Bacteria: What you need to know

Bacteria are microscopic, single-celled organisms that exist in their millions, in every environment, both inside and outside other organisms.

Some bacteria are harmful, but most serve a useful purpose. They support many forms of life, both plant and animal, and they are used in industrial and medicinal processes.

Bacteria are thought to have been the first organisms to appear on earth, about 4 billion years ago. The oldest known fossils are of bacteria-like organisms.

Bacteria can use most organic and some inorganic compounds as food, and some can survive extreme conditions.

A growing interest in the function of the gut microbiome is shedding new light on the roles bacteria play in human health.

What are bacteria?

Bacteria are single-celled organisms.

Bacteria are single-cell organisms that are neither plants nor animals.

They usually measure a few micrometers in length and exist together in communities of millions.

A gram of soil typically contains about 40 million bacterial cells. A milliliter of fresh water usually holds about one million bacterial cells.

The earth is estimated to hold at least 5 nonillion bacteria, and much of the earth’s biomass is thought to be made up of bacteria.


There are many different types of bacteria. One way of classifying them is by shape. There are three basic shapes.

Spherical: Bacteria shaped like a ball are called cocci, and a single bacterium is a coccus. Examples include the streptococcus group, responsible for “strep throat.”

Rod-shaped: These are known as bacilli (singular bacillus). Some rod-shaped bacteria are curved. These are known as vibrio. Examples of rod-shaped bacteria include Bacillus anthracis (B. anthracis), or anthrax.

Spiral: These are known as spirilla (singular spirillus). If their coil is very tight they are known as spirochetes. Leptospirosis, Lyme disease, and syphilis are caused by bacteria of this shape.

There are many variations within each shape group.


Bacterial cells are different from plant and animal cells. Bacteria are prokaryotes, which means they have no nucleus.

A bacterial cell includes:

Capsule: A layer found on the outside of the cell wall in some bacteria.

Cell wall: A layer that is made of a polymer called peptidoglycan. The cell wall gives the bacteria its shape. It is located outside the plasma membrane . The cell wall is thicker in some bacteria, called Gram positive bacteria.

Plasma membrane: Found within the cell wall, this generates energy and transports chemicals. The membrane is permeable, which means that substances can pass through it.

Cytoplasm: A gelatinous substance inside the plasma membrane that contains genetic material and ribosomes.

DNA: This contains all the genetic instructions used in the development and function of the bacterium. It is located inside the cytoplasm.

Ribosomes: This is where proteins are made, or synthesized. Ribosomes are complex particles made up of RNA-rich granules.

Flagellum: This is used for movement, to propel some types of bacteria. There are some bacteria that can have more than one.

Pili: These hair-like appendages on the outside of the cell allow it to stick to surfaces and transfer genetic material to other cells. This can contribute to the spread of illness in humans.

Bacteria can thrive even in extreme environments, such as glaciers.

Bacteria can be found in soil, water, plants, animals, radioactive waste, deep in the earth’s crust, arctic ice and glaciers, and hot springs. There are bacteria in the stratosphere, between 6 and 30 miles up in the atmosphere, and in the ocean depths, down to 32,800 feet or 10,000 meters deep.

Aerobes, or aerobic bacteria, can only grow where there is oxygen. Some types can cause problems for the human environment, such as corrosion, fouling, problems with water clarity, and bad smells.

Anaerobes, or anaerobic bacteria, can only grow where there is no oxygen. In humans, this is mostly in the gastrointestinal tract. They can also cause gas gangrene, tetanus, botulism, and most dental infections.

Facultative anaerobes, or facultative anaerobic bacteria, can live either with or without oxygen, but they prefer environments where there is oxygen. They are mostly found in soil, water, vegetation and some normal flora of humans and animals. Examples include Salmonella.

Mesophiles, or mesophilic bacteria, are the bacteria responsible for most human infections. They thrive in moderate temperatures, around 37 degrees Celsius. This is the temperature of the human body.

Examples include Listeria monocytogenes, Pesudomonas maltophilia, Thiobacillus novellus, Staphylococcus aureus, Streptococcus pyrogenes, Streptococcus pneumoniae, Escherichia coli, and Clostridium kluyveri.

The human intestinal flora, or gut microbiome, contains beneficial mesophilic bacteria, such as dietary Lactobacillus acidophilus.

Extremophiles, or extremophilic bacteria, can withstand conditions considered too extreme for most life forms.

Thermophiles can live in high temperatures, up to 75 to 80 degrees Celsius (C), and hyperthermophiles can surivive in temperatures up to 113 degrees C.

Deep in the ocean, bacteria live in total darkness by thermal vents, where both temperature and pressure are high. They make their own food by oxidizing sulfur that comes from deep inside the earth.

Other extremophiles include:

Halophiles, found only in a salty environment

Acidophiles, some of which live in environments as acidic as pH0

Alkaliphiles, living in alkiline environments up to pH 10.5

Psychrophiles, found in cold temperatures, for example, in glaciers

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