The bacteria ( Bacteria ) are living organisms Unicellulaire S Procaryotes , characterized by an absence of core and of Organite S. the majority of the bacteria have a cellular Paroi glucidic, the Peptidoglycane. The bacteria measure some micrometers length and can present various forms: spherical forms (hulls), forms lengthened or in sticks (bacilli), more or less spiral forms. The study of the bacteria is the Bactériologie, a branch of the Microbiologie.

The bacteria are ubiquitaires and are present in all the types of Biotope S met on Ground. They can be isolated from the ground, of fresh water, marines or brackish, of the air, the oceanic depths, the earth's crust, on the skin and in the intestine of the animals. There is forty million bacterial cells in one gram of ground and a million bacterial cells in one millilitre of fresh water. In all, there would be five quintillions (5 × 10³⁰) of bacteria in the world. However, a great number of these bacteria are not characterized yet because noncultivable in laboratory. The bacteria have a considerable importance in the biogeochemical cycles like the Cycle of carbon and the fixing of the nitrogen of the atmosphere.

At the Man, it was calculated that 10¹² bacteria colonizes the skin, 10¹⁰ bacteria colonize the mouth and 10¹⁴ bacteria lives in the intestine, with the result that there are ten times more bacterial cells than of human cells in the human body. The majority of these bacteria are inoffensive or beneficial for the organization. There exists however of many species Pathogène S at the origin of much of infectious illness like the Choléra, the Syphilis, the Peste, the carbuncle, the Tuberculose. Bacteria can involve respiratory or intestinal disorders whereas others can be responsible for the infection of wound. The bacterial infections can be treated thanks to the Antibiotique S which generally inhibit one of their vital function (for example the Pénicilline blocks the synthesis of the cellular wall).

The bacteria can be very useful for the Man during the treatments of waste water, in the agroalimentary one during manufacture of the Yaourt S or the Fromage and in the industrial production of many chemical compounds.

History

The bacteria being microscopic, they are thus visible only with a Microscope. Antoine van Leeuwenhoek was the first to observe bacteria, thanks to a microscope of its manufacture, in 1668.

The word “bacterium” appears for the first time with the German microbiologist Christian Gottfried Ehrenberg in 1828. This word derives from the Greek βακτηριον, who means “stick”.

At the 19th century, work of Louis Pasteur revolutionized the Bactériologie. He showed in 1859 that the processes of Fermentation are caused by Micro-organisme S and that this growth was not due to the spontaneous Generation. He showed also the role of the micro-organisms like infectious agents. Pasteur also designed culture media, processes of destruction of the micro-organisms like the Autoclave and the Pasteurization.

The German doctor Robert Koch and its collaborators reflects at the point the farming techniques of the bacteria on solid medium. Robert Koch is one of the pioneers of medical microbiology, it worked on the Choléra, the Maladie of coal (carbuncle) and the Tuberculose. He showed in a clear way that a bacterium could be the agent responsible for an infectious illness and he proposed a series of postulates (the Postulats of Koch) confirming the role etiologic of a micro-organism in a disease. He obtained the Nobel Prize of medicine and physiology in 1905.

The microbiologists Martinus Beijerinck and Sergei Winogradsky initiated the first work of microbiology of the environment and microbial ecology by studying the microbial communities of the ground and water and the relations between these micro-organisms.

If the bacteria were known at the century, there did not exist yet of antibactérien treatment. In 1909, Paul Ehrlich developed a treatment against the Syphilis before the use of the Pénicilline into therapeutic suggested by Ernest Duchesne in 1897 and studied by Alexander Fleming in 1929.

Into 1977, Carl Woese thanks to his work of molecular phylogeny divided the procaryotes into two groups: Bacteria and the Archaea.

Cellular structure

As a Procaryotic (organization without core), the bacteria are relatively simple cells, characterized by an absence of core and Organite S like the Mitochondrie S and the Chloroplaste S.

An important characteristic of the bacteria is the cellular Paroi. The bacteria can be divided into two groups (negative Gram and positive Gram) based on the difference of the structure and the chemical composition of the cellular wall highlighted thanks to the Coloration of Gram. The bacteria with coloring of positive Gram have a cellular wall containing a Peptidoglycane (or muréine) thick and teichoïques acid whereas bacteria with coloring of negative Gram present a fine peptidoglycane localized in the periplasm between the cytoplasmic membrane and an external cellular membrane. The wall gives to the bacterium its form and protects it from the bursting under the effect of the very strong osmotic pressure of the cytoplasm. The peptidoglycane ensures the rigidity of the wall. There exist however bacteria without wall: they are the Mycoplasme S.

