Bacteria are microscopic, single-celled organisms found almost everywhere on Earth. They live in soil, water, air, food, and inside and on the bodies of plants and animals. Some cause disease, but most do not. Many bacteria are essential to ecosystems, food production, agriculture, and human health.
Despite their tiny size, bacteria are remarkably diverse. They can obtain energy from sunlight, organic matter, inorganic chemicals, or interactions with other organisms. Some thrive in ordinary environments such as freshwater and soil, while others live in places with extreme heat, acidity, salinity, or pressure.
Understanding bacteria starts with a basic distinction: bacteria are living cells, but they are not built like the cells of plants, animals, fungi, or other eukaryotes. Their simpler cellular organization gives them a different way of storing genetic information, reproducing, obtaining nutrients, and adapting to changing environments.
What are bacteria?
Bacteria are prokaryotic microorganisms, meaning their cells do not have a membrane-bound nucleus. Their DNA is located in a region of the cell called the nucleoid rather than enclosed inside a nucleus.
A typical bacterial cell contains a cell membrane, cytoplasm, ribosomes, and DNA. Many also have a cell wall, and some possess additional structures such as capsules, flagella, or pili. Although a bacterial cell is structurally simpler than a typical animal or plant cell, it can carry out all the basic processes necessary for life.
Bacteria belong to the domain Bacteria, one of the major branches of cellular life. They are distinct from archaea, another group of prokaryotic organisms. Archaea may resemble bacteria in their overall cell organization, but they differ in important aspects of their genetics, cell membranes, and biochemistry.
The word “bacteria” refers to an enormous range of organisms rather than a single type of microbe. Some bacterial species are harmless residents of the human body, some are important decomposers, some produce useful compounds, and others can cause infections.
How big are bacteria?
Most bacteria are measured in micrometers, or millionths of a meter. Individual cells are generally too small to see without a microscope.
Their small size has important biological consequences. A bacterium has a large surface area relative to its volume, allowing substances such as nutrients and gases to move efficiently across its cell membrane. It also means that bacterial populations can contain enormous numbers of cells without occupying much physical space.
Bacterial cells come in several characteristic shapes. Cocci are roughly spherical, bacilli are rod-shaped, and spirilla or related forms have curved or spiral shapes. Some bacteria form chains, clusters, pairs, or other distinctive arrangements after dividing.
Shape is useful for identifying bacteria, but it does not by itself determine how a bacterium lives or what it does.
How are bacterial cells organized?
Although bacterial cells lack the complex internal compartments found in eukaryotic cells, they have specialized structures that perform essential functions.
The cell membrane
The cell membrane forms the boundary between the bacterium and its surroundings. It controls which substances enter and leave the cell and plays a central role in energy production.
Bacteria do not have mitochondria. Instead, important energy-generating processes associated with cellular respiration take place at the cell membrane.
The cell wall
Many bacteria have a cell wall outside the cell membrane. It helps maintain the cell’s shape and protects it from environmental stresses, including changes in water pressure.
A major component of the cell walls of many bacteria is peptidoglycan, a mesh-like material made from sugars and amino-acid derivatives. The structure of the cell envelope differs between major bacterial groups, which is one reason laboratory staining methods can distinguish them.
The cytoplasm and ribosomes
The cytoplasm is the cell’s internal environment, where many chemical reactions occur. Bacterial cells contain ribosomes, molecular machines that build proteins from genetic instructions.
Bacterial ribosomes differ structurally from those of human cells. This difference is important in medicine because some antibiotics can interfere with bacterial protein production without targeting human ribosomes in the same way.
The chromosome and plasmids
Most bacteria have a primary chromosome consisting of DNA. Unlike the chromosomes inside a eukaryotic nucleus, bacterial chromosomes are generally not enclosed by a nuclear membrane.
Many bacteria also contain plasmids, small circular DNA molecules separate from the main chromosome. Plasmids can carry genes that provide useful traits under particular conditions. Some plasmids, for example, contain genes that contribute to resistance to certain antibiotics.
Flagella and pili
Some bacteria have flagella, thread-like structures used for movement. A bacterium can use rotating flagella to move through liquid or across surfaces.
Pili are hair-like structures with several functions. Certain pili help bacteria attach to surfaces or other cells, while specialized structures can participate in the transfer of DNA between bacterial cells.
Some bacteria also produce a capsule or other external layer that can help them attach to surfaces or resist environmental challenges.
How do bacteria reproduce?
Most bacteria reproduce through binary fission. A cell copies its DNA, grows, and divides into two daughter cells.
Under favorable conditions, some bacterial populations can increase very rapidly because each generation can produce another round of cell division. Rapid reproduction does not mean that every bacterium survives, however. Temperature, available nutrients, competition, immune defenses, antibiotics, predators, and other environmental factors can sharply limit population growth.
