Bacteria

? Bacteria
Escherichia coli
Scientific classification
Domain: Bacteria
Subgroups
Actinobacteria Aquificae Bacteroidetes/Chlorobi Chlamydiae/Verrucomicrobia Chloroflexi Chrysiogenetes Cyanobacteria Deferribacteraceae|Deferribacteres Deinococcus-Thermus Dictyoglomi Fibrobacteres/Acidobacteria Firmicutes Fusobacteria Gemmatimonadetes Nitrospirae Planctomycetes Proteobacteria Spirochaetes Thermodesulfobacteria Thermomicrobia Thermotogae

Bacteria (singular: bacterium) are a group of microscopic, single-celled prokaryotes—that is, organisms characterized by a lack of a nucleus and membrane-bound organelles. The term "bacteria" has been variously applied to all prokaryotes, or to a major group of them exclusive of the anaerobic archaebacteria. In the five kingdom system of classification (see taxonomy, bacteria are placed in the Kingdom Monera. In the Domain system of classification, bacteria are usually considered synonomous with the Bacteria or Eubacteria domain, although the term sometimes includes the Archaea (originally labelled Archaebacteria domain). The largely photosynthetic blue-green algae, or Cyanobacteria, were once classified as algae, but are now considered bacteria. The study of bacteria is known as bacteriology, a subfield of microbiology.

Bacteria bodies may be spherical, rod-shaped, or spiral/curved shaped. Although unicellular, some bacteria form groupings of cells, such as clusters, filaments, or chains.

Bacteria are the most abundant of all organisms. They are ubiquitous in soil, water, and as symbionts of other organisms. Many pathogens, disease-causing organisms, are bacteria. Most are minute, usually only 0.5-5.0 μm in their longest dimension, although giant bacteria like Thiomargarita namibiensis and Epulopiscium fishelsoni may grow past 0.5 mm in size. They generally have cell walls, like plant and fungal cells, but with a very different composition (peptidoglycans). Many move around using flagella, which are different in structure from the flagella of other groups.

Although bacteria are often viewed negatively, given this groups connection to diseases, bacteria also perform invaluable, beneficial functions. These include decompostion of plants and animals, aid in digestion, nutrient absorption, yogurt production, sewage treatment, and as sources of medicines.

Cellular structure

As prokaryotes, all bacteria have a relatively simple cell structure lacking a cell nucleus and membrane-bound organelles such as mitochondria and chloroplasts. The DNA of prokaryotes floats freely around the cell.

The most prominent bacterial structural characteristic is the cell wall. Bacteria can be divided into two groups (Gram positive and Gram negative) based on differences in cell wall structure as revealed by Gram staining. In a Gram stain, a series of dyes are applied to stain cell walls. Gram negative bacteria have an outer, lipopolysaccharide-containing membrane and stain pink. They also have a thin peptidoglycan layer located in the periplasm (the region between the outer and cytoplasmic membranes). Gram positive bacteria lack this covering, but possess a cell wall containing a thick peptidoglycan (called Murein in older sources) layer and teichoic acids; they stain purple. Gram-positive bacteria are more susceptible to antibiotics, while gram-negative bacteria are difficult to treat with antibiotics.

Many bacteria contain other extracellular structures such as flagella, fimbriae, and pili, which are used respectively for motility (movement), attachment, and conjugation (transmission of DNA between bacterial cells by contact other than fusion). Some bacteria also contain capsules or slime layers that facilitate bacterial attachment to surfaces and biofilm formation.

Bacteria contain relatively few intracellular structures compared to eukaryotes but do contain a tightly supercoiled chromosome, ribosomes, and several other species-specific structures such as intracellular membranes, nutrient storage structures, gas vesicles and magnetosomes (membranous organelles that contain magnetite crystals to orient magnetotactic bacteria in geomagnetic fields). Some bacteria are capable of forming endospores that allow them to survive extreme environmental and chemical stresses. This property is restricted to specific Gram positive organisms such as Bacillus and Clostridium.

Reproduction

Bacteria reproduce through asexual reproduction (binary fission) that results in cell division. Two identical clone daughter cells are produced. Bacterial population growth is thus sometimes said to follow an approximate exponential growth phase.

Certain types of bacteria are also capable of sexual reproduction through bacterial conjugation. In conjugation, one bacterium transfers genetic material to another through a long, hollow tube called a sex pilus. The genetic material transferred may be either chromosomal or from a plasmid. Conjugation increases the genetic variability of bacterial populations and facilitates the emergences of antibiotic resistance.

