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.

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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 feed in different ways.

Heterotrophic bacteria, or heterotrophs, get their energy through consuming organic carbon. Most absorb dead organic material, such as decomposing flesh. Some of these parasitic bacteria kill their host, while others help them.

Autotrophic bacteria (or just autotrophs) make their own food, either through either:

  • photosynthesis, using sunlight, water and carbon dioxide, or
  • chemosynthesis, using carbon dioxide, water, and chemicals such as ammonia, nitrogen, sulfur, and others

Bacteria that use photosynthesis are called photoautotrophs. Some types, for example cyanobacteria, produce oxygen. These probably played a vital role in creating the oxygen in the earth's atmosphere. Others, such as heliobacteria, do not produce oxygen.

Those that use chemosynthesis are known as chemoautotrophs. These bacteria are commonly found in ocean vents and in the roots of legumes, such as alfalfa, clover, peas, beans, lentils, and peanuts.

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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°C. 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°C, and hyperthermophiles can surivive in temperatures up to 113°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 pH 0
  • alkaliphiles, living in alkiline environments up to pH 10.5
  • psychrophiles, found in cold temperatures, for example, in glaciers

Extremophiles can survive where no other organism can.

Bacteria may reproduce and change using the following methods:

  • Binary fission: An asexual form of reproduction, in which a cell continues to grow until a new cell wall grows through the center, forming two cells. These separate, making two cells with the same genetic material.
  • Transfer of genetic material: Cells acquire new genetic material through processes known as conjugation, transformation, or transduction. These processes can make bacteria stronger and more able to resist threats, such as antibiotic medication.
  • Spores: When some types of bacteria are low on resources, they can form spores. Spores hold the organism's DNA material and contain the enzymes needed for germination. They are very resistant to environmental stresses. The spores can remain inactive for centuries, until the right conditions occur. Then they can reactivate and become bacteria.
  • Spores can survive through periods of environmental stress, including ultraviolet (UV) and gamma radiation, desiccation, starvation, chemical exposure, and extremes of temperature.

Some bacteria produce endospores, or internal spores, while others produce exospores, which are released outside. These are known as cysts.

Clostridium is an example of an endospore-forming bacterium. There are about 100 species of Clostridium, including Clostridium botulinim (C. botulinim) or botulism, responsible for a potentially fatal kind of food poisoning, and Clostridium difficile (C. Difficile), which causes colitis and other intestinal problems.

Bacteria are often thought of as bad, but many are helpful. We would not exist without them. The oxygen we breathe was probably created by the activity of bacteria.

Human survival

Many of the bacteria in the body play an important role in human survival. Bacteria in the digestive system break down nutrients, such as complex sugars, into forms the body can use.

Non-hazardous bacteria also help prevent diseases by occupying places that the pathogenic, or disease-causing, bacteria want to attach to. Some bacteria protect us from disease by attacking the pathogens.

Nitrogen fixation

Bacteria take in nitrogen and release it for plant use when they die. Plants need nitrogen in the soil to live, but they cannot do this themselves. To ensure this, many plant seeds have a small container of bacteria that is used when the plant sprouts.

Food technology

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Cheesemaking involves bacteria.

Lactic acid bacteria, such as Lactobacillus and Lactococcus together with yeast and molds, or fungi, are used to prepare foods such as as cheese, soy sauce, natto (fermented soy beans), vinegar, yogurt, and pickles.

Not only is fermentation useful for preserving foods, but some of these foods may offer health benefits.

For example, some fermented foods contain types of bacteria that are similar to those linked with gastrointestinal health. Some fermentation processes lead to new compounds, such as lactic acid, which that appear to have an anti-inflammatory effect.

More investigation is needed to confirm the health benefits of fermented foods.

Bacteria in industry and research

Bacteria can break down organic compounds. This is useful for activities such as waste processing and cleaning up oil spills and toxic waste.

The pharmaceutical and chemical industries use bacteria in the production of certain chemicals.

Bacteria are used in molecular biology, biochemistry and genetic research, because they can grow quickly and are relatively easy to manipulate. Scientists use bacteria to study how genes and enzymes work.

Bacteria are needed to make antibiotics.

Bacillus thuringiensis (Bt) is a bacterium that can be used in agriculture instead of pesticides. It does not have the undesirable environmental consequences associated with pesticide use.

Some types of bacteria can cause diseases in humans, such as cholera, diptheria, dysentery, bubonic plague, pneumonia, tuberculosis (TB), typhoid, and many more.

If the human body is exposed to bacteria that the body does not recognize as helpful, the immune system will attack them. This reaction can lead to the symptoms of swelling and inflammation that we see, for example, in an infected wound.

In 1900, pneumonia, TB, and diarrhea were the three biggest killers in the United States. Sterilization techniques and antibiotic medications have led to a significant drop in deaths from bacterial diseases.

However, the overuse of antibiotics is making bacterial infection harder to treat. As the bacteria mutate, they become more resistant to existing antibiotics, making infections harder to treat. Bacteria transform naturally, but the overuse of antibiotics is speeding up this process.

"Even if new medicines are developed, without behaviour change, antibiotic resistance will remain a major threat."

World Health Organization (WHO)

For this reason, scientists and health authorities are calling on doctors not to prescribe antibiotics unless it is necessary, and for people to practice other ways of preventing disease, such as good food hygiene, hand washing, vaccination, and using condoms.

Recent research has led to a new and growing awaress of how the human body interacts with bacteria, and particularly the communities of bacteria living in the intestinal tract, known as the gut microbiome, or gut flora.

In 2009, researchers published findings suggesting that women with obesity were more likely to have a particular kind of bacteria, Selenomonas noxia (S. noxia), in their mouth.

In 2015, scientists at the University of North Carolina found that the intestines of people with anorexia contain "very different" bacteria, or microbial commiunities, compared with people who do not have the condition. They suggest that this may have a psychological impact.

Over 2,000 years ago, a Roman author, Marcus Terentius Varro, suggested that disease may be caused by tiny animals that floated in the air. He advised people to avoid marshy places during building work because they might contain insects too small for the eye to see that entered the body through the mouth and nostrils and cause diseases.

In the 17th century, a Dutch scientist, Antonie van Leeuwenhoek created a single-lens microscope with which he saw what he called animalcules, later known as bacteria. He is considered to be the first microbiologist.

In the 19th century, the chemists Louis Pasteur and Robert Koch said that diseases were caused by germs. This was known as the Germ Theory.

In the 1910, the scientist Paul Ehrlich announed the development of the first antibiotic, Salvarsan. He used it to cure syphilis. He was also the first scientist to detect bacteria by using stains.

In 2001, Joshua Lederburg coined the term "gut microbiome," and scientists worldwide are currently seeking to describe and understand more precisely the structures, types, and uses of "gut flora," or bacteria in the human body.

In time, this work is expected to shed new light a wide range of health conditions.