Bacteria also acquire genetic variation through processes other than ordinary reproduction. Mutations can alter DNA, and bacteria can exchange genetic material with one another.
Three important forms of bacterial gene transfer are transformation, in which a bacterium takes up DNA from its surroundings; transduction, in which viruses transfer bacterial DNA; and conjugation, in which DNA is transferred directly between bacterial cells through specialized cellular machinery.
These processes can spread useful traits through bacterial populations, including some forms of antibiotic resistance.
How do bacteria get energy and nutrients?
Bacteria have extraordinarily varied metabolisms. They do not all “eat” in the same sense that animals do.
Some bacteria obtain energy by breaking down organic molecules. Others use light as an energy source. Still others obtain energy from inorganic substances, including particular forms of sulfur, nitrogen, iron, or hydrogen.
Bacteria also differ in how they obtain carbon, the element required to build many of their cellular molecules. Some use organic carbon from other organisms or their remains. Others can use carbon dioxide as their primary carbon source.
This metabolic diversity allows bacteria to occupy ecological niches that would be inaccessible to many other organisms.
Photosynthetic bacteria illustrate this diversity. Cyanobacteria can use light energy to drive photosynthesis and produce oxygen as part of the process. They are not algae, although they were historically called “blue-green algae.”
Where do bacteria live?
Bacteria are found in virtually every environment that can support life.
They inhabit freshwater and oceans, soils and sediments, plants and animals, buildings, food, and many other environments. Certain species can survive conditions that would kill most familiar organisms, including very high or low temperatures, intense salinity, or highly acidic surroundings.
Bacteria can live independently, but many live in close relationships with other organisms. Some relationships benefit both partners; others benefit one organism without substantially helping or harming the other.
Their ecological success comes partly from their enormous metabolic and genetic diversity. Different bacteria can specialize in different nutrients, temperatures, oxygen levels, and chemical conditions.
What role do bacteria play in ecosystems?
Bacteria are fundamental to the cycling of matter through ecosystems.
Decomposition
Many bacteria break down dead organisms and organic waste. Decomposition releases nutrients from complex biological material, making elements such as carbon, nitrogen, and phosphorus available for reuse.
Without microbial decomposers, nutrients would become locked in dead organic matter rather than continually circulating through ecosystems.
The nitrogen cycle
Bacteria are especially important in the nitrogen cycle. Different bacterial processes convert nitrogen between chemical forms that organisms can use or that can return to the atmosphere.
Some bacteria carry out nitrogen fixation, converting atmospheric nitrogen into forms that can enter biological systems. Other bacteria participate in nitrification and denitrification, helping move nitrogen through soil, water, and the atmosphere.
Photosynthesis and global cycles
Photosynthetic bacteria contribute to carbon cycling and, in the case of cyanobacteria, oxygen production. Bacterial activity in oceans and soils also influences the movement of carbon and other elements on a global scale.
Their influence is therefore much larger than their individual size suggests.
Are bacteria harmful to humans?
Some bacteria cause disease, but pathogenic bacteria represent only a fraction of bacterial diversity.
Disease-causing bacteria can harm the body in several ways. They may invade tissues, multiply in places where they should not be, damage cells directly, or produce toxins. The immune system normally detects and controls many bacterial threats, but certain bacteria have evolved mechanisms that help them evade immune defenses or establish infections.
Examples of bacterial diseases include strep throat, tuberculosis, cholera, and some forms of pneumonia and foodborne illness. Different diseases are caused by different bacterial species or strains, and their effects vary substantially.
It is important not to treat “bacteria” as synonymous with “germs.” A bacterium may be harmless, beneficial, or harmful depending on its species, strain, location, and interaction with its host.
What are beneficial bacteria?
Humans depend on bacteria in numerous ways.
The human body hosts microbial communities on the skin and in parts of the digestive tract. These communities include many bacterial species that coexist with us. In the intestine, bacteria can help break down substances that human digestive enzymes cannot fully process and produce certain metabolic products that interact with the host.
Bacteria are also central to food production. Carefully controlled bacterial fermentation is used to make foods such as yogurt, many cheeses, sauerkraut, kimchi, pickles, and other fermented products.
Outside the body, bacteria support agriculture by participating in nutrient cycling and forming relationships with plants. In industry and biotechnology, bacterial cells are used to produce enzymes, chemicals, medicines, and other useful substances.
What is the microbiome?
A microbiome is the community of microorganisms living in a particular environment together with, in many scientific uses, their collective genetic material.
The human microbiome includes bacteria as well as archaea, fungi, viruses, and other microscopic life. Different body sites support different microbial communities because temperature, moisture, acidity, available nutrients, and other conditions vary.
The microbiome is not a fixed collection of organisms. Its composition can change with diet, environment, age, illness, medications, and other factors.
