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bacteria, singular bacterium, any of a group of microscopic single-celled organisms that live in enormous numbers in almost every environment on Earth, from deep-sea vents to deep below Earth's surface to the digestive tracts of humans.

Bacteria lack a membrane-bound nucleus and other internal structures and are therefore ranked among the unicellular life-forms called prokaryotes. Prokaryotes are the dominant living creatures on Earth, having been present for perhaps three-quarters of Earth history and having adapted to almost all available ecological habitats. As a group, they display exceedingly diverse metabolic capabilities and can use almost any organic compound, and some inorganic compounds, as a food source. Some bacteria can cause diseases in humans, animals, or plants, but most are harmless and are beneficial ecological agents whose metabolic activities sustain higher life-forms. Other bacteria are symbionts of plants and invertebrates, where they carry out important functions for the host, such as nitrogen fixation and cellulose degradation. Without prokaryotes, soil would not be fertile, and dead organic material would decay much more slowly. Some bacteria are widely used in the preparation of foods, chemicals, and antibiotics. Studies of the relationships between different groups of bacteria continue to yield new insights into the origin of life on Earth and mechanisms of evolution.

All living organisms on Earth are made up of one of two basic types of cells: eukaryotic cells, in which the genetic material is enclosed within a nuclear membrane, or prokaryotic cells, in which the genetic material is not separated from the rest of the cell. Traditionally, all prokaryotic cells were called bacteria and were classified in the prokaryotic kingdom Monera. However, their classification as Monera, equivalent in taxonomy to the other kingdoms-Plantae, Animalia, Fungi, and Protista-understated the remarkable genetic and metabolic diversity exhibited by prokaryotic cells relative to eukaryotic cells. In the late 1970s American microbiologist Carl Woese pioneered a major change in classification by placing all organisms into three domains-Eukarya, Bacteria (originally called Eubacteria), and Archaea (originally called Archaebacteria)-to reflect the three ancient lines of evolution. The prokaryotic organisms that were formerly known as bacteria were then divided into two of these domains, Bacteria and Archaea. Bacteria and Archaea are superficially similar; for example, they do not have intracellular organelles, and they have circular DNA. However, they are fundamentally distinct, and their separation is based on the genetic evidence for their ancient and separate evolutionary lineages, as well as fundamental differences in their chemistry and physiology. Members of these two prokaryotic domains are as different from one another as they are from eukaryotic cells.

Prokaryotic cells (i.e., Bacteria and Archaea) are fundamentally different from the eukaryotic cells that constitute other forms of life. Prokaryotic cells are defined by a much simpler design than is found in eukaryotic cells. The most-apparent simplification is the lack of intracellular organelles, which are features characteristic of eukaryotic cells. Organelles are discrete membrane-enclosed structures that are contained in the cytoplasm and include the nucleus, where genetic information is retained, copied, and expressed; the mitochondria and chloroplasts, where chemical or light energy is converted into metabolic energy; the lysosome, where ingested proteins are digested and other nutrients are made available; and the endoplasmic reticulum and the Golgi apparatus, where the proteins that are synthesized by and released from the cell are assembled, modified, and exported. All of the activities performed by organelles also take place in bacteria, but they are not carried out by specialized structures. In addition, prokaryotic cells are usually much smaller than eukaryotic cells. The small size, simple design, and broad metabolic capabilities of bacteria allow them to grow and divide very rapidly and to inhabit and flourish in almost any environment.

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Prokaryotic and eukaryotic cells differ in many other ways, including lipid composition, structure of key metabolic enzymes, responses to antibiotics and toxins, and the mechanism of expression of genetic information. Eukaryotic organisms contain multiple linear chromosomes with genes that are much larger than they need to be to encode the synthesis of proteins. Substantial portions of the ribonucleic acid ( RNA) copy of the genetic information (deoxyribonucleic acid, or DNA) are discarded, and the remaining messenger RNA (mRNA) is substantially modified before it is translated into protein. In contrast, bacteria have one circular chromosome that contains all of their genetic information, and their mRNAs are exact copies of their genes and are not modified.

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bacteria | Cell, Evolution, & Classification

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bacteria, singular bacterium, any of a group of microscopic single-celled organisms that live in enormous numbers in almost every environment on Earth, from deep-sea vents to deep below Earth's surface to the digestive tracts of humans. Bacteria lack a membrane-bound nucleus and other internal structures and are therefore ranked among the unicellular life-forms called prokaryotes.

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Molecular Expressions Cell Biology: Bacteria Cell Structure

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One of the earliest prokaryotic cells to have evolved, bacteria have been around for at least 3.5 billion years and live in just about every environment imaginable. Explore the structure of a bacteria cell with our three-dimensional graphics.

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How Bacteria Rule Over Your Body – The Microbiome

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Where do you think these drugs or methods will go? Do you think that we will mainly rely on the development of new antibiotics that target different mechanisms or on the development of novel methods to fight off resistant bacteria?

