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 [bak-te´re-um] (pl. bacte´ria) (L.)
any prokaryotic organism. adj., adj bacte´rial.

Bacteria are single-celled microorganisms that differ from all other organisms (the eukaryotes) in lacking a true nucleus and organelles such as mitochondria, chloroplasts, and lysosomes. Their genetic material consists of a single loop of double-stranded DNA, whereas the genetic material of eukaryotes consists of multiple chromosomes, which are complex structures of DNA and protein.

Bacteria reproduce by cell division about every 20 minutes, giving them a very high rate of population growth and evolution. Genetic material can be transferred between bacteria by three processes: transformation (absorption of naked DNA), transduction (transfer by a virus), and conjugation (transfer by independently replicating DNA molecules, called plasmids, which can be inserted into the bacterial DNA). Some bacteria can also form spores, dehydrated forms that are relatively resistant to heat, cold, lack of water, toxic chemicals, and radiation.

Most bacteria have a rigid cell wall outside of the cell membrane primarily composed of a dense layer of peptidoglycan, a network of polysaccharide chains with polypeptide crosslinks. Some antimicrobials, the penicillins and cephalosporins, act by interfering with peptidoglycan synthesis.

Bacteria can have any of three types of external structures: flagella (whiplike locomotor organelles), pili (minute filamentous appendages), or a capsule (a layer of gelatinous material around the cell). Various types of pili are involved in conjugation and in the adherence of bacteria to mucosal surfaces. The capsule protects the bacterium from phagocytosis.
Bacterial structure and morphology. From Hart and Shears, 1997.
Classification of Bacteria. Bacteria are classified into two major groups, gram-positive and gram-negative, based on their reaction to Gram stain. Other important classification characteristics are morphology and metabolic reactions. A spherical bacterium is called a coccus. Some species do not always completely separate when the cells divide and characteristically occur in pairs (see diplococcus), clusters (see staphylococcus), or chains (see streptococcus). A rod-shaped bacterium is called a bacillus. Some species have tapered ends (fusiform bacilli) or are shaped like long threads (filamentous bacilli) or spirals (spirochetes).

On the basis of their requirements for atmospheric oxygen, bacteria can be divided into obligate aerobes, which require oxygen; obligate anaerobes, which grow only in the absence of oxygen; and facultative anaerobes, which adapt to either environment. On the basis of their growth on a specific medium under aerobic and anaerobic conditions, certain groups are divided into oxidizers, those that use oxygen to metabolize sugars; fermenters, those that metabolize sugars in the absence of oxygen; and nonutilizers, which do not grow on the medium.

Two groups of prokaryotic organisms are sometimes not classified as bacteria. These are Cyanobacteria (the blue-green bacteria), which have aerobic photosynthesis like plants; and Mycoplasma, which lack cell walls.
Bacterial Infection. The skin, respiratory tract, and gastrointestinal tract are inhabited by a variety of bacteria. These normal flora are harmless or even helpful, protecting their host by interference with the growth of harmful bacteria. An opportunistic infection occurs when an organism indigenous to one part of the body invades another part where it is pathogenic. A commonly occurring example is infection of the urinary tract with Escherichia coli or other enteric bacilli.

There are many mechanisms by which pathogenic bacteria can be transmitted from person to person, including airborne infection, direct contact, contact with animals, transmission by insect vectors, or indirect transmission in drinking water, milk, or food or on inanimate objects. Although some diseases, such as cholera and botulism, are caused by toxins absorbed in the intestine, most diseases occur from bacteria that can attach to a mucosal surface, multiply, and invade tissue. To be pathogenic, bacteria must also be able to resist the host defenses: bactericidins, such as complement and lysozyme in the blood, and phagocytosis and subsequent intracellular destruction by leukocytes.

Bacteria can cause disease by producing toxins, by causing inflammation or the formation of granulomas, or by inducing a hypersensitivity reaction. exotoxins are extremely potent poisons produced by some gram-positive bacteria. These include neurotoxins, such as tetanus toxin and botulinum toxin; enterotoxins, such as cholera toxin; and diphtheria toxin, which blocks protein synthesis, thereby causing tissue necrosis. endotoxins are lipopolysaccharides that are components of the outer membrane of gram-negative cell walls and are released on cell lysis. They can cause hypotension, fever, disseminated intravascular coagulation, and shock. Other toxins include hemolysins and leukocidins, which destroy red and white blood cells; kinases, which lyse blood clots; and enzymes that attack tissue.

