Bacteria are ancient, microscopic organisms that are found everywhere on Earth. They are one of the three main branches of the tree of life and have been around for over 3.5 billion years.

All bacteria are single-celled organisms. They have prokaryotic cells so they don’t have a nucleus or organelles.

They are an extremely diverse and abundant group of organisms and are important for a range of reasons. Most life on Earth would not exist if it weren’t for bacteria.

Why are bacteria important?

Bacteria are important for a number of reasons. They perform functions that make them important to other life forms plus they are important economically in a number of industries.

Some species of bacteria cause disease and infections, but far more commonly, bacteria are beneficial. They are so important in our own bodies that if all of the bacteria from inside our bodies were removed, we would die without their help.

Bacteria that live inside the stomachs and intestines of animals help with digestion. Bacteria have enzymes that are able to digest tough foods, such as plant fibers, that animals are unable to digest.

Bacteria also play a role in important ecological processes. They are one of the main players in the decomposition of dead plants and help to recycle nutrients back into ecosystems. They are also able to take gases from the atmosphere and turn them into usable nutrients such as carbohydrates and nitrates.

Certain industrial processes utilize the metabolism of bacteria. Bacteria are responsible for the fermentation that leads to the production of foods such as cheese and yogurt. Bacteria are also used in waste treatment facilities to help speed up the process of breaking down human waste, food waste and cleaning products.

Where are bacteria found?

Bacteria growthBacteria are found pretty much everywhere on Earth. Through the oceans, in soil, in air and inside other organisms.

A large number of bacteria live on and within our very own bodies. They are in our stomachs and intestines, and our throats, ears and nose. They can also be found on our skin and hair. Bacteria can be literally be found everywhere.

Structure of bacteria cells

Bacteria are prokaryotic organisms. They all have only one cell and that cell doesn’t have a ‘true’ nucleus or organelles.


Instead of having their DNA enclosed in a nucleus, the DNA is tightly coiled into a region of the cell called a nucleoid. The nucleoid isn’t a true nucleus because it isn’t surrounded by a membrane. A bacteria cell has less than 1% as much DNA as a eukaryotic cell.

Cell wall

A key feature of bacteria cells is a cell wall. The cell wall surrounds a bacteria cell and provides protection. It also maintains the shape of the cell and prevents it from bursting open.

Compared to the cell walls of plants which are made from cellulose, bacteria cell walls have multiple layers of made from different compounds. Different species of bacteria have cell walls with different structural make up. Differences in the structure of cell walls is what separate bacteria into gram-positive and gram-negative bacteria.

It is the cell wall that is often targeted by antibiotics to kill bacteria.


Although bacteria cells don’t have organelles they do have sub-cellular structures called ribosomes and flagella. Ribosomes are used to produce proteins using information provided by DNA.


Flagella are long, thin structures that extend out from some bacterial cells and are used for movement. The flagella of bacteria are completely different to the flagella found in eukaryotic cells, although they perform the same function. A bacterial flagella is constantly moving at full speed and a bacterium has very little control over where it moves to.

Gram-positive and gram-negative bacteria

Differences in the structure of cell walls can separate bacteria into two different groups: gram positive and gram negative. To identify which group a bacteria belongs to it is stained with a crystal violet dye. Gram-positive and gram-negative bacteria can be distinguished by the color they become after being stained.

Gram-positive bacteria stains a blue or purple color after the dye is washed off. Gram-negative bacteria become red or pink in color.

The two groups of bacteria stain different colors because of different thicknesses of a compound in their cell walls called ‘peptidoglycan’. Gram-positive bacteria have a thick layer of peptidoglycan in their cell wall. The thick layer of peptidoglycan stains blue or purple after being exposed to a crystal violet dye.

Gram-negative bacteria don’t have a thick layer of peptidoglycan in their cell wall. When they are stained with a crystal violet dye their cell walls are unable to retain the color of the dye and instead turn red or pink.

The cell walls of gram-negative bacteria are more complex than those of gram-positive bacteria. Gram-negative bacteria have an outer membrane that surrounds the cell wall. This outer membrane makes gram negative bacteria harder to kill with antibiotics.

