Bacteria (bacterium, singular), microorganisms that lack internal cell membranes. The most common and ancient organisms on earth, bacteria are intimately connected to the lives of all organisms.
Most bacteria are less than 1 micron, or one millionth of a meter (0.00025 in), in length. Hundreds of thousands of bacteria can fit into a space the size of the period at the end of this sentence. However, colonies of bacteria, such as on a laboratory culture plate or on the surface of salt marsh muds, can easily be viewed without a microscope.
 

Classification 
Grouping organisms helps scientists study, understand, and discuss them more effectively. Life on earth is frequently described as being either prokaryotic (microscopic and lacking cells with internal membranes) or eukaryotic (macroscopic or microscopic but with clearly defined internal compartments). Bacteria are prokaryotic organisms, or prokaryotes. They make up the kingdom Monera, which has also been called Prokaryotae.
Most microbiologists (scientists who study life forms that can only be observed with the aid of a microscope) classify bacteria into two large groups, the archaebacteria and the eubacteria. The two groups developed from a common ancestor more than 3.5 billion years ago. Most archaebacteria live in high-temperature environments where no other life forms can grow, such as volcanically active regions and hot springs. They contain fatty substances known as lipids. Archaebacteria also lack a chemical, peptidoglycan, found in the walls of all other bacteria. Common bacteria today that are included in this ancient group include thermophiles (heat lovers), halophiles (salt lovers), and certain methanogens (gas releasers).
The eubacteria include all other bacteria. A large number of the eubacteria, such as the green bacteria, purple bacteria, and cyanobacteria, are called phototrophs. These bacteria are able to convert the sun's energy into food in a process called photosynthesis. Phototrophic bacteria have dominated earth seas and landscapes for hundreds of millions of years and remain common today.
Microbiologists also classify bacteria according to whether or not they require oxygen to survive. Bacteria that require oxygen are called aerobic bacteria, or aerobes. Bacteria that live without oxygen are called anaerobic bacteria, or anaerobes. Both aerobes and anaerobes can be either phototrophic or nonphototrophic.
Microbiologists further classify bacteria according to their basic shapes. Spherical bacteria are called cocci, corkscrew-shaped are called spirilla or spirochetes, rod-shaped are called bacilli, and threadlike bacteria are called filamentous. Some bacteria, called pleiomorphic, take various forms depending on conditions. Microbiologists have discovered that bacteria are more complex than previously believed. For example, some groups of filamentous bacteria grow into stalked structures nearly big enough to be seen with the unaided eye, while members of the cyanobacteria group feature filaments with specialized cells. These examples suggest that some bacteria can organize themselves into a kind of multicellular system. Moreover, many bacteria have very complex metabolic systems. Some even can live on iron and other metal deposits.
Finally, many bacteria may also be classified as gram-negative or gram-positive according to the composition of their cell wall. This classification is done by means of a laboratory staining technique invented by the Danish microbiologist Hans Christian Gram. Gram's stain consists of the dye crystal violet mixed with iodine. After a slide with bacteria on its surface has been heated so that the organisms adhere to the glass, the stain is applied to the slide, and the cell walls of the bacteria become stained. Alcohol is then applied to the slide. In bacteria with multilayered cell walls, the alcohol removes most of the stain. These bacteria appear reddish and are called gram-negative. In bacteria with a thicker, single-layer cell wall, the alcohol dehydrates the stained walls and causes the pores in the cell walls to close, preventing the stain from escaping. These bacteria appear purple and are called gram-positive.

Bacteria and Viruses
Many bacteria become directly infected by viruses known as bacteriophages. Some types of bacteriophages enter and kill the host bacterial cell, whereas others become integrated into the genetic machinery of the host bacterium. In a process called transduction, bacteriophages can move genetic material from one bacterium to another and even between different species of bacteria. Through natural selection, gene transfer can promote genetic traits such as resistance to antibiotics. Over the long run, bacteria have gained greatly from viral infection, for gene transfer and incorporation have enhanced their adaptability to environmental conditions.
The British scientist Frederick W. Twort and the French-Canadian scientist Félix H. d'Hérelle first discovered bacterial viruses in 1915. Today viruses are essential in genetic engineering, in which scientists duplicate natural processes to introduce favorable genetic characteristics to an organism.