At the intracellular level, the bacteria have a chromosome in the form of filament of DNA, support of heredity. The bacterial chromosome is in general circular. In addition to this DNA genomic, the bacterial cells often contain molecules of circular DNA extra-chromosomal called Plasmide S. the cells contain also many Ribosome S allowing the proteinic synthesis thanks to the mechanism of the translation. The cytoplasm of the procaryotes often contains intracellular substances of reserve which are stocks of nutrients in the form of Glycogène, Amidon or Poly-b-hydroxybutyrate (PBH). Certain species of watery bacteria have blisters with gases which ensure the buoyancy of the cells. Other species, the magnetotactic bacteria, have the characteristic to present a magnétosome.

Many bacteria have extracellular structures as Flagelle S used for the mobility of the cells, and the Fimbria E allowing the attachment or the phenomenon of conjugation. The bacteria Hétérotrophe S can use their whip to move towards zones rich in organic substances (nutrients) thanks to the phenomenon called Chimiotactisme.

Some bacteria can manufacture fine external layers with the cellular wall, generally consisted of Polysaccharide S (of sugars). When the layer is compact, one speaks about capsule. The capsules constitute for example a barrier of protection of the cell against the external environment and also against phagocytosis. It facilitates also the attachment on the surfaces and the formation of Biofilm S. Klebsiella , Bacillus anthracis , Streptococcus pneumoniae are examples of capsuled bacteria. When the layer is diffuse, one speaks about mucoïde layer. When the layer is diffuse, one speaks about Glycocalyx. The glycocalyx makes it possible the bacteria to adhere to a support.
Certain qualified bacteria of engainées bacteria produce a dense and rigid external layer: the sheath. This phenomenon is current in the bacteria of water which form filamentous chains ( Sphaerotilus natans for example). The sheath protects the cells against turbulences from water. Bacteria of the group Cytophaga - Flavobacterium produces a mucous layer which enables them to remain in close contact with a solid medium. Other bacteria as the Spirillum can wrap of a called proteinic layer the layer S.

Some bacteria like Bacillus or Clostridium can manufacture Endospore S enabling them to resist certain conditions of environmental or chemical stress.

Growth and Reproduction

The cellular division

Two identical cells are produced starting from a cell mother. The cell multiplication appears by an increase in cellular volume, followed synthesis of a transverse septum in the middle of the cell, leading to the separation of the two cells girls. Bacterial division is preceded by duplication by the bacterial Chromosome thanks to the Réplication by the DNA.

Some bacteria have more complex reproductive structures but always in an asexual way, facilitating dispersion: Myxococcus works out fructifications, while air Streptomyces form of the hyphas.

When they are in a favourable medium the bacteria can multiply with a vertiginous pace. A population of bacterium can double every 20 minutes according to: the availability in nutrients, the presence of concurrent bacteria, the presence of predatory (for example of the Paramécie S), the presence of Bacteriophage S, the Antibiotic presence of S (inhibiting for example the synthesis of the bacterial wall, thus resulting in their death) products by mushrooms or Actinomycète S (filamentous bacteria).

Growth and culture of the bacteria

At the laboratory, the bacteria can be cultivated in Culture medium liquid or in solid medium. The culture medium must bring the nutritive elements or elementary nutrients to the bacterium. The solid gélosés culture media are used to isolate from the pure cultures of bacterial cells. In the case of the bacteria dividing quickly, a bacterial cell dispersed on a gélosé medium will multiply and, at the end of 24 to 48 hours, will become a cluster of bacteria, called a bacterial colony, visible with the naked eye.

The generation time is the time necessary with a bacterium to divide. The generation time thus corresponds to the time necessary so that a population of cells doubles of number. This time is very variable according to the species of bacteria and the conditions environmental. At the laboratory, under ideal conditions, it is for example 20 minutes for Escherichia coli , 100 minutes for Lactobacillus acidophilus , 1000 minutes for Mycobacterium tuberculosis .

The growth of a bacterial population in a liquid culture medium not renewed, can be observed in time. The cells divide, and their number increases with time. If one notes the number of bacteria with various intervals during the growth, one obtains a curve of growth. It presents four principal phases:

• the phase of latency corresponds to one period of adaptation of the bacterium to the medium
• During the phase of exponential growth, the bacteria develop in a maximum way with a growth rate maximum and constant
• After a transitional stage of deceleration, the number of bacteria does not evolve/move any more: it is the stationary phase. Bacterial divisions which are still done are compensated by the death of bacteria
• the last phase is the phase of mortality or decline. The bacteria do not divide any more, they die and can be lysed. The culture medium does not bring any more the requirements to the development of the bacteria. One observes a curve of progressive exponential decay.

