Order Bifidobacteriales Bifidobacterium
Order Bifidobacteriales Bifidobacterium a All classes are listed, but not all orders. Only some representative, important, and interesting genera are included.
as resistant. The endospore develops within the vegetative cell (Figure 10.27) and is then released through cell lysis. Under suitable conditions, the endospore can later germinate to produce a new vegetative cell. This ability is of obvious survival value in the soil, where conditions suitable for growth may alternate with unsuitable conditions. The degree of resistance shown by endospores is quite amazing, including survival in boiling water and autoclaving (steam heating at 121°C for 20 minutes) of soil. Probably the most incredible, however, was the recent finding of viable Bacillus endospores in the gut of a bee preserved in amber for at least 25 million years!
Low G + C The genus Clostridium contains anaerobic endosporeforming rods. A variety of fermentation products (some have been used commercially) may be produced from carbohydrates by different species, including carbon dioxide, hydrogen, acetate, acetone, butanol, butyric acid, ethanol, isopropanol, lactic acid, propionic acid, and succinic acid. Cellulose-degrading species are probably the major decomposers of cellulose in anaerobic soils. Similarly, nitrogen fixation by some species probably represents the major source of new fixed nitrogen in anaerobic soils. Several species can ferment amino acids, producing the highly unpleasant odors characteristic of putrefying protein by forming compounds such as putrescine and cadaverine. Some species are thermophilic.
Several common and normally free-living soil species of Clostridium produce toxins that may be fatal to humans, (Sections 12.3.1, 12.7.1, and 12.7.2). Botulism results from production (usually in food) of an exotoxin (a toxin released outside the cells) by C. botu-linum. If infected food is eaten, the fatality rate is close to 100%. Also, the reason that infants should not be fed honey is the possibility of infection of the immature intestinal tract with C. botulinum, leading to toxin production. C. tetani can grow in deep, anaerobic wounds, releasing the exotoxin that causes tetanus, or "lockjaw." C. perfringens can be a normal inhabitant of the intestines (and has even been used as an indicator organism for sewage sludge). However, it can also cause gastroenteritis and is one of the clostridia responsible for gas gangrene.
Heliobacterium, a close relative of Clostridium, is of interest in that it is an endospore-forming phototroph (Table 10.4). It is also an important nitrogen-fixing soil bacteria, especially in paddies (rice fields). Desulfotomaculum is an endospore-forming sulfate-reducing bacteria. Epulopiscium fishelsoni, a rod-shaped symbiont in surgeonfish, is of interest because of its incredible cell size: 50 mm in diameter and up to 600 mm long!
Eubacterium is an anaerobic, non-spore-forming, rod-shaped bacteria that produces a mixture of organic acids as fermentation products. It is found in soil but is also one of the predominant intestinal bacteria. Some species are pathogens.
The Mollicutes lack a cell wall and hence are pleomorphic (of variable shape). They include the smallest (0.2 mm) known free-living cells. This small size and lack of rigidity means that they are sometimes able to pass through filtration systems intended to sterilize water or media. There are both facultative and obligate anaerobes. A number, including many Mycoplasma, are pathogenic.
Bacillus is a large, heterogeneous group of aerobic and facultative, rod-shaped endo-spore-formers. They are very common in soil, degrading a wide variety of compounds, and some are among the major species active during thermophilic composting (optimum temperature for B. stearothermophilus is 65°C). Some produce antibiotics (e.g., bacitra-cin), others are used commercially for industrial enzyme production. Several species attack insects, and in fact the insecticide Bt, effective against a number of pests, is actually B. thuringiensis. B. anthracis causes anthrax (Section 12.7.5), which is usually a disease of animals such as sheep. However, because it is so easy to culture and store (since it forms endospores), and because as an aerosol it can be very infective and nearly always fatal in humans, it is also of concern as a biological warfare agent.
The lactic acid bacteria, which include Lactobacillus, Streptococcus, and Enterococ-cus, grow mainly on sugars and produce lactic acid as their major fermentation product. Although they are obligate anaerobes, they tend to be aerotolerant. They require complex organic media, as they are unable to produce many basic cell constituents themselves from only simple compounds. All are cocci, except for Lactobacillus (rods), and tend to grow in chains. Lactobacillus is used to produce a number of foods, including some cheeses, yogurt, sourdough, sauerkraut, pickles, and acidophilus milk (for lactose-intolerant people), as well as being of great importance (because of the acids it produces) in the agricultural crop preservation technique known as ensilage. It can still grow well at pH 5 and below. It is also a common inhabitant of the mouth, intestines, and vagina of humans and other warm-blooded animals, but is not pathogenic.