Movement and senses

A-Monotrichous; B-Lophotrichous; C-Amphitrichous; D-Peritrichous;

Motile bacteria can move about, either using flagella, bacterial gliding, or changes of buoyancy. A unique group of bacteria, the spirochaetes, have structures similar to flagella, called axial filaments, between two membranes in the periplasmic space. They have a distinctive helical body that twists about as it moves.

Bacterial flagella are arranged in many different ways. Bacteria can have a single polar flagellum at one end of a cell, clusters of many flagella at one end. or flagella scattered all over the cell, as with Peritrichous. Many bacteria (such as E.coli) have two distinct modes of movement: forward movement (swimming) and tumbling. The tumbling allows them to reorient and introduces an important element of randomness in their forward movement.

Motile bacteria are attracted or repelled by certain stimuli, behaviors called taxes—for instance, chemotaxis, phototaxis, mechanotaxis and magnetotaxis. In one peculiar group, the myxobacteria, individual bacteria attract each other to form swarms and may differentiate to form fruiting bodies. The myxobacteria move only when on solid surfaces, unlike E. coli which is motile in liquid or solid media.

Some bacteria have the ability to perceive the concentration gradient of stimuli to a resolution of one part per 10,000 (equivalent to a human being able to discern between two coin-filled jars, one filled with 9,999 coins, the other with 10,000).

Groups and identification

Bacteria come in a wide variety of shapes:
A. Bacillus
B. Cocci in chains
C. Cocci in clusters
D. Cocci in twos
E. Spirillum
F. Vibrio

Historically, bacteria, as originally studied by botanists, were classified in the same way as plants, that is, mainly by shape. Bacteria come in a variety of different cell morphologies (shapes), including bacillus (rod-shape), coccus (spherical), spirillum (helical), and vibrio (curved bacillus). However, because of their small size, bacteria are relatively uniform in shape and therefore classification of species based on morphology was unsuccessful.

The first formal classification scheme originated following the development of the Gram stain by Hans Christian Gram. This separates bacteria based on the structural characteristics of their cell walls. This scheme included:

• Gracilicutes—Gram negative staining bacteria with a second cell membrane
• Firmicutes—Gram positive staining bacteria with a thick peptidoglycan wall
• Mollicutes—Gram negative staining bacteria with no cell wall or second membrane
• Mendosicutes—atypically staining strains now classified as cyanobacteria in the domain Archaea

Further developments essentially based on this scheme included comparisons of bacteria based on differences in cellular metabolism, as determined by a wide variety of specific tests. Bacteria were also classified based on differences in cellular chemical compounds such as fatty acids, pigments and quinones, for example. While these schemes allowed for the differentiation between bacterial strains, t was unclear whether these differences represented variation between distinct species or between strains of the same species. It was not until the utilization of genome-based techniques such as %guanine+cytosine ratio determination, genome-genome hybridization, and gene sequencing (in particular the rRNA gene) that microbial taxonomy developed (or at least is developing) into a stable, accurate classification system. It should be noted, however, that due to the existence of numerous historical classification schemes and our current poor understanding of microbial diversity, bacterial taxonomy remains a changing and expanding field.

Bacteria that form clusters are described by the prefix staphylo", such as a cluster of spherical bacterial cells would be called a staphylococcus and a cluster of rod-shaped bacterial cells would be called a staphylobacillus. When bacteria form filaments, they prefex strepto is used, such that a cluster of rod-shaped bacterial cells would be called a streptobacillus.

History

Fossil bacteria have been found in deposits in present-day Australia that are estimated to be 3.5 billion years old. The great antiquity of the bacteria has enabled them to evolve a great deal of genetic diversity. They are far more diverse than the mammals or insects, for instance. Indeed, the genetic distance between E. coli and the bacterium Thermus aquaticus is greater than the distance between humans and oak trees.

The first bacteria were observed by Anton van Leeuwenhoek in 1676 using a single-lens microscope of his own design. The name bacterium was introduced much later, by Christian Gottfried Ehrenberg in 1828, derived from the Greek word]] βακτηριον meaning "small stick". Because of the difficulty in describing individual bacteria and the importance of their discovery to fields such as medicine, biochemistry, and geochemisty, the history of bacteria is generally described as the history of microbiology.

Two organelles, mitochondria and chloroplasts, are generally theorized to have been derived from endosymbiotic bacteria.

Benefits and dangers

Microorganisms are widely distributed and are most abundant where they have food, moisture, and the right temperature for their multiplication and growth. They can be carried by air currents from one place to another. The human body is home to billions of microorganisms; they can be found on skin surfaces, in the intestinal tract, in the mouth, nose, and other body openings. They are in the air one breathes, the water one drinks, and the food one eats.