Parameters influencing the microbial growth

Certain environmental conditions (physicochemical parameters) influence the growth of the micro-organisms. Among those appear the pH (acidity and alkalinity), the temperature, the presence of O₂, of CO₂, the availability of the eau.
The majority of the micro-organisms tolerate a range of pH allowing the growth. The optimal pH of growth of much of bacteria is close to neutrality (pH 7). The micro-organisms Acidophile S develop with pH acids, whereas the micro-organisms Alcalinophile S develop with pH basiques.
In the same way, the bacteria can be distinguished according to their aptitude to grow according to the temperature. The Mésophile S generally develop with temperatures ranging between 20 and 45°C. The Psychrophile S have optimal temperatures of growth lower than 15°C, whereas the bacteria Thermophile S grow in an optimal way at temperatures ranging between 45 and 70°C. The micro-organisms having optimal temperatures of growth higher than 70°C are described as hyperthermophiles.

Genetics

Genetic material

The majority of the bacteria have single a circular Chromosome. There exists however of rare example of bacteria, like Rhodobacter sphaeroides having two chromosomes. The size of the Génome can be very variable according to the species of studied bacteria. The genome of the stock of Escherichia coli sequence in 1997 consists of 4,6 Mpb (4  600  000 even of bases), it codes 4200 Protéine S. The genome of another stock of E. coli sequence in 2001 includes/understands 5,5 Mpb coding 5400 proteins. Certain bacteria present a very small genome, like the bacterium parasitizes Mycoplasma genitalium with a genome of 580  000 pairs of bases and the endosymbiotic bacterium of insect, Candidatus Carsonella ruddii with a genome of only 160  000 pairs of bases. On the contrary, the bacterium of the ground Sorangium cellulosum has a genome made up of 12  200  000 pairs of bases. Common thing not very, the Spirochetes as of the Streptomyces has the effect of having a linear chromosome.

The bacteria also often contain one or more Plasmide S, which are molecules of extra-chromosomal DNA. These plasmides can confer certain advantages on the bacteria, like resistance to antibiotics or factors of virulence. The plasmides are generally DNA doubles circular bit. They are retorted independently of the chromosome. The bacterial chromosome can in addition integrate DNA of bacterial virus (Bactériophage). These bacteriophages can contribute to the Phénotype of the host. For example, the bacteria Clostridium botulinum and Escherichia coli O157: H7 synthesizes a toxin coded by a gene which comes from a phage which was integrated into the genome of these bacteria during the evolution.

Genetic variation

The bacteria are asexual organizations, after bacterial division, the cells girls inherit an identical copy of the genome of their relative. However, all the bacteria are able to evolve/move by modification of their genetic material caused by genetic recombinations or changes. The changes (random specific change of the genetic Information of a cell) come from error during the replication of the DNA or from the exposure to agents Mutagène S. the rate of change varies largely according to the species or the bacterial strains.

Some bacteria can also transfer from the genetic material between the cells. There exist 3 mechanisms of transfer of genes between the cells: the transformation, the transduction, and the conjugation.
During the transformation, it is a plasmide which is transferred in the bacterial cell, whereas during transduction, the transfer of DNA takes place via a bacteriophage. During the conjugation, two bacteria can approach, thanks to special structures, the pili, and there is then a transfer of DNA of a bacterium to another. The foreign DNA can be integrated in the Génome and be transmitted to the following generations. This acquisition of genes, coming from a bacterium or environment, is called horizontal transfer of gene (horizontal HGT for obstructs transfer ). The transfer of genes is particularly important in the mechanisms of Résistance to the antibiotics.

Morphology and association of the bacteria

The bacteria present a great diversity of sizes and forms. The typical bacterial cells have a size ranging between 0,5 and 5 µm length, however, some species like Thiomargarita namibiensis and Epulopiscium fishelsoni can measure up to 500 µm (0,5 mm) length and be visible with the naked eye. Among the smallest bacteria, the Mycoplasmes measure 0,3 µm, that is to say a size comparable with certain large viruses.

The majority of the bacteria are spherical, is called hulls ( pl . cocci, of Greek the kókkos , grain), or is in the shape of sticks, called bacilli ( pl . baccili, of Latin baculus , stick). There exist also intermediate forms: cocobacilles. Some bacteria in the shape of sticks are slightly curved like the Vibrio . Other bacteria are helicoid. They are Spirille S if the form is invariable and rigid, of the Spirochète S if the organization is flexible and can change form. The great diversity of forms is determined by the cellular wall and the Cytosquelette. The various shapes of bacteria can influence their capacity to acquire nutrients, to stick to surfaces, to swim in a liquid and to escape the predation.