Disease
Of the thousands of bacterial species on the earth, only a small fraction cause disease. For example, cholera results from infection by Vibrio cholerae, a bacterium that reproduces quickly in drinking and bathing water that has been extensively contaminated with human feces.
Pathogens are microorganisms that cause disease. Bacterial pathogens are frequently disabled or killed by the immune systems of organisms. Large cells called macrophages attack and destroy bacteria that are not normally present in the body, while cells called lymphocytes bring about other immune responses, including the production of antibodies.
Nevertheless, the history of human cultures around the world has been greatly shaped by those bacterial infections that overcame such immune responses. Outbreaks of plague, the deadly disease caused by the bacterium Yersinia pestis, have killed hundreds of millions of people throughout recorded history. The wall-less bacterium Mycobacterium tuberculosis also has had a major effect on human life, causing tuberculosis in millions of people through the centuries and bringing about premature deaths to many important literary and arts figures, including Honoré de Balzac, John Keats, and George Orwell. Although the incidence of tuberculosis greatly declined during the mid-twentieth century, the disease has recently become a major problem again. One of the primary drugs originally used to cure it, streptomycin, is ineffective today because the overuse of antibiotics has allowed resistant strains of bacteria to evolve.
For decades, human beings' primary means of controlling microbial growth has been pasteurization, sterilization, and other heating processes. Pasteurization is the use of mild heat to reduce bacterial populations in foods, whereas sterilization is the complete killing off of bacteria. Sterilization is necessary to destroy highly resistant bacterial structures such as endospores.

History
In the late 17th century, Antoni van Leeuwenhoek, a custodian and town caretaker of Delft, Holland, became the first person to make a systematic study of bacteria. Leeuwenhoek spent hundreds of hours to make the finest ground glass for his simple microscopes. Considered the founder of microbiology, he was the first to discover and describe a variety of very minute organisms, many of which we now know were bacteria. Leeuwenhoek's work set the stage for later researchers such as Louis Pasteur of France, who showed that microbes do not arise from nonliving matter, as scientists of his day believed, and Robert Koch of Germany, who showed that bacteria could cause disease.
As scientists have recognized the importance of bacteria in the functioning of the earth and all life forms, other microbiology researchers have taken on equal historical prominence. Sergei Winogradsky, a Russian soil investigator of the late 19th century, described important energy-yielding metabolic reactions in bacteria and discovered many anaerobic microorganisms. He is considered a founder of microbial ecology. Winogradsky's status is shared by his colleague Martinus Beijerinck, who discovered the role of microorganisms in the cycling of nutrients, especially nitrogen. He was one of the first scientists to study symbiotic organisms-that is, organisms that have combined with other organisms for survival advantage.
In the 20th century, scientists began to study certain bacteria for clues about how life originated and maintains itself. Entirely new views about the significance of bacteria to human cultures and to the earth have evolved. In the 1940s, American researcher Selman Waksman pioneered the development of antibiotics by discovering a wide range of filamentous soil bacteria that produce these substances. This discovery meant that diseases that once were crippling or fatal could be controlled or even cured.
American microbiologist Lynn Margulis resurrected the forgotten early-20th-century studies by biologists Konstantin S. Mereschkovsky and B. M. Kozo-Polyansky of Russia and Ivan Wallin of the United States by showing the fundamental prokaryotic nature of eukaryotes like plants and animals. In the 1960s, Margulis's studies led to the conclusion that such key eukaryotic features as the energy-generating centers called mitochondria in all animals, plants, and fungi and the photosynthesizing centers called plastids in all algae and plants were derived from ancient bacteria that had been acquired through symbiosis. In the 1980s American microbiologist Carl Woese promoted a revolutionary new way to classify and study bacteria. Developing a technique based on analyzing the sequence of mutations in genetic material, Woese showed that animals, plants, and fungi are more closely related than many species of bacteria are to one another.

Bacteria in Our Daily Lives
Bacteria are like living paint, covering nearly every surface imaginable and living within other living and nonliving things. Many exist in a symbiotic condition in which they function as partners with other organisms. This symbiosis has profound consequences on people's lives. For example, the agricultural industry depends on the existence of bacteria that can transform the nitrogen gas from the atmosphere into ammonia in the soil that plants can use in a process called nitrogen fixation.
The influence of bacteria may be most pronounced in their recycling ability. Like fungi, bacteria feed on dying material and convert it back into basic substances. This process of decomposition is as significant as photosynthesis, for without it food chains would cease, and fallen trees, leaves, and other refuse would simply pile up. Bacteria also strongly influence the movement of key elements, such as sulfur, iron, phosphorus, and carbon, around the globe. The weathering of rocks, which releases elements back into life systems for use, is substantially enhanced by the breakdown processes of bacteria.
Bacteria are the main digesters of cellulose within cows and other mammals. Cattle have evolved special stomachs that maintain bacteria and protista and allow for fermentation of the grass products. Considering that the manufacture of cheeses is dependent on bacterial fermentation, it is evident that the dairy industry would be nonexistent without bacteria.
The main cleansing agents in sewage treatment are a variety of specialized bacteria that convert, mostly through fermentation, the organic materials of sewage into carbon dioxide, methane, and hydrogen gases. There is a bacterial species involved with the production of nearly every familiar product. For example, vinegar, which is used as both a flavor enhancer and an important food preservative, results from the conversion of ethyl alcohol to acetic acid by acetic-acid bacteria. Specific enzymes extracted from bacteria are used in spot removers, meat tenderizers, laundry starches, and household detergents. Bacteria are now used throughout the growing biotechnology industry in the development of new products for medical treatment. Bacteria that can digest petroleum are even used in oil-spill cleanups.