Bacteria are both harmful and useful to the environment, and animals, including humans. The role of bacteria in disease and infection is important. Some bacteria act as pathogens and cause tetanus, typhoid fever, pneumonia, syphilis, cholera, foodborne illness, leprosy, and tuberculosis(TB). Sepsis, a systemic infectious syndrome characterized by shock and massive vasodilation, or localized infection, can be caused by bacteria such as Streptococcus, Staphylococcus, or many gram-negative bacteria. Some bacterial infections can spread throughout the host's body and become systemic. In plants, bacteria cause leaf spot, fireblight, and wilts. The mode of infection includes contact, air, food, water, and insect-borne microorganisms. The hosts infected with the pathogens may be treated with antibiotics, which can be classified as bacteriocidal and bacteriostatic, which at concentrations that can be reached in bodily fluids either kill bacteria or hamper their growth, respectively. Antiseptic measures may be taken to prevent infection by bacteria, for example, prior to cutting the skin during surgery or swabbing skin with alcohol when piercing the skin with the needle of a syringe. Sterilization of surgical and dental instruments is done to make them sterile or pathogen-free to prevent contamination and infection by bacteria. Sanitizers and disinfectants are used to kill bacteria or other pathogens to prevent contamination and risk of infection.

In soil, microorganisms which reside in the rhizosphere (a zone that includes the root surface and the soil that adheres to the root after gentle shaking) help in the transformation of molecular dinitrogen gas as their source of nitrogen, converting it to nitrogenous compounds in a process known as nitrogen fixation. This serves to provide an easily absorbable form of nitrogen for many plants, which cannot fix nitrogen themselves. Many other bacteria are found as symbionts in humans and other organisms. For example, the presence of the gut flora in the large intestine can help prevent the growth of potentially harmful microbes.

The ability of bacteria to degrade a variety of organic compounds is remarkable. Highly specialized groups of microorganisms play important roles in the mineralization of specific classes of organic compounds. For example, the decomposition of cellulose, which is one of the most abundant constituents of plant tissues, is mainly brought about by aerobic bacteria that belong to the genus Cytophaga. This ability has also been utilized by humans in industry, waste processing, and bioremediation. Bacteria capable of digesting the hydrocarbons in petroleum are often used to clean up oil spills. Some beaches in Prince William Sound were fertilized in an attempt to facilitate the growth of such bacteria after the infamous 1989 Exxon Valdez oil spill. These efforts were effective on beaches that were not too thickly covered in oil.

Bacteria, often in combination with yeasts and molds, are used in the preparation of fermented foods such as cheese, pickles, soy sauce, sauerkraut, vinegar, wine, and yogurt. Using biotechnology techniques, bacteria can be bioengineered for the production of therapeutic drugs, such as insulin, or for the bioremediation of toxic wastes.

Sources

• Some text in this entry was merged with the Nupedia article entitled Bacteria, written by Nagina Parmar; reviewed and approved by the Biology group (editor: Gaytha Langlois, lead reviewer: Gaytha Langlois, lead copyeditors: Ruth Ifcher and Jan Hogle)
• This article contains material from the Science Primer published by the NCBI, which, as a US government publication, is in the public domain at http://www.ncbi.nlm.nih.gov/About/disclaimer.html.

Further reading

• Alcamo, I. Edward. Fundamentals of Microbiology. 5th ed. Menlo Park, California: Benjamin Cumming, 1997.
• Atlas, Ronald M. Principles of Microbiology. St. Louis, Missouri: Mosby, 1995.
• Holt, John.G. Bergey's Manual of Determinative Bacteriology. 9th ed. Baltimore, Maryland: Williams and Wilkins, 1994.
• Hugenholtz P, Goebel BM, Pace NR (1998). "Impact of culture-independent studies on the emerging phylogenetic view of bacterial diversity". J Bacteriol 180 (18): 4765-74. PMID 9733676.
• Koshland, Daniel E., Jr. (1977). "A Response Regulator Model in a Simple Sensory System". Science 196: 1055-1063. PMID 870969.
• Stanier, R.Y., J. L. Ingraham, M. L. Wheelis, and P. R. Painter. General Microbiology. 5th ed. Upper Saddle River, New Jersey: Prentice Hall, 1986.

External links

• Bacterial Nomenclature Up-To-Date from DSMZ
• Bacterial Growth and Cell Wall (Ger)
• Microminds
• The largest bacteria
• Tree of Life
• Videos of bacteria swimming and tumbling, use of optical tweezers and other fine videos.
• Planet of the Bacteria by Stephen Jay Gould
• Major Groups of Prokaryotes

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