Many bacterial species can be observed in isolated unicellular form whereas other species are associated in pairs (diploïdes) like the Neisseria or in of chain, characteristic of the Streptocoques. In these cases, the hulls divide according to a single axis and the cells remain dependant after division. Certain hulls divide according to a perpendicular axis and are arranged systematically to form layers. Others divide in a disordered way and form clusters as the members of the kind Staphylococcus who present a characteristic regrouping in bunch of grapes. Other bacteria can pass and form filaments made up of several cells like the Actinomycetes. Other organizations as the Cyanobactéries form chains called trichomes. In this case, the cells are in close relationship and the physiological exchanges are favoured.

In spite of their apparent simplicity, the bacteria can also form complex associations. They can stick to surfaces and to form aggregations called Biofilm S. the bacteria present in the biofilm can present a complex arrangement of cells and extracellular components, forming secondary structures like microcolonies, in which a canal system facilitating is formed the diffusion of the nutrients.

Within the biofilms of the relations are established between bacteria, leading to an integrated cellular answer. Molecules of the cellular communication or " Quorum sensing " are either of Homosérine lactones for the bacteria with negative Gram, or of short peptides for the bacteria with positive Gram. Moreover within established biofilms, the physicochemical characteristics (pH, oxygenation, metabolites) are harmful with the good bacterial development and constitute stressing conditions. The bacteria set up answers of stresses which are as much of adaptation to these adverse conditions. In general the answers of stress make the bacteria more resistant to any form of destruction by mechanical agents or molecules biocides.

Mobility of the bacteria

Certain bacteria are mobile and can move thanks to one or more Flagelle S, other bacteria can move by slip.

Whip bacteria are long flexible proteinic appendices. Their number and their position can differ according to the species from bacteria. Scourging (or ciliature) polar monotriche corresponds to the presence of only one whips with a pole of the bacterium (example of the Vibrio ). Polar scourging lophotriche corresponds to the presence of several whip with the pole of the bacterium ( Pseudomonas for example). Other bacteria as Escherichia coli produces whip on all cellular surface and thus have a scourging péritriche.

The filament of whips consists of a protein, the Flagelline. The type of rotation of whips can determine the type of movement of the bacterium.

The mobile bacteria can react to stimuli, be attracted by nutritive substances like sugars, the amino-acids, oxygen, or be pushed back by harmful substances. This behavior is called the Chimiotactisme. Chimiorécepteurs of proteinic nature are present at the level of the plasmic membrane and the periplasm of the bacteria and can detect various gravitational or harmful substances.

The copper ions block the rotation of whip. To make it set out again, one resorts to the acid ethylenediaminetetraacetic, able to capture the ions and thus to release whips it.

Metabolism

The Métabolisme of a cell is the whole of the chemical reactions which occur on the level of this cell. To carry out this process, the bacteria, like all the other cells, have energy requirement. ATP is the universal biochemical energy source. The ATP is common to all forms of life, but the reactions of Oxydo-réduction implied in its synthesis are very varied according to the organizations and in particular in the bacteria. The bacteria live in practically all the environmental niches of the Biosphère. They can thus use a very broad variety of source of carbon and/or énergie.
The bacteria can be classified according to their type of metabolism, according to the sources of carbon and energy used for the growth, the donors of electrons and the acceptors of electrons.
The cellular energy of the Chimiotrophe S is of origin chemical whereas that of the phototrophes is of luminous origin. The source of carbon of the Autotrophe S is CO₂, while organic substrates are the source of carbon of the Hétérotrophe S. It is also possible to distinguish two possible sources from Proton S (H+) and from electron S (E): the bacteria reducing of the mineral compounds are Lithotrophe S whereas those reducing of the organic substances are organotrophe s.

The bacteria can be divided into four great nutritional types according to their sources of carbon and energy:

• the Photoautotrophe S use the light like energy source and the CO₂ like source of carbon.
• the Photohétérotrophe S develop by Photosynthèse. They assimilate CO₂ in the presence of a donor of electrons.
• the Chimioautotrophe S use reduced inorganic substrates for the reducing assimilation of CO₂ and like energy source.
• the Chimiohétérotrophe S use organic substrates like source of carbon and energy.