Different shapes of bacteria cells

Rod-shaped bacteriaThere are three common shapes bacteria cells are known to take: round, rod-shaped and spiral shaped. Round, or cocci, are spherical shaped prokaryotic cells. They include a number of very small bacteria, often smaller than 1 μm, and often live in association with other round shaped bacteria cells.

Rod shaped cells are known as bacilli. Being elongated means that bacilli have larger surface areas relative to their volume so they are able to grow larger. Well-known bacillus bacteria include those found in yogurt marketed as ‘probiotics’ and Bacillus anthracis, the bacteria that causes anthrax.

Spiral shaped cells are known as spirilla or spirochetes. Spirilla bacterial cells vary in the amount of spiralling between species. Some bacteria may be almost a crescent shape whilst others will form loose and tight coils.

Reproduction of bacteria

The reproduction of bacteria cells is not yet fully understood. Bacteria are able to reproduce in a number of ways and they can reproduce very quickly. Research done on the E. coli bacteria has shown that they can reproduce every 20 minutes in ideal conditions.

Bacteria cells commonly reproduce via binary fission, where one cell splits into two cells. Some species are able to spit into a number of fragments and each fragment has the potential to grow into a fully developed bacteria.

Some bacteria produce long-living cells called ‘endospores’ that are able to survive through harsh environmental conditions for long periods. Once the environment improves the endospore reproduces normal bacteria cells and the bacterial population can begin to grow again.

Bacteria are highly evolved

Short generations and large genetic variability allow bacteria to evolve quickly to changing environments. Their ability to adapt to their environment is verified by the fact that they have persisted for over 3.5 billions years.

Bacteria are simple in structure compared to eukaryotes but they are not primitive with respect to evolution. They are highly evolved organisms and well-adapted to a massive suite of environments.


Cyanobacteria, also called blue-green algae, are one of the main lineages of bacteria. They are thought to be one of the first life forms on Earth and have played a crucial role in the evolution of life. Cyanobacteria are still found almost everywhere on Earth including some of the world’s hottest deserts and Antarctica.

The importance of cyanobacteria in the history of life on Earth is due to the fact that they have changed the Earth’s atmosphere and environment. Cyanobacteria are both photosynthesizers and nitrogen fixers. They can convert carbon dioxide and water into sugar and oxygen plus they can turn nitrogen gas in the atmosphere into usable nitrate nutrients.

By fixing atmospheric nitrogen and carbon dioxide, cyanobacteria changed the environment of Earth. They put oxygen into the atmosphere and made nitrates available to other organisms.

It is also believed that cyanobacteria are the predecessor to all plants. The most popular theory is that green algae evolved from a eukaryotic cell engulfing a cyanobacterium cell. The cyanobacteria became an organelle called a ‘chloroplast’ and over time green algae evolved into land plants.


Cyanobacteria are still one of the few organisms that can fix nitrogen from the atmosphere and turn it into nitrates. The fixing of nitrogen is done by special cells called ‘heterocytes’. The heterocytes share the nitrates they produce with neighbouring cyanobacteria cells. In return the heterocytes receive carbohydrates and other compounds from the other cells.

In good conditions, cyanobacteria populations can grow incredibly fast and form algal blooms. Algal blooms can completely cover the surface of lakes and coastal seas. They are often toxic and in some cases they can be lethal.


The term pathogen is used to describe an organism that causes disease or illness of another organism. Many pathogens are in fact bacteria but they can also be fungi, viruses or protists.

The vast majority of bacteria are not pathogens and many are actually very important to the well-being of the organism they live in. The few species of bacteria that are pathogenic however do manage to cause serious problems to their host. Some bacteria species that are harmful pathogens to humans cause illnesses and diseases such as anthrax, lyme disease, gonorrhea, strep throat, whooping-cough, meningitis, tuberculosis and the pneumonic plague.

Interesting facts

  • Chloroplasts and mitochondria of eukaryotic cells are thought to have originally been bacteria that were absorbed by eukaryotic cells and used as organelles for photosynthesis and respiration

Last edited: 18 March 2016