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What causes antibiotic resistance? - Kevin Wu

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Right now, you are inhabited by trillions of microorganisms. Many of these bacteria are harmless (or even helpful!), but there are a few strains of 'super bacteria' that are pretty nasty -- and they're growing resistant to our antibiotics. Why is this happening?

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Foodborne illness (commonly known as food poisoning) is often caused by consuming food contaminated by bacteria and while the American food supply is among the safest in the world, the Federal government estimates that there are about 48 million cases of foodborne illness each year. This estimate is equivalent to 1 in 6 Americans becoming sick from consuming food contaminated with bacteria, which result in an estimated 128,000 hospitalizations and 3,000 deaths.

Foodborne illness occurs when contaminated food is consumed, which causes an infection resulting in illness. There are several factors that contribute to the symptoms and severity of food poisoning, including a weakened immune system and age.

E. coli (Escherichia coli)

Escherichia coli (E. coli) bacteria normally live in the intestines of people and animals. Most and actually are an important part of a healthy human intestinal tract. However, some E. coli are pathogenic, meaning they can cause illness, either diarrhea or illness outside of the intestinal tract. The types of E. coli that can cause illness can be transmitted through contaminated water or food, or through contact with animals or persons

Salmonella (Salmonella Enteritidis)

Salmonella spp. can cause serious and sometimes fatal infections in young children, frail or elderly people, and others with weakened immune systems. Healthy people infected with Salmonella spp. often experience fever, diarrhea (which may be bloody), nausea, vomiting, and abdominal pain.

In rare circumstances, infection with Salmonella spp. can result in the organism getting into the blood stream and producing more severe illnesses such as arterial infections (i.e., infected aneurysms), endocarditis, and arthritis.

Direct-human-contact animal foods contaminated with Salmonella spp. pose a significant health risk to humans who have direct contact with the foods at homes, petting zoos, agricultural fairs, or similar venues.

Listeria (Listeria monocytogenes or L. Monocytogenes)

Listeria is a harmful bacterium that can be found in refrigerated, ready-to-eat foods (meat, poultry, seafood, and dairy - unpasteurized milk and milk products or foods made with unpasteurized milk), and produce harvested from soil contaminated with L. monocytogenes.

Many animals can carry this bacterium without appearing ill, which means that it can be found in foods made from animals. L. monocytogenes is unusual because it can grow at refrigerator temperatures where most other foodborne bacteria do not. When eaten, it may cause listeriosis, an illness to which pregnant women and their unborn children are very susceptible.

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Foodborne illness (commonly known as food poisoning) is often caused by consuming food contaminated by bacteria and while the American food supply is among the safest in the world, the Federal government estimates that there are about 48 million cases of foodborne illness each year .

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(Image credit: Centers for Disease Control)

Bacteria are microscopic, single-celled organisms that thrive in diverse environments. These organisms can live in soil, the ocean and inside the human gut.

Humans' relationship with bacteria is complex. Sometimes bacteria lend us a helping hand, such as by curdling milk into yogurt or helping with our digestion. In other cases, bacteria are destructive, causing diseases like pneumonia and methicillin-resistant Staphylococcus aureus (MRSA).

Structure

Bacteria (singular: bacterium) are classified as prokaryotes, which are single-celled organisms with a simple internal structure that lacks a nucleus, and contains DNA that either floats freely in a twisted, thread-like mass called the nucleoid, or in separate, circular pieces called plasmids. Ribosomes are the spherical units in the bacterial cell where proteins are assembled from individual amino acids using the information encoded in ribosomal RNA.

Bacterial cells are generally surrounded by two protective coverings: an outer cell wall and an inner cell membrane. Certain bacteria, like the mycoplasmas, do not have a cell wall at all. Some bacteria may even have a third, outermost protective layer called the capsule. Whip-like extensions often cover the surfaces of bacteria - long ones called flagella or short ones called pili - that help bacteria to move around and attach to a host.

Classification

A few different criteria are used to classify bacteria. The organisms can be distinguished by the nature of their cell walls, by their shape, or by differences in their genetic makeup.

The Gram stain is a test used to identify bacteria by the composition of their cell walls, named for Hans Christian Gram, who developed the technique in 1884. The test stains Gram-positive bacteria, or bacteria that do not have an outer membrane. Gram-negative bacteria don't pick up the stain. For example, Streptococcus pneumoniae ( S. pneumoniae), which causes pneumonia, is a Gram-positive bacterium, but Escherichia coli () and Vibrio cholerae, which causes cholera, are Gram-negative bacteria.

There are three basic bacterial shapes: Round bacteria called cocci (singular: coccus), cylindrical, capsule-shaped ones known as bacilli (singular: bacillus); and spiral bacteria, aptly called spirilla (singular: spirillum). The shapes and configurations of bacteria are often reflected in their names. For example, the milk-curdling Lactobacillus acidophilus are bacilli, and pneumonia-causing S. pneumoniae are a chain of cocci. Some bacteria take other shapes, such as stalked, square or star.