At the chimiohétérotrophes, the substrates are degraded in smaller molecules to give intermediate metabolites (Pyruvate, AcétylCoA…) who themselves are degraded with production of CO₂, H₂O and of energy. These reactions energy producers are reactions of oxidation of a hydrogenated substrate, with release of protons and of electrons thanks to Déshydrogénase S. the transfer of protons and electrons to a final acceptor is carried out by a whole series of enzymes which form a chain of electronic transport. Energy thus produced is released by small stages with an aim of being transferred in chemical bonds rich in energy (ATP, NADH, NADPH). According to the nature of the final acceptor of electrons, one distinguishes the processes of the Respiration and the Fermentation. Breathing can be Aérobie when O₂ is the final acceptor of protons and electrons, or Anaérobie (breathing nitrates, and breathing fumarate for example). In all the cases, the final acceptor of electrons must be an oxidized molecule (O₂, NO − , SO − ).
At the organizations Aerobic S, the Oxygène is used like acceptor of electrons. At the anaerobic organizations, others inorganic compounds like the Nitrate, the Sulfate or the Carbon dioxide are used like acceptors of electrons. These organizations take part in very important ecological processes at the time of the Dénitrification, the reduction of sulfates and the Acétogénèse. These processes are also important at the time of biological answers to pollution, for example, the bacteria reducing sulfates are responsible for the production for made up highly toxic starting from the mercury (methyl and diméthylmercure) present in the environment. The anaerobes (nonrespiratory) use the Fermentation to provide energy to the growth of the bacteria. During fermentation, an organic compound (the substrate or the energy source) is the donor of electrons while an other made up organics is the acceptor of electrons. The principal substrates used during fermentation are Glucide S, amino-acid , Purine S and Pyrimidine S. Divers compounds can be salted out by the bacteria during fermentations. For example, alcoholic fermentation led to the formation of ethanol and CO₂. The optional anaerobic bacteria are able to modify their metabolism between fermentation and various final acceptors of electrons, according to the conditions of the medium where they are.

According to their lifestyle, the bacteria can be classified in various groups:

• the strict Aérobie S can live only in the presence of Dioxygène or oxygenates molecular (O₂).
• the aéro-anaerobes optional can live in presence or absence of dioxygene;
• the Anaérobie S can live only in absence of dioxygene. The aérotolérants are anaerobic organizations which can survive all the same in the presence of oxygen.
• the Microaérophile S require oxygen to survive but a weak concentration.

The bacteria Lithotrophe S can use inorganic compounds like energy source. The Hydrogen, the Carbon monoxide, the Ammonia (NH₃), ferrous ions as well as other reduced metal ions and some compounds of the reduced Sulfur. The Méthane can be used by the Méthanotrophe S like source of carbon and electrons. At the phototrophes aerobic and the Chimiolithotrophe, oxygen is used like final acceptor of electrons, whereas in anaerobic condition, in fact inorganic compounds are used.

In addition to the fixing of CO₂ during photosynthesis, some bacteria can fix the Azote N₂ (fixing of the nitrogen by using an enzyme: the Nitrogénase. Aerobic, anaerobic and photosynthetic bacteria are able to fix nitrogen. The Cyanobactérie S which fix nitrogen, have specialized cells (the Hétérocyste S).

Bacteria and ecosystem

The bacteria, with the others Micro-organisme S take part to a very large extent in biological balance existing in the surface of the Ground. They colonize all indeed the ecosystem S and are at the origin of fundamental chemical conversions at the time of the biogeochemical processes responsible for the cycle of the elements on planet.

Watery ecosystem

Natural water like marine water (Ocean S) or the fresh water (Lake S, Pond S, pond S, River S…) are very important microbial habitats. The Organic matters in solution and the minerals dissolved allow the development of the bacteria. The bacteria take part in these mediums in the self-purification of water. They are also the prey of the Protozoaire S. the bacteria composing the Plancton of the aquatic environments are called the Bactérioplancton.

Bacterium of the ground

The ground is composed of mineral matter coming from the erosion of the rocks and of organic matter (the Humus) coming from the decomposition partial of the Végétaux. The microbial flora there is very varied. It includes/understands Bactérie S, Champignon S, Protozoaire S, Algue S, Virus, but the bacteria are the most important representatives quantitatively. One can find all the types of bacteria there, of the Autotrophe S, the Hétérotrophe S, the Aérobie S, the Anaérobie S, the Mésophile S, the Psychrophile S, the Thermophile S. Certaines bacteria, like mushrooms, are able to degrade insoluble substances of vegetable origin like the Cellulose, the Lignine, to reduce the Sulfate S, to oxidize the Soufre, to fix the atmospheric Azote and to produce Nitrate S. the bacteria play a part in the cycle of the nutrients of grounds, and are in particular able to fix the nitrogen. They thus have a role in the fertility of the grounds for the Agriculture. The bacteria abound on the level of the roots of the plants with which they live in mutualism.
With the difference in the aquatic environments, water is not always available in the grounds. The bacteria set up strategies to adapt to the dry periods. The Azotobacter produce Cyste S, the Clostridium and the Bacillus of the Endospore S or other types of spores at the Actinomycète S.