Streptococcus species also are involved in ensilage, as well as production of some foods, such as buttermilk. Others are important pathogens, causing strep throat, scarlet fever, rheumatic fever, and pneumococcal pneumonia (Sections 12.4.1 and 12.4.2). Some species are normal inhabitants of the respiratory tract, intestines, or mouth, where they can be involved in dental caries (cavities). Enterococcus species (formerly, fecal streptococci) are common intestinal bacteria of warm-blooded animals; in fact, they are typically more common than Escherichia coli and other coliforms, except in humans.
Staphylococcus (Figure 10.9) are aerobic and facultatively anaerobic, typically grow in clusters, and are halotolerant, usually able to grow in 15% NaCl. They are common inhabitants of the skin and mucous membranes. S. aureus, which is yellow, can be pathogenic, causing pimples, abscesses, boils, and impetigo of the skin, pneumonia, meningitis, and toxic shock syndrome. It also produces enterotoxins (exotoxins that affect the intestines), which makes it the most common cause of food poisoning (Section 12.3.1).
Listeria is widespread in nature, being found in soil, vegetation, and fecal material, and as an animal pathogen. One species causes listeriosis, which is most commonly a food-borne illness (Section 12.3.2). The short rods are aerobic or microaerophilic and are able to grow at refrigerator temperatures (4°C).
High G + C The Actinobacteria include the actinomycetes and a number of related organisms. Most are soil bacteria (in fact, the earthy odor of soil comes from the geos-mins produced by many species of Actinobacteria and some cyanobacteria), but some are aquatic, and a number are pathogens. Actinomycetes develop filamentous masses referred to as mycelia (singular, mycelium), which superficially resemble fungal growths. The individual filaments are called hyphae (singular, hypha). Special sporeforming structures, called sporangia (singular, sporangium), may be produced, or filaments may fragment into spores in some species, but endospores are not formed. Many actinomycetes produce antibiotics, including such important ones (from Streptomyces) as streptomycin, tetracycline, chloramphenicol, and neomycin.
Among the Actinobacteria that are not actinomycetes are the Corynebacteria, which are aerobic or facultative, and include Corynebacterium, Cellulomonas, and Arthrobacter. Corynebacterium are irregular or club-shaped rods, often in V-shaped groups. It includes both free-living and pathogenic species, including C. diphtheriae, which causes diphtheria. Cellulomonas are very similar, but degrade cellulose and are nonpathogenic. Arthrobacter, strict aerobes unable to utilize cellulose, are among the most common soil bacteria. They begin as small cocci, which then elongate into irregular rods under growth conditions, forming cocci again as the medium is exhausted.
Propionic acid bacteria, such as Propionibacterium, are anaerobes that produce propionic acid as a main fermentation product. They help give Swiss cheese its characteristic taste, as well as its holes (from carbon dioxide production).
Mycobacterium are rod-shaped, sometimes forming filaments and even branches; however, they do not produce true mycelia. Included are free-living forms from soil and water, as well as several important pathogens. A key characteristic of this group is that they are acid fast; when stained with basic fuchsin and phenol, they are not decolorized by an acid-alcohol mixture. This property has been found to be due to the presence of mycolic acids on the cell surface, which occur only in this group. Mycobacterium tend to be slow growing, but also resistant to many environmental stresses. M. tuberculosis is the cause of tuberculosis (Section 12.4.3), a once-devastating disease that is still a major problem worldwide and is increasing again in the United States. M. leprae causes leprosy (Section 12.7.6).
Nocardioform bacteria (Figure 10.28) are usually considered the simplest of the acti-nomycetes, as well as being similar in some ways to the Corynebacteria and Mycobacteria. Mycelia are typically formed, but as the culture ages they tend to fragment into small sporelike elements. Almost all are strictly aerobic (a few are facultatively anaerobic), and most grow relatively slowly. Nocardia are very common in soils, where they seem to play a role in degradation of some recalcitrant compounds. A few are known to be opportunistic pathogens. They also have been found to be involved in foaming problems in activated sludge wastewater treatment plants, as have Rhodococcus, Gordonia, Skerma-nia, and Tsukamurella (Section 16.1.3).
Actinomyces are anaerobic or facultative actinomycetes which produce easily fragmenting hyphae and no extensive mycelium. Frankia is microaerophilic, forms true mycelia, and is able to fix nitrogen. Streptomyces are very common, with the genus containing over 500 species; they are aerobic, with extensive mycelia that produce spores. A number of actinomycetes are thermophilic, including, for example, Thermomonospora, and some Microbispora and Streptomyces, and several species may be important in composting.