Host resistance to infection is lowered in weak and debilitated patients and in those with a decreased ability to mount an effective immune response because of disease or the effects of drugs (such as corticosteroids, immunosuppressive agents, or cytotoxic agents).

A major problem in antimicrobial therapy is the evolution of antibiotic-resistant strains of bacteria, which are an important cause of serious nosocomial infections. Unnecessary overuse of antimicrobial agents speeds up the evolution of resistant strains. This problem is exacerbated by the transfer of resistance between different species by plasmids, producing multiple drug-resistant strains.
Diseases Caused by Bacteria. The different kinds of bacteria tend to affect different organs and systems of the body, producing infectious diseases, each with its own group of symptoms.

Staphylococci are generally found on the surface of the skin. When they invade the body tissue, for instance through a cut, they usually produce a local infection with inflammation and pus. Occasionally a strain of staphylococcus develops that can cause an infection affecting more than a local area of the body, but this is relatively rare.

The diseases produced by streptococci are often more serious. Streptococci tend to resist localization and may spread through the bloodstream. Among the diseases caused by streptococci are streptococcal sore throat, rheumatic fever, and scarlet fever.

Pneumonia, meningitis, and gonorrhea are produced by different types of diplococci. The pneumococcus, which produces pneumonia, has its special effect on the lungs; the meningococcus has an affinity for the meninges of the brain and spinal cord. Both types of bacteria enter the body via the respiratory tract. gonorrhea, spread by bacteria called gonococci, is usually spread by coitus.

Cholera, caused by a spirillum and spread by unsanitary water supplies, was formerly a dread epidemic disease. syphilis, like gonorrhea, is spread most often by coitus. It also is caused by a spirillum.

Bacilli are responsible for many serious diseases, including plague, diphtheria, leprosy, tuberculosis, and typhoid fever. Prevention and control of the spread of many infectious diseases can be accomplished through immunization and proper sanitary conditions.
acid-fast bacterium one that is not readily decolorized by acids after staining, especially Mycobacterium and Nocardia.
blue-green bacteria see Cyanobacteria.
coliform bacterium one of the gram-negative rod-shaped bacteria that are normal inhabitants of the intestinal tract of humans and animals.
hemophilic bacteria bacteria that have a nutritional affinity for constituents of fresh blood or whose growth is significantly stimulated on blood-containing media.
lactic acid bacteria bacteria that, in suitable media, produce fermentation of carbohydrate materials to form lactic acid.
lysogenic bacterium any bacterial cell harboring in its genome the genetic material (prophage) of a temperate bacteriophage and thus reproducing the bacteriophage in cell division; occasionally the prophage develops into the mature form, replicates, lyses the bacterial cell, and is free to infect other cells.
water bacterium a gram-negative bacterium capable of rapid growth in all types of water and producing pyrogenic infections, especially in immunocompromised hospital patients, occurring as contaminants in hemodialysis fluids and in flood waters. Genera of medical importance include Aeromonas, Flavobacterium, and Pseudomonas.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.


(bak-tēr'ē-ŭm), The singular form is bacterium, not bacteria.
A bacterial generic name placed on the list of rejected names by the Judicial Commission and the International Committee on Systematic Bacteriology of the International Association of Microbiological Societies. As a consequence, Bacterium is no longer used in bacteriology. Identifiable organisms formerly placed in the genus Bacterium have all been transferred to other genera. Specifically, Bacterium anitratum is now known as Acinetobacter calcoaceticus; Bacterium coli is now called Escherichia coli.
[Mod. L. fr. G. baktērion, dim. of baktron, a staff or club]


A unicellular prokaryotic microorganism that usually multiplies by cell division and has a cell wall that provides a constancy of form; they may be aerobic or anaerobic, motile or nonmotile, and free living, saprophytic, commensal, parasitic, or pathogenic.
See also: Cyanobacteria.
[Mod. L. fr. G. baktērion, dim. of baktron, a staff]
Farlex Partner Medical Dictionary © Farlex 2012


n. pl. bac·teria (-tîr′ē-ə)
1. Any of various prokaryotic microorganisms of the domain Bacteria that may be free-living, saprophytic, commensal, or pathogenic and that vary widely in terms of morphology, oxygen tolerance, nutritional and temperature requirements, and motility. Also called eubacterium.
2. Any of the prokaryotic organisms, such as an archaeon. Not in scientific use.
The American Heritage® Medical Dictionary Copyright © 2007, 2004 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.