Extreme environments

The bacteria can also be met in more extreme environments. They are described as Extrémophile S. Of the bacteria Halophile S are met in salted lakes, bacteria Psychrophile S are isolated from cold environments like oceans Arctique and the Antarctic, Banquise S. Of the bacteria Thermophile S are isolated from the hot sources or the hydrothermal chimneys.

Interactions with other organizations

In spite of their apparent simplicity, the bacteria can maintain complex associations with other organizations. These associations can be indexed in Parasitisme, mutualism and Commensalisme. Because of their small sizes, the bacteria commensales are ubiquitaires and are met on the surface and inside the plants and of the animals.

Mutualists

In the ground, the bacteria of the Rhizosphère (soil horizon fixed at the roots of the plants) fix the Azote and produce nitrogenized compounds used by the plants (example of the bacterium Azotobacter or Frankia ). In exchange, the plant excretes on the level of the roots of sugars, the amino-acids and the vitamins which stimulate the growth of the bacteria. Other bacteria as Rhizobium are associated with the plants Légumineuses on the level of Nodosité S on the roots.

There exist many symbiotic relations or mutualists of bacteria with Invertébré S. For example, the animals which develop near the hydrothermal chimneys of the oceanic funds like the tubicolous worms Riftia pachyptila , the moulds Bathymodiolus or the shrimp Rimicaris exoculata live in symbiosis with chimiolitho-autotrophic bacteria.
Buchnera is a bacterium Endosymbiote Aphide S (plant louse). She lives inside the cells of the insect and provides him essential amino-acids. The bacterium Wolbachia is lodged in the testicles or the ovaries of certain insects. This bacterium can control the capacities of reproduction of sound hôte.
Bacteria are associated with the Termite S and sources of nitrogen and carbon bring to him.

Bacteria colonizing the Rumen Herbivore S allow the digestion of the Cellulose by these animals. The presence of bacteria in the intestine of the Man contributes to the digestion of food but the bacteria also manufacture Vitamine S like the pholic acid , the Vitamine K and the biotine.
Bacteria colonize the jabot of a bird folivore (consuming sheets), the Hoazin ( Opisthocomus hoazin ). These bacteria allow the digestion of cellulose of the sheets, in the same manner as in the rumen of the ruminants.

Bioluminescent bacteria as Photobacterium are often associated with marine fish or invertebrates. These bacteria are points of disjunction in specific bodies among their hosts and emit a luminescence thanks to a particular protein: the Luciférase. This luminescence is used by the animal at the time of various behavior like the reproduction, the attraction of preys or the dissuasion from predatory.

Pathogenic

The pathogenic bacteria are responsible for human diseases and cause infections. The infectious organizations can be distinguished in three types: pathogenic obligatory the, accidental ones or opportunistes.
Pathogenic obligatory cannot survive apart from its host. Among the obligatory pathogenic bacteria, Corynebacterium diphtheriae involves the Diphtérie, Treponema pallidum is the agent of the Syphilis, Mycobacterium tuberculosis causes the Tuberculose, Mycobacterium leprae the Lèpre, Neisseria gonorrhoeae the Gonorrhée. The Rickettsia at the origin of the Typhus are parasitic bacteria intracellulaires.
Pathogenic an accidental present in nature can infect the Man under certain conditions. For example, Clostridium tetani causes the Tétanos while penetrating in a wound. Vibrio cholerae involves the Choléra following the consumption of a water contaminée.
Pathogenic opportunist infects individuals weakened or reached by another disease. Bacteria like Pseudomonas aeruginosa , of the species of the normal flora, like Staphylococcus of the cutaneous flora, can become the pathogenic opportunist ones under certain conditions. One especially meets this type of infection in hospitalier.
medium

The capacity of a bacterium to cause a disease is its pathogenic capacity. The intensity of the pathogenic capacity is the Virulence. The result of the relation bacterium-host and the evolution of the disease depend on the number of pathogenic bacteria present in the host, of the virulence of this bacterium, defenses of the host and his degree of résistance.
To start a disease, the infectious bacteria must initially penetrate in the organization and adhere to a fabric. Factors of adhesion allow the fixing of the bacteria a cell. The invasive capacity is the capacity of the bacterium to be spread and to multiply in fabrics of the host. The bacteria can produce lytic substances enabling him to be disseminated in fabrics. Certain bacteria present also a capacity toxinogene which is the capacity to produce Toxine S, chemical substances carrying damage with the host. One can distinguish the Exotoxin S released during the multiplication of the bacteria and the Endotoxine S fixed in the membrane of the bacteria.

