Bacterial world

Some of the key genera of Proteobacteria:

Nitrosomonas and Nitrobacter: Bacteria able to grow hemolithotrophically at the expense of reduced inorganic nitrogen compounds are called nitrifying bacteria. These two are belongs to nitrifying bacteria. Nitrification results from the sequential activities of two physiological groups of organisms, the ammonia-oxidizing bacteria (which oxidize NH3 to nitrite, NO2-) and the nitrite-oxidizing bacteria, the actual nitrate-producing bacteria, which oxidize NO2 - toNO3-. Ammonia-oxidizing bacteria typically have genus names beginning in Nitroso-, whereas genus names ofnitrate producers begin with Nitro-.

Pseudomonas: The cells are straight or slightly curved gram negative, chemoorganotrophic rods with polar flagella. Some major distinguishing characteristics of the pseudomonad group include an obligately respiratory metabolism, the absence of gas formation from glucose and a positive oxidase test, all of which help to distinguish pseudomonads from enteric bacteria. Pseudomonads typically have very simple nutritional requirements and one of their characteristic properties is the ability to use many different organic compounds as carbon and energy sources; some species utilize over 100 different compounds. The pseudomonads are ecologically important in soil and water and are probably responsible for the degradation of many low-molecular-weight compounds derived from the breakdown of plant and animal materials in oxic habitats. They are also capable of catabolizing many xenobiotic (not naturally occurring) compounds, such as pesticides and other toxic chemicals, and are thus important agents of bioremediation in the environment. Pseudomonas fluorescens is a plant-associated rhizobacteria helping the plant growth. Pseudomonas aeruginosa is frequently associated with infections of the urinary and respiratory tracts in humans.

Azotobacter: Azotobacter is a free-living chemoorganotrophic bacteria inhabit soil and is capable of aerobic nitrogen (N2) fixation. When they are growing on N2 as a nitrogen source, extensive capsules or slime layers. Azotobacter can form resting structures called cysts. Like bacterial endospores, Azotobacter cysts show negligible endogenous respiration and are resistant to desiccation, mechanical disintegration, and ultraviolet and ionizing radiation. Cysts are not very heat resistant, and they are not completely dormant because they rapidly oxidize carbon sources if supplied.

Enteric bacteria (Escherichia): The enteric bacteria comprise a relatively homogeneous phylogenetic group within the Gammaproteobacteria and consist of facultatively aerobic, gram-negative, nonsporulating rods that are either nonmotile or motile by peritrichous flagella. Among the enteric bacteria are many species pathogenic to humans, other animals, or plants, as well as other species of industrial importance. Escherichia coli, the best known of all microorganisms, is the classic example of an enteric bacterium. Escherichia, Salmonella, Proteus, Enterobacter are the key genera belongs to this group.

Spirilla: The spirilla are gram-negative, motile, spiral-shaped bacteria that show a wide variety of physiological attributes. Some key taxonomic criteria used are cell shape, size, and number of flagella, relation to oxygen (obligately aerobic, microaerophilic), relationship to higher organisms, and certain other physiological characteristics, such as nitrogen fixation and halophilism. Azospirillum lipoferum is a nitrogen-fixing organism and of considerable interest because it enters into a symbiotic relationship with tropical grasses and grain crops such as corn. Spirillum, Aquaspirillum, Oceanospirillum, and Azospirillum are the key genera belong to this group.

Vibrio: The Vibrio group contains gram-negative, facultatively aerobic rods and curved rods that employ a fermentative metabolism. Most species of Vibrio are polarly flagellated, although some are peritrichously flagellated. One key difference between the Vibrio group and enteric bacteria is that members of the former are oxidase-positive, a test for the presence of cytochrome c, whereas members of the latter are oxidase-negative. The best-known genera in this group are Vibrio, Aliivibrio, and Photobacterium. Most vibrios and related bacteria are aquatic, found in marine, brackish, or freshwater habitats. Vibrio cholerae is the specific cause of the disease cholera in humans. Cholera is one of the most common human infectious diseases in developing countries and is transmitted almost exclusively via water.