An obsolete genus of bacteria, the species of which have been placed in other genera—e.g., Acinetobacter, Clostridium, Escherichia, etc.
Segen's Medical Dictionary. © 2012 Farlex, Inc. All rights reserved.


, pl. bacteria (bak-tēr'ē-ŭm, -ă)
A unicellular prokaryotic microorganism that usually multiplies by cell division and has a cell wall that provides a constancy of form; may be aerobic or anaerobic, motile or nonmotile, and free-living, saprophytic, parasitic, or pathogenic.
See also: Cyanobacteria
[Mod. L. fr. G. baktērion, dim. of baktron, a staff]
Medical Dictionary for the Health Professions and Nursing © Farlex 2012


(bak-ter'e-um) (-ter'e-a) plural.bacteria [L. bacterium, fr Gr. bakterion, a small staff]
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A one-celled organism without a true nucleus or cell organelles, belonging to the kingdom Procaryotae (Monera). The cytoplasm is surrounded by a rigid cell wall composed of carbohydrates and other chemicals that provide the basis for the Gram stain. Some bacteria produce a polysaccharide or polypeptide capsule, which inhibits phagocytosis by white blood cells. Bacteria synthesize DNA, RNA, and proteins, and they can reproduce independently but may need a host to provide food and a favorable environment. Millions of nonpathogenic bacteria live on human skin and mucous membranes; these are called normal flora. Bacteria that cause disease are called pathogens. bacterial (-al), adjective See: table


Shape: There are three principal forms of bacteria. Spherical or ovoid bacteria occur as single cells (micrococci) or in pairs (diplococci), clusters (staphylococci), chains (streptococci), or cubical groups (sarcinae). Rod-shaped bacteria are called bacilli, more oval ones are called coccobacilli, and those forming a chain are called streptobacilli. Spiral bacteria are rigid (spirilla), flexible (spirochetes), or curved (vibrios). See: illustration

Size: On average, bacilli measure about 1 µm in diameter by 4 µm in length. They range in size from less than 0.5 to 1.0 µm in diameter to 10 to 20 µm in length for some of the spirilla.

Reproduction: Binary fission is the usual method of reproduction, but some bacteria exchange genetic material with members of the same species or different species. Reproductive rate is affected by changes in temperature, nutrition, and pH. If the environment becomes unfavorable, some bacilli form spores, in which their genetic material is condensed and surrounded by a thick wall. Spores are highly resistant to heat, drying, and disinfectants. When the environment again becomes favorable, the spores germinate.

Mutation: Bacteria, like all living things, undergo mutations, and the environment determines which mutations are beneficial and have survival value. Certainly beneficial to bacteria, though not at all to humans, are the mutations that provide resistance to the potentially lethal effects of antibiotics.

Motility: None of the cocci are capable of moving, but most bacilli and spiral forms can move independently. Locomotion depends on the possession of one or more flagella, slender whiplike appendages that work like propellers.

Food and oxygen requirements: Most bacteria are heterotrophic (require organic material as food). If they feed on living organisms, they are called parasites; if they feed on nonliving organic material, they are called saprophytes. Bacteria that obtain their energy from inorganic substances, including many of the soil bacteria, are called autotrophic (self-nourishing). Bacteria that require oxygen are called aerobes; those that grow only in the absence of oxygen are called anaerobes. Bacteria that grow both with and without oxygen are facultative anaerobes. Most bacteria in the human intestines are anaerobic. See: infection, opportunistic

Temperature requirements: Although some bacteria live at very low or very high temperatures, the optimum temperature for most human pathogens is 97° to 99°F (36° to 38°C).