The pathogenic bacteria trying to invade a host meet however many mechanisms of defense ensuring the organization a protection with the infections. A good food and a hygiene of correct life constitute the first protection. The skin, the mucous membranes form a first line of defense against the penetration of pathogenic organizations. The bacteria of the normal flora constitute also a barrier of protection. When a micro-organism penetrated these first lines of defense, it meets specialized cells which are mobilized against invasion: they are the Phagocyte S. the Inflammation is a nonspecific defensive reaction. A second very effective defense system is the Immune system specific, able to recognize Antigène S carried or secreted by the bacteria, and to work out Anticorps and specific immunizing cells of these antigens.

Importance of the bacteria in industry and technologies

The origin of industrial microbiology dates from the prehistoric time . The first civilizations used without the knowledge of the micro-organisms to produce alcoholic drinks, Pain and Fromage.

The bacteria like Lactobacillus , Lactococcus or Streptococcus , combined with the Yeast S and Moisissure S intervene in making of food fermented like cheeses, the Yaourt S, the Bière, the Vin, the sauce of soya, the Vinaigre, the Choucroute.

The acetic bacteria ( Acetobacter , Gluconobacter ) can produce acetic Acid starting from the ethanol. They are met in the alcoholic juices and are used in the production of the Vinaigre. They are also exploited for the production of ascorbic acid (Vitamine C) starting from the Sorbitol transformed into Sorbose.

The Heterotrophic capacity of the bacteria S to degrade a broad variety of organic compounds is exploited in treatments of waste like the Bioremédiation or the treatment of waste water. Bacteria are also used in the septic tanks to ensure purification of it. Bacteria, able to degrade Hydrocarbon S of the Oil, can be used during the cleaning of a Oil slick. The process of cleaning of mediums polluted by micro-organisms is the Bioremédiation.

Bacteria can be used to recover metals of economic interests starting from ores. It is the Biolixiviation. The activity of bacteria is thus exploited for the recovery of copper.

Bacteria can be used in the place of Pesticide S in biological Lutte to fight parasites of the plants. For example, Bacillus thuringiensis produces a protein LT which is toxic for certain Insecte S. This toxin is used in Agriculture to fight insects which nourish Plante S.

Because of their capacity to be multiplied quickly and of their relative facility to being handled, certain bacteria as Escherichia coli are tools very much used in Molecular biology, Génétique and Biochimie. The scientists can determine the function of Gène S, Enzyme S or identify metabolic ways necessary to the fundamental comprehension of living and also allowing to implement new applications in Biotechnologie.

Many a Enzyme S used in various industrial processes was isolated from micro-organisms. The enzymes of the Détergent S are Protéase S of certain stocks of Bacillus . Amylase S able to hydrolize the starch are very much used in food industry. The Taq polymerase used in the reactions of polymerization in chain (PCR) for the amplification of the DNA comes from a thermophilous bacterium Thermus aquaticus .

The genetically modified bacteria are very much used for the production of medicinal products. It is the case for example of the Insuline, the Growth hormone, some Vaccin S, of the Interféron S… Certain bacteria as Streptomyces are very employed for the production of Antibiotique S.

Classification and Identification

The Taxonomie makes it possible to classify the living organisms in a rational way. In the bacteria, the Taxon S in the hierarchical order are the following: phyla (or divisions), classes, subclasses, orders, sub-orders, families, subfamilies, tribes, subgroups, kinds, sub-genera, species and subspecies. Various approaches allow the classification of the bacteria.

phenotypical classification

• morphological Criteria (form and grouping of the bacteria, presence or absence of whip, nature of the wall, type of mobility, presence of Endospore).
• physiological Criteria (metabolic standard, energy source, of carbon, nitrogen, type of substrate used, capacity to produce certain molecules, products of fermentation, metabolites secondary…).
• Criteria of pathogenicity.
• Criteria of Sérogroupage.

Chimiotaxonomy

It is about the chemical analysis of cellular components (structure and composition of the wall, the plasmic membranes, the peptidoglycane).

Molecular classification

• Composition in bases of the DNA. The percentage of Guanine + Cytosine varies from an organization with another, but is relatively constant within the same species. The percentage of G + C varies from 25 to 70% at the procaryotes.
• Hybridization DNA - DNA. This technique makes it possible to compare the totality of the bacterial genome and to estimate the degree of homology between two bacteria. This characteristic is important in the definition of a bacterial species.
• Sequencing of ribosomal ARN S or sequencing of genes coding ARNr. At the procaryotes, the comparison of the nucleotide sequence of the ARN 16S makes it possible to evaluate the degree of relationship between these organizations. These genes are described as molecular clock and allow phylogenetic classification .

Identification of the bacterial species

The genetic determination of the species is based on the study of genes of the ribosomal ARN. The choice of genes of ARNr 16S is justified for the following reasons:

• ARNr 16S are molecules ubiquists;

• their structure is well preserved because any modification could harm the proteinic synthesis. It results a very slow evolution from it from these genes.