Rhizobium: Rhizobia are species of Alpha- or Betaproteobacteria that can grow freely in soil or can infect leguminous plants and establish a symbiotic relationship. The same genus (or even species) can contain both rhizobial and nonrhizobial strains. Infection of legume roots by rhizobia leads to the formation of root nodules in which the bacteria fix gaseous nitrogen. Nitrogen fixation in root nodules accounts for a fourth of the N2 fixed annually on Earth and is of enormous agricultural importance, as it increases the fixed nitrogen content of soil. Nodulated legumes can grow well on unfertilized bare soils that are nitrogen deficient, while other plants grow only poorly on them.

The genus Bacillus and Paenibacillus: Species of Bacillus and Paenibacillus grow well on defined media containing any of a number of carbon sources. Many bacilli produce extracellular hydrolytic enzymes that break down complex polymers such as polysaccharides, nucleic acids, and lipids, permitting the organisms to use these products as carbon sources and electron donors. Many bacilli produce antibiotics, including bacitracin, polymyxin, tyrocidine, gramicidin, and circulin. In most cases the antibiotics are released when the culture enters the stationary phase of growth and is committed to sporulation.

The genus Clostridium: The clostridia lack a respiratory chain; unlike Bacillus species, they obtain ATP only by substrate-level phosphorylation. Many anaerobic energy-yielding mechanisms are known in the clostridia. A number of clostridia are saccharolytic and ferment sugars, producing butyric acid as a major end product. Some of these also produce acetone and butanol, such as Clostridium pasteurianum, which is also a vigorous nitrogen-fixing bacterium. One group of clostridia ferments cellulose with the formation of acids and alcohols, and these are likely the major organisms decomposing cellulose anaerobically in soil.

The genus Lactobacillus: Lactobacilli are typically rod-shaped, varying from long and slenderto short, bent rods. Lactobacilli are common in dairy products, and some strains are used in the preparation of fermented milk products. For instance,Lactobacillus acidophilus is used in the production of acidophilus milk; Lactobacillus delbrueckii is used in the preparation of yogurt; and other species are used in the production of sauerkraut, silage, and pickles. The lactobacilli are usually more resistant to acidic conditions than are the other lactic acid bacteria and are able to grow well at pH values as low as 4. Because of this, they can be selectively enriched from dairy products and fermenting plant material by use of acidic carbohydrate-containing media. The acid resistance of the lactobacilli enables them to continue growing during natural lactic fermentations, even when the pH value has dropped too low for other lactic acid bacteria to grow. The lactobacilli are therefore typically responsible for the final stages of most lactic acid fermentations.

Mollicutes

It is a class of bacteria distinguished by the absence of a cell wall. The word "Mollicutes" is derived from the Latin mollis (meaning "soft" or "pliable"), and cutis (meaning "skin"). Individuals are very small, typically only 0.2–0.3 μm (200-300 nm) in size and have a very small genome size. They vary in form, although most have sterols that make the cell membrane somewhat more rigid. Many are able to move about through gliding, but members of the genus Spiroplasma are helical and move by twisting. The best-known genus in the Mollicutes is Mycoplasma.

Mollicutes are parasites of various animals and plants, living on or in the host's cells. Many cause diseases in humans, attaching to cells in the respiratory or urogenital tracts, particularly species of Mycoplasma and Ureaplasma. Phytoplasma and Spiroplasma are plant pathogens associated with insect vectors.

Whereas formerly the trivial name "mycoplasma" has commonly denoted any member of the class Mollicutes, it now refers exclusively to a member of the genus Mycoplasma.