(Image credit: Kateryna Kon/Shutterstock)

Reproduction

Most bacteria multiply by a process called binary fission, according to the Cornell University College of Agriculture and Life Sciences. In this process, a single bacterial cell, called the "parent," makes a copy of its DNA and grows larger by doubling its cellular content. The cell then splits apart, pushing the duplicated material out and creating two identical "daughter" cells.

Some bacterial species, such as cyanobacteria and firmicutes, reproduce via budding. In this case, the daughter cell grows as an offshoot of the parent. It starts off as a small nub, grows until it is the same size as its parent, and splits off.

The DNA found in parents and offspring after binary fission or budding is exactly the same. Therefore, bacterial cells introduce variation into their genetic material by integrating additional DNA, often from their surroundings, into their genome. This is known as horizontal gene transfer; the resulting genetic variation ensures that bacteria can adapt and survive as their environment changes. [ Watch Strange, Glowing Bacteria Harpoon and Swallow DNA to Evolve]

There are three ways horizontal gene transfer occurs: transformation, transduction and conjugation.

Transformation is the most common process of horizontal gene transfer and occurs when short DNA fragments are exchanged between donors and recipients. Transduction, which typically only occurs between closely related bacteria, requires the donor and recipient to transfer DNA by sharing cell surface receptors. Conjugation requires physical contact between the cell walls of bacteria; the DNA transfers from the donor cell to the recipient. Through conjugation, a bacterial cell can transfer DNA to eukaryotic cells (multi-celled organisms). Conjugation aids in the spread of antibiotic-resistance genes.

Bacteria in human health and disease

Bacteria can be beneficial as well as detrimental to human health. Commensal, or "friendly" bacteria, share space and resources within our bodies and tend to be helpful. There are about 10 times more microbial cells than human cells in our bodies; the highest numbers of microbial species are found in the gut, according to microbiologist David A. Relman's 2012 article in Nature.

The human gut is a comfortable setting for bacteria, with plenty of nutrients available for their sustenance. In a 2014 review article published in the American Journal of Gastroenterology, the authors mention that gut bacteria and other microorganisms, such as helpful strains of E.coli and Streptococcus, aid in digestion, stave off colonization by harmful pathogens, and help to develop the immune system. Moreover, the disruption of gut bacteria has been linked to certain disease conditions. For instance, patients with Crohn's disease have an increased immune response against gut bacteria, according to a 2003 review published in the journal The Lancet.

Other bacteria can cause infections. Several bacteria - ranging from so-called group A Streptococcus, Clostridium perfringens ( C. perfringens), E. coli and S. aureuscan cause a rare but severe soft tissue infection called necrotizing fasciitis (sometimes called flesh-eating bacteria). According to the Centers for Disease Control and Prevention (CDC), this infection affects the tissues surrounding muscles, nerves, fat and blood vessels; it can be treated, especially when caught early.

(Image credit: Alissa Eckert and Jennifer Oosthuizen/CDC)

Antibiotic resistance

Antibiotics are typically used to treat bacterial infections. However, in recent years, improper and unnecessary use of antibiotics has promoted the spread of several strains of antibiotic-resistant bacteria.

In cases of antibiotic resistance, the infectious bacteria are no longer susceptible to previously effective antibiotics. According to the CDC, at least 2 million people in the U.S. are infected with antibiotic-resistant bacteria every year, leading to the death of at least 23,000 people.

"Pretty much any infection you can think of now has been identified as being associated with some level of resistance," said Dr. Christopher Crnich, an infectious disease physician and hospital epidemiologist at the University of Wisconsin Hospitals and Madison Veterans Affairs Hospital. "There's very few infections that we now treat where infections caused by resistant bacteria is not a clinical concern."

MRSA, for example, is one of the more notorious antibiotic-resistant bacterial strains; it resists methicillin and other antibiotics used to treat Staphylococcus infections, which are acquired primarily through skin contact. MRSA infections occur in health-care settings such as hospitals and nursing homes, where it can lead to pneumonia or bloodstream infections. MRSA also spreads in the community, especially in situations where there is a lot of exposed skin, other physical contact, and the use of shared equipment - for example, among athletes, in tattoo parlors, and in day care facilities and schools. Community-acquired MRSA most often causes serious skin infections.

An important facet of combating antibiotic resistance is to be careful about their use. "It's so important for us to use antibiotics intelligently," Crnich told LiveScience. "You only want to use an antibiotic when you have a clear-cut bacterial infection."
Additional resources: This article was updated on Apr. 25, 2019, by Live Science Contributor Rachel Ross.

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(Image credit: Centers for Disease Control) Bacteria are microscopic, single-celled organisms that thrive in diverse environments. These organisms can live in soil, the ocean and inside the human gut. Humans' relationship with bacteria is complex. Sometimes bacteria lend us a helping hand, such as by curdling milk into yogurt or helping with our digestion.