Some authors break this kingdom into two, one for the Planctomycetales (containing Planctomyces and a few other genera), the other for the Chlamydiales (containing only
the genus Chlamydia). Planctomyces is a stalked, budding bacteria, but it is unrelated to the more common appendaged and budding bacteria such as Caulobacter (see earlier discussion of proteobacteria; Section 10.5.6), with which it was previously grouped. The stalk is made of protein (rather than cell wall and cytoplasm), and in fact the cell lacks the peptidoglycan that in other bacteria gives the cell wall much of its strength. Plancto-mycetes are heterotrophic chemoorganotrophs found mainly at the surface of freshwater lakes but also occur in marine and other aquatic environments. Recent genetic probe studies suggest that they are present in activated sludge wastewater treatment systems.
Chlamydia are obligate intracellular parasites, usually of birds or mammals, and so were originally grouped with the Rickettsias (a-proteobacteria). Trachoma, caused by C. trachomatis, is the leading worldwide cause of human blindness. Strains of this species also cause chlamydial infections of the genitourinary tract, which is probably the most widespread sexually transmitted disease, and apparently can be involved in heart disease through their role in the buildup of deposits on the interior walls of arteries. Other species may cause pneumonia in humans, and the disease psittacosis in birds and sometimes mammals, including humans. The bacteria have an infective form that invades a host cell, then grows inside it.
The spirochetes are heterotrophic chemoorganotrophs with distinctive, long, thin, tightly coiled spiral cells. They have a "wriggling" type of motility that is now known to result from an unusual arrangement of endoflagella. Instead of sticking out from the cell, spirochete flagella bend back and run along the cell, and together with it are enclosed in a flexible sheath.
Members of the genus Spirochaeta are free-living obligate or facultative anaerobes. They are probably the spirochetes occasionally seen in activated sludge wastewater treatment systems (Figure 10.29), where they are indicative of anaerobic conditions. In some species the length may exceed 200 mm.
Several other spirochetes are important pathogens of humans and other animals. One species of Treponema (T. pallidum), which is microaerophilic or anaerobic, is the cause of the important venereal disease syphilis (Section 12.6.1). Other Treponema species cause yaws and other diseases in humans and animals, and many live commensally in the mouth or digestive tract, including in the rumen (the cellulose-digesting forestomach of ruminant animals, such as cattle, sheep, and deer). Microaerophilic Borrelia is the cause of Lyme disease (Section 12.5.6) and relapsing fever. Leptospira is aerobic and can be free living or pathogenic, causing leptospirosis (Section 12.2.3).
This kingdom contains only a few known species. Fibrobacter are gram-negative, rod-shaped (usually), non-spore-forming anaerobes that ferment cellulose and a few other compounds. They are found in the intestines or rumen.
Bacteroids are obligately anaerobic chemoorganotrophic heterotrophs. Members of the genus Bacteroides are the most common bacteria in the human lower digestive tract, with concentrations in feces usually exceeding 1010 per gram. Bacteroids are often an important part of anaerobic environments (including the rumen) in which organics are being degraded to simpler compounds that can then be utilized by methanogens (archaea that generate methane; Section 10.6.3).
Flavobacteria also are chemoorganotrophic heterotrophs, but they are aerobic or occasionally facultatively anaerobic and are widely distributed in aquatic, marine, and soil systems. Colonies of the common genus Flavobacterium are often yellowish.
The Sphingobacteria are aerobic or facultatively anaerobic chemoorganotrophic hetero-trophs. Cytophaga are common in water and soil, have gliding motility, and can produce small spherical resting stages called microcysts. Cytophaga are able to digest cellulose and chitin, making them very important in the cycling of carbon in aerobic environments. They can also digest the agar that is used routinely for solidifying media in microbiology labs. Haliscomenobacter is a common small filament in activated sludge.
The Fusobacteria are gram-negative, anaerobic or facultative, non-spore-forming rods. Fusobacterium is typically found in the mouth or intestines and is sometimes pathogenic. Streptobacillus is usually found in the mouth of rats and can cause one type of rat bite fever.
There are only two known genera of Verrucomicrobia, both unicellular nonmotile che-moorganotrophic heterotrophs. Prosthecobacter has a single prostheca, is strictly aerobic, and is oligotrophic (grows at low levels of available nutrients). It is found in aquatic systems, soils, and wastewater treatment plants. Verrucomicrobium has numerous prosthecae, is facultatively anaerobic, and has been found in soils and a eutrophic (nutrient-rich) lake.
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