Enzyme production: Bacteria produce enzymes that act on complex food molecules, breaking them down into simpler materials; they are the principal agents of decay and putrefaction. Putrefaction, the decomposition of nitrogenous and other organic materials in the absence of air, produces foul odors. Decay is the gradual decomposition of organic matter exposed to air by bacteria and fungi.

Toxin production: Cell wall molecules called adhesins bind bacteria to the host cells. Once attached, the bacteria may produce poisonous substances called toxins. There are two types: exotoxins, enzymes that are released by bacteria into their host, and endotoxins, which are parts of the cell walls of gram-negative bacteria and are toxic even after the death of the cell. Exotoxins include hemolysins, leukocidins, coagulases, and fibrinolysins. Endotoxins stimulate production of cytokines that can produce widespread vasodilation and shock. See: endotoxin; sepsis

Miscellaneous: Some bacteria produce pigments; some produce light. Soil bacteria are essential for the nitrogen cycle in the processes of nitrogen fixation, nitrification, and denitrification.


Several methods are used to identify bacteria in the laboratory:

Culture: Bacteria are grown on various culture media; a visible colony containing millions of cells may be visible within several hours. A colony is usually composed of the descendants of a single cell. Each species of bacteria grows in colonies with a characteristic color, shape, size, texture, type of margin or edge, and particular chemical features. Groups of cells can then be examined under a microscope, usually with Gram's stain. In addition, colonies can be separated and antibiotics applied to assess their sensitivity to different drugs.

Hanging drop: Unstained bacteria in a drop of liquid are examined under ordinary or dark-field illumination.

Gram's stain: Gram-positive bacteria retain dye, turning purple; gram-negative bacteria can be decolorized by alcohol and colored red by a second dye; acid-fast bacteria retain the dye even when treated with an acid alcohol decolorizer. Bacteria are often described by a combination of their response to Gram's stain and their appearance. For example, “gram-positive staphylococcus” indicates a cluster of spheres that stain purple, whereas gram-negative bacilli are rod-shaped and pink.

Immunofluorescence: Bacteria stained with fluorescein and examined under a microscope equipped with fluorescent light appearing yellow-green.

acetic acid bacteria

Any of a family of bacteria that oxidize alcohol and convert it to acetic acid (vinegar).

antibody-coated bacterium

1. A bacterium coated with an antibody that acts as an opsonin to make the bacterium more susceptible to phagocytosis.
2. A laboratory test using fluorescein-labeled antibodies to locate antigens with which the antibody links. See: opsonin

flesh-eating bacterium

A colloquial name given in the popular media to a rare invasive infection of the skin and underlying soft tissue by group A streptococcus. The infection is difficult to treat with antibiotics alone because it progresses rapidly through tissue planes. Emergency surgical debridement is required. See: necrotizing fasciitis

heterotrophic bacteria

Any of the bacteria that rely on organic compounds to grow and reproduce.

mucophob bacteria

Any of the bacteria that avoid, or cannot survive in, mucus.

probiotic bacterium

A bacterium that prevents illness, e.g., the Lactobacillus. species found in yogurt.
OrganismType and/or Site of Infection
Gram-Positive Bacteria
Clostridium difficilePseudomembranous colitis
Staphylococcus aureusPneumonia, cellulitis, boils, impetigo, toxic shock, postoperative bone/joints, eyes, peritonitis
Staphylococcus epidermidisPostoperative bone/joints, IV line–related phlebitis
Streptococcus pneumoniae (pneumococcus)Pneumonia, meningitis, otitis media, sinusitis, septicemia
Streptococcus pyogenesScarlet fever, pharyngitis, impetigo, rheumatic fever, erysipelas
viridans group streptococciEndocarditis
Gram-Negative Bacteria
Campylobacter jejuniDiarrhea (most common worldwide cause)
Escherichia coliUrinary tract, pyelonephritis, septicemia, gastroenteritis, peritonitis
Haemophilus influenzaePneumonia, meningitis, otitis media, epiglottitis
Klebsiella pneumoniaePneumonia, wounds
Legionella pneumophiliaPneumonia
Neisseria gonorrhoeaeGonorrhea
Neisseria meningitidis (meningococcus)Meningitis
Pseudomonas aeruginosaWounds, urinary tract, pneumonia, IV lines
Salmonella enteritidisGastroenteritis, food poisoning
Salmonella typhiTyphoid fever
Shigella dysenteriaeDysentery
Vibrio choleraeCholera
Medical Dictionary, © 2009 Farlex and Partners