Researchers study these communities because microorganisms can influence digestion, metabolism, immune function, and other aspects of human biology. At the same time, claims about individual bacterial species should be interpreted carefully: an organism associated with a health effect in one context may behave differently in another.
What are biofilms?
Bacteria often live as communities rather than as isolated, free-floating cells.
A biofilm is a structured community of microorganisms attached to a surface and surrounded by a self-produced matrix of substances. Dental plaque is a familiar example of a microbial biofilm.
Biofilms can form on natural surfaces, medical devices, pipes, rocks, and many other materials. Living in a biofilm can change how bacteria interact with their environment and can make bacterial communities more difficult to eliminate than individual free-floating cells.
Biofilms are important in both health and environmental science because bacteria in these communities can exchange chemical signals, share resources, and interact with one another in ways that differ from life as individual cells.
How do antibiotics affect bacteria?
Antibiotics are drugs used to treat certain bacterial infections. They work by interfering with biological processes that bacteria need to survive or reproduce.
Different antibiotics target different features of bacterial biology. Some interfere with cell-wall construction, while others disrupt protein production, DNA-related processes, or other essential functions.
Antibiotics do not treat viral infections such as the common cold or influenza because viruses do not have the bacterial structures and metabolic systems that these drugs target.
What is antibiotic resistance?
Antibiotic resistance occurs when bacteria acquire traits that allow them to survive exposure to an antibiotic that would otherwise inhibit or kill them.
Resistance is a property of bacteria, not of a person’s body. It can arise through genetic changes and can spread between bacteria through gene transfer. When antibiotics eliminate susceptible bacteria, resistant bacteria may have an advantage in surviving and multiplying.
Resistance does not mean that every antibiotic has become ineffective against a particular bacterium. It means that susceptibility to one or more drugs has been reduced. Treatment therefore depends on the specific bacterium, infection, and available antibiotics.
How are bacteria identified?
Scientists identify bacteria using a combination of physical, chemical, genetic, and biochemical characteristics.
Microscopy can reveal cell shape and arrangement. Laboratory tests can examine how bacteria metabolize particular substances or respond to specific conditions. Staining techniques can reveal differences in cell-envelope structure.
Modern microbiology also relies heavily on DNA sequencing. Comparing genetic sequences can help researchers determine how organisms are related and identify bacteria that are difficult or impossible to distinguish using traditional methods.
No single characteristic is enough to describe bacterial diversity. Two bacteria may look similar under a microscope while differing substantially in their genetics, metabolism, and effects on other organisms.
How are bacteria different from viruses?
Bacteria and viruses are both microscopic, but they are fundamentally different.
A bacterium is a living cell capable of carrying out metabolism, maintaining itself, and reproducing independently under suitable conditions. A virus is not a cell. It consists of genetic material surrounded by a protein-based structure and, in some viruses, an additional membrane envelope.
Viruses cannot reproduce on their own. They must enter a suitable host cell and use the host’s cellular machinery to make new viral particles.
This difference explains why antibiotics can act against bacteria but not viruses.
How are bacteria different from fungi?
Fungi, including yeasts and molds, are eukaryotic organisms. Their cells contain nuclei and other membrane-bound organelles. Bacteria are prokaryotes and lack these structures.
Fungal cells also differ from bacterial cells in their cell walls and other biochemical features. These differences matter medically because antibacterial drugs generally do not work against fungal infections, while antifungal medicines target biological features specific to fungi.
Why are bacteria so successful?
Bacteria have persisted and diversified because they combine simple cellular organization with extraordinary biochemical flexibility.
Their small size allows rapid exchange with the environment. Their ability to reproduce by cell division can support fast population growth when conditions are favorable. Their genomes can change through mutation, and bacteria can exchange genetic material. Their diverse metabolisms allow them to exploit energy sources ranging from organic compounds to sunlight and inorganic chemicals.
They can also enter specialized survival states. Some bacterial groups produce highly resistant structures called endospores when conditions become unfavorable. An endospore is not a method of reproduction; it is a dormant survival structure that allows the bacterial cell to withstand stresses that would otherwise be lethal.
Taken together, these traits allow bacteria to survive environmental changes, colonize new habitats, and participate in nearly every major biological and chemical process on Earth.
Why bacteria matter
Bacteria are not simply disease-causing organisms that need to be eliminated. They are one of the central forms of life on Earth, with roles ranging from decomposing organic matter and cycling nutrients to supporting human digestion and enabling biotechnology.
Some bacteria cause serious disease and require treatment. Others are essential partners in ecosystems or in the human body. Still others are useful because humans have learned to harness their metabolism and genetic machinery.
The key to understanding bacterial life is therefore diversity. A bacterium is a microscopic cell, but “bacteria” describes an enormous biological world—one that operates in every corner of the environment and is deeply intertwined with the lives of larger organisms.