The choice of genes of ARNr rather than ARNr themselves is based on the choice of the technique of amplification by PCR. This technique allows, starting from a colony of bacteria, to obtain fragments of DNA corresponding to gene or part of gene. The genetic analyzes also relate to the intergenic area 16S-23S operate ribosomal ARN. The latter is a variable area length according to the organizations. It gives an immediate indication on the fact that two stocks given belong or not to the same species.

All the micro-organisms have at least a copy of genes coding the ribosomal ARN. These molecules are essential to the synthesis of proteins, reason for which this sequence of DNA is very preserved within the species (more than 99%). This conservation of sequence makes it possible to use this area for the determination of the species. Indeed, the degree of similarity of the sequences of ARNr between two organizations indicates their relative relationship. The procedure using ARNr 16S as factor of identification implies the extraction of the DNA of the bacteria of a colony. Then starters recognizing of the very preserved zones of gene make it possible to amplify by PCR most of the gene ARNr 16S, which thereafter is sequence. The data on the nucleotidic sequence are compared with databases of already known sequences. The sequences of gene coding ARNr 16S are known for more than 4000 bacterial strains. These sequences can be consulted by interrogation of data banks such as EMBL and GenBank by the software Fasta and Blast. The Ribosomal one Dated Project II (RDP) is also interesting insofar as its base of data is specific ARN 16S. This software is accessible on line on the Internet. According to the various authors, the degree of homology between two bacteria so that they belong to the same species must be higher than 97%, even 99%.

As the genes of the intergenic area 16S-23S are preserved, they differ from one stock to another as well on the level of the sequence as the length. This results from what many bacteria have of the multiple copies by genome of the opéron of ARNr, it results during amplification a characteristic reason from it. As for gene of ARNr 16S, the systematic study of the intergenic area 16S-23S requires the amplification of this area by PCR. The utility of the intergenic area 16S-23S is that it makes it possible to distinguish from the different species and sometimes various stocks within the same species. Indeed the intergenic area being less preserved, of variabilities on the level them sequences can arise for stocks of the same species but pertaining to different biovars.

The sequences of the intergenic area 16S-23S are compared by interrogation of the data bases IWoCS which are specific intergenic area 16S-23S. The base of GenBank data also is very well provided. The degree of homologies should ideally be close to 100% for identical stocks.

Anecdotes

Oldest bacteria in life

September 4th, 2007 a drilling in the Pergélisol of the Canadian North-West allowed scientists of the University of California directed by professor Eske Willerslev (University of Copenhagen) to put at the day a old Bactérie of approximately 500  000 years and always alive.

One found a bacterium deadened inside a Abeille which was in Ambre (fossil resin - coming from Conifère S of the time Oligocène, which pushed on the site of current the the Baltic - being presented in the form of pieces hard and breakable, more or less transparent, yellow or reddish) since 25 to 40 million years.

In the same way, a remained bacterium deadened since 250 million years was discovered in a salt crystal. She was discovered by Russell Vreeland of the university of West Chester in Pennsylvania in a bed of salt to approximately 600 meters under ground, close to Carlsbad to the Nouveau Mexico.

In space, the bacteria would become almost three times more virulent. It is at least the case of Salmonella typhimurim, a bacterium responsible for food poisoning. Those went on a journey on board the Atlantis shuttle in 2006. To their return, the bacteria which had been preserved in a tight container, were transmitted to mice. One has needed only one the third of the usual amount to kill half of the group of mouse which had been infected.

Search for extraterrestrial bacteria

One currently seeks to know if there were a bacterial life on the planet Mars. Certain elements of analysis of the Martian ground seem to be directed in this direction, and the abundant presence of water over Mars formerly perhaps could constitute a ground extremely favorable to the development of the bacterial life, if it appeared. If the thing had suddenly been confirmed, it would be an important component in favor of the assumption of Panspermie.

A thing seems certain today (2006): the various American apparatuses and European envoys over Mars with an aim of exploration of planet left there a great quantity of bacteria of terrestrial origin. If these terrestrial bacteria are able to survive by finding the water Martian deep (which sometimes flashes back on the surface or accumulates in ice around the poles) and to adapt to the physico-chemical and climatic medium of this planet (in particular of the extrémophiles able to use carbon and iron oxides), they could contaminate and quickly colonize grounds which one seeks to explore today, and even produce rather quickly new specifically Martian species, in the long term producing deep changes in the chemistry of the grounds, even on the very thin atmosphere of Mars.

Other research is also interested in the ices of the moon jovienne Europe which shelter liquid water under their surface.

See too

• bacterial Structure
• Archaea
• microbial Ecology
• List of important bacteria on the clinical level

References

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