ryujiniota"
• "Spongiamicota"
• "Sumerlaeota"
• "Synechomicrobiota"
• "Tangaroaeota"
• "Tectimicrobiota"
• "Tianyaibacteriota"
• "Wirthibacterota"
• "Zhuqueibacterota"
• "Zhurongbacterota"

Interactions with other organisms

Further information: Microbes in human culture

Bacteria can have many different relationships with other living things. Some bacteria live on or in other organisms without causing harm. These are called commensals. For example, many bacteria live on our skin, in our airways, and in our guts. They usually do not hurt us but can sometimes cause infections if they move to other parts of the body.

Some bacteria can even help other organisms. For instance, in the soil, certain bacteria help plants by changing nitrogen in the air into a form plants can use. In our guts, bacteria help us make vitamins and break down food. Without these helpful bacteria, many animals, including humans, could not survive.

However, some bacteria can make us sick. These are called pathogens. They can cause diseases like tetanus, typhoid fever, and tuberculosis. Our bodies have ways to fight off these bacteria, but sometimes they can still make us ill. Doctors treat bacterial infections with medicines called antibiotics, which stop the bacteria from growing or kill them. It is important to use these medicines properly to prevent the bacteria from becoming resistant to them.

Significance in technology and industry

Bacteria, like certain types such as Lactobacillus and Lactococcus, have been used for thousands of years along with yeasts and moulds to make foods such as cheese, pickles, soy sauce, sauerkraut, vinegar, wine, and yogurt.

Bacteria can break down many different organic materials, which is useful for cleaning up waste and helping the environment. Some bacteria can eat the hydrocarbons in petroleum and are used to clean up oil spills. They are also used to break down industrial toxic wastes. In the chemical industry, bacteria help make pure chemicals that are used in medicines and farm chemicals.

Bacteria can also be used instead of pesticides to control pests. One example is Bacillus thuringiensis (BT), a soil bacterium used as a safe insecticide for certain insects. Because it only affects specific insects, it is considered friendly to the environment and does not harm humans, wildlife, pollinators, or most other helpful insects.

Bacteria are also very important in scientific research. Because they grow quickly and are easy to study, scientists use them to learn about genes, enzymes, and how cells work. This knowledge helps scientists understand more complex living things. Researchers also use bacteria to produce important proteins like insulin, growth factors, and antibodies. Samples of bacterial strains are kept in special centers so scientists all over the world can study them.

History of bacteriology

For the history of microbiology, see Microbiology. For the history of bacterial classification, see Monera § History, and Bacterial taxonomy. For the natural history of Bacteria, see Last universal common ancestor.

Bacteria were first seen by a Dutch scientist named Antonie van Leeuwenhoek in 1676. He used a microscope he made himself to look at tiny living things, which he called animalcules. These included bacteria and other small organisms.

Later, scientists began to study bacteria more closely. Louis Pasteur showed that tiny living things cause fermentation, helping us understand how they work. Robert Koch proved that specific bacteria cause diseases like tuberculosis, which helped doctors treat infections better.