Actinobacteria

Actinobacteria are Gram-positive bacteria with high G+C DNA content that constitute one of the largest bacterial phyla, and they are ubiquitously distributed in both aquatic and terrestrial ecosystems. Many Actinobacteria have a mycelial lifestyle and undergo complex morphological differentiation. They also have an extensive secondary metabolism and produce about two-thirds of all naturally derived antibiotics in current clinical use, as well as many anticancer, anthelmintic, and antifungal compounds. Consequently, these bacteria are of major importance for biotechnology, medicine, and agriculture. Actinobacteria play diverse roles in their associations with various higher organisms, since their members have adopted different lifestyles, and the phylum includes pathogens (notably, species of Corynebacterium, Mycobacterium, Nocardia, Propionibacterium, and Tropheryma), soil inhabitants (e.g., Micromonospora and Streptomyces species), plant commensals (e.g., Frankia spp.), and gastrointestinal commensals (Bifidobacterium spp.). Actinobacteria also play an important role as symbionts and as pathogens in plant-associated microbial communities.

Most of the Actinobacteria (the streptomycetes in particular) are saprophytic, soil-dwelling organisms that spend the majority of their life cycles as semi dormant spores, especially under nutrient limited conditions. However, the phylum has adapted to a wide range of ecological environments: actinomycetes are also present in soils, fresh and salt water, and the air. They are more abundant in soils than other media, especially in alkaline soils and soils rich in organic matter, where they constitute an important part of the microbial population. Actinobacteria can be found both on the soil surface and at depths of more than 2 m below ground.

The population density of Actinobacteria depends on their habitat and the prevailing climate conditions. They are typically present at densities on the order of 10^6 to 10^9 cells per gram of soil; soil populations are dominated by the genus Streptomyces, which accounts for over 95% of the Actinomycetales strains isolated from soil. Other factors, such as temperature, pH, and soil moisture, also influence the growth of Actinobacteria. Like other soil bacteria, Actinobacteria are mostly mesophilic, with optimal growth at temperatures between 25 and 30°C. However, thermophilic Actinobacteria can grow at temperatures ranging from 50 to 60°C. Vegetative growth of Actinobacteria in the soil is favored by low humidity, especially when the spores are submerged in water. In dry soils where the moisture tension is greater, growth is very limited and may be halted. Most Actinobacteria grow in soils with a neutral pH. They grow best at a pH between 6 and 9, with maximum growth around neutrality. However, a few strains of Streptomyces have been isolated from acidic soils (pH3.5).

The genus Mycobacterium. This group shares an unusual waxy cell envelope that contains mycolic acids, meaning these bacteria are unusual in being acid fast and alcohol fast. The mycobacterial cell wall contains various polysaccharide polymers, including arabinogalactan, lipomannan, lipoarabinomannan, and phosphatidylinositol mannosides. Mycobacteria are generally free-living saprophytes and they are the causative agents of a broad spectrum of human diseases. Mycobacterial diseases are very often associated with immunocompromised patients, especially those with AIDS. In addition, M. bovis and M. tuberculosis, isolated initially from infected animals, are most likely obligate parasites of humans. Both species can survive within macrophages and cause pulmonary disease, although organs other than lungs may be affected. M. leprae, which causes leprosy, lives in Schwann cells and macrophages; infection with this species results in a chronic granulomatous disease of the skin and peripheral nerves. Interestingly, the pathogenic M. ulcerans, which is the third most common causative agent of mycobacterial disease, has also been isolated as a soil inhabitant in symbiosis with roots of certain plants living in tropical rain forests and similar environments.

The genus Nocardia. The genus Nocardia is a ubiquitous group of environmental bacteria that is most widely known as the causative agent of opportunistic infection in immunocompromised hosts. Nocardia species are ubiquitous soilborne aerobic actinomycetes, with more than 80 different species identified, of which at least 33 are pathogenic. The pathogen can spread to the brain, kidneys, joints, bones, soft tissues, and eyes, causing disseminated nocardiosis in humans and animals. Moreover, Nocardia species produce industrially important bioactive molecules, such as antibiotics and enzymes.