The singular of BACTERIA.
Collins Dictionary of Medicine © Robert M. Youngson 2004, 2005
Bacteriumclick for a larger image
Fig. 60 Bacterium . Diagrammatic representation of a typical bacterium.


(pl. bacteria) a unicellular or (more rarely) multicellular PROKARYOTE organism. Some are AUTOTROPHIC and contain BACTERIOCHLOROPHYLLS and bacterioviridin, carrying out photosynthesis anaerobically Bacteria have various shapes, occurring as cocci (spherical), bacilli (rod-shaped), spirilla (helical) and vibrios (curved rods) which range in size from 1 μm to about 500 μm in diameter, but are usually between 1 and 10 μm. They are present in soil, water, air and as free-living SYMBIONTS, PARASITES and PATHOGENS. While some bacteria are useful in NITROGEN and SULPHUR CYCLES, many cause diseases of plants, animals and man, e.g. ANTHRAX, TETANUS. Reproduction occurs asexually but genetic transfer can take place by CONJUGATION, TRANSFORMATION and TRANSDUCTION; other genetic changes may be brought about by MUTATION, RECOMBINATION or the acquisition of a PLASMID (see Fig. 60 ). Traditionally, identification has been based on morphological characters and biochemical tests. Increasingly, however, molecular tests are being carried out, using, for example, PROBES.
Lambda(λ) Head and tail Double-stranded DNA E. coli
Mu Head and tail Double-stranded DNA E. coli
T2,T4,T6 Head and tail Double-stranded DNA E. coli
SP01 Head and tail Double-stranded DNA Bacillus subtillis
M13 Filament Single-stranded DNA E. coli
φX174 lcosahedral Single-stranded DNA E. coli
MS2,Qβ lcosahedral Single-stranded RNA E. coli
φ6 lcosahedral with envelope Double-stranded RNA Pseusdomonas phaseolica
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005


A microscopic one-celled organism. Haemophilus influenzae is a specific bacterium.
Gale Encyclopedia of Medicine. Copyright 2008 The Gale Group, Inc. All rights reserved.

Patient discussion about bacterium

Q. How the bacterias are produced?

A. The Bacteria are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals. (The name comes from the Greek bakterion, meaning small staff.) Bacteria are ubiquitous in every habitat on Earth, growing in soil, acidic hot springs, radioactive waste,[2] water, and deep in the Earth's crust, as well as in organic matter and the live bodies of plants and animals. There are typically 40 million bacterial cells in a gram of soil and a million bacterial cells in a millilitre of fresh water; in all, there are approximately five nonillion (5×1030) bacteria on Earth,[3] forming much of the world's biomass.[4] Bacteria are vital in recycling nutrients, with many important steps in nutrient cycles depending on these organisms, such as the fixation of nitrogen from the atmosphere and putrefaction. Hope this helps.

Q. I am wondering why did he prescribe me a bacteria? My doctor has prescribed some probiotic medicines for diarrhea recently and these medicines are actually bacteria…..I am wondering why did he prescribe me a bacteria?

A. Some bacteria like Lactobacillius and Bifidobacterium are normally present in the guts of the humans and they are beneficial bacteria for humans. If not present or if they reduce in the guts then other harmful bacteria may invade your guts and will lead to other bowel problems, low immunity and ulcers. So these bacteria are friendly. Diarrhea would flush out these gut bacteria which have to be supplemented immediately by giving probiotics in some.

Q. Dog waste bacteria. How bad is it.Dose it ever die? My husband has invented a Pet Waste-A-Way it liquefies the dog waste & it goes into the earth with in 8 seconds.We are worried about the environment & what it would do. We have gotten info back both ways. Some say it is there if you have dogs & some say it is bad & never dies.


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