The genus Corynebacterium: Among the known pathogenic members of Corynebacterium are C. diphtheria, which is a notorious strictly human-adapted species and the causative agent of the acute, communicable disease diphtheria, which is characterized by local growth of the bacterium in the pharynx along with the formation of an inflammatory pseudomembrane. The virulence factor in diphtheria is an exotoxin that targets host protein synthesis. Another important Corynebacterium pathogen is C. ulcerans, which is increasingly acknowledged as an emerging pathogen in various countries; infections with this species can mimic diphtheria.

The genus Bifidobacterium. Bifidobacterium has different shapes, including curved, short and bifurcated Y shapes. The cells have no capsule and they are non-sporeforming, nonmotile and nonfilamentous bacteria. The genus encompasses bacteria with health-promoting or probiotic properties, such as antimicrobial activity against pathogens that is mediated through the process of competitive exclusion and also bile salt hydrolase activity, immune modulation, and the ability to adhere to mucus or the intestinal epithelium. This bacterium is considered as one of the Probiotic bacteria.

The genus Streptomyces. Among the various mycelial genera of Actinobacteria, Streptomyces has received particular attention for several reasons: (1) Streptomyces are abundant and important in the soil, where they play major roles in the cycling of carbon trapped in insoluble organic debris, particularly from plants and fungi (Carbon sequestration). This action is enabled by the production of diverse hydrolytic exoenzymes. (2) the genus exhibits a fairly wide phylogenetic spread. Third, streptomycetes are among Nature’s most competent chemists and produce a stunning multitude and diversity of bioactive secondary metabolites; consequently, they are of great interest in medicine and industry.

The genus Frankia. Frankia is the only nitrogen-fixing actinobacterium and can be distinguished by its ability to enter into symbiotic associations with diverse woody angiosperms known collectively as actinorhizal plants. The most notable plant genera in this group are Alnus, Casuarina, and Elaeaginus, and their symbiosis with Frankia enables them to grow well in nitrogen-poor soils.

Cyanobacteria

Cyanobacteria comprise a large, morphologically and ecologically heterogeneous group of oxygenic, phototrophic Bacteria. Cyanobacteria represent one of the major phyla of Bacteria and show a distant relationship to gram-positive bacteria. These organisms were the first oxygen-evolving phototrophic organisms on Earth, and over billions of years converted the originally anoxic atmosphere of Earth to the highly oxic atmosphere we see today.

The morphological diversity of the cyanobacteria is impressive. Both unicellular and filamentous forms are known, and there is considerable variation within these morphological types. Cyanobacteria can be divided into five morphological groups: (1) unicellular, dividing by binary fission; (2) unicellular, dividing by multiple fission (colonial); (3) filamentous, containing differentiated cells called heterocysts that function in nitrogen fixation; (4) filamentous non-heterocystous forms; and (5) branching filamentous species

Cyanobacterial cells range in size from 0.5–1 µm in diameter to cells as large as 40 µm in diameter. Phylogenetically, cyanobacteria group along morphological lines in most cases. Filamentous, heterocystous, and nonheterocystous species form distinct groups, as do the branching forms. However, unicellular cyanobacteria are highly diverse, with different representatives showing phylogenetic relationships to different morphological groups. The cell wall of cyanobacteria is similar to that of gram negative bacteria, and peptidoglycan is present in the walls. Many cyanobacteria produce extensive mucilaginous envelopes. The photosynthetic membrane system is often complex and multilayered, although the thylakoid membranes are regularly arranged in concentric circles around the periphery of the cytoplasm in some of the structurally simpler cyanobacteria. Cyanobacteria produce chlorophyll a, and all of them also have characteristic biliprotein pigments, phycobilins, which function as accessory pigments in photosynthesis. One class of phycobilins, phycocyanins, are blue and together with the green chlorophyll a, are responsible for the blue-green color of most cyanobacteria (in past these group of bacteria were called as Blue Green Algae). However, some cyanobacteria produce phycoerythrin, a red phycobilin, and species producing phycoerythrin are red or brown.