what does it mean for an organism to be anaerobic

General Concepts

Clinical Manifestations

Symptoms are related to the absence of oxygen from the affected surface area: hence, abscesses, devitalized tissue, and penetration of strange affair lead to clinical infection.

Oxygen Toxicity

Low or undetectable levels of superoxide dismutase and catalase allow oxygen radicals to class in anaerobic leaner and to inactivate other bacterial enzyme systems.

Pathogenic Anaerobes

Anaerobes are potentially pathogenic when displaced from normal environments (human colon, soil) and implanted in dead or dying tissue; abscesses, pneumonias, and oral and pelvic infections result.

Processing of Clinical Specimens

Anaerobic conditions are required for sample collection, culturing, and identification.

Introduction

The broad classification of bacteria every bit anaerobic, aerobic, or facultative is based on the types of reactions they employ to generate energy for growth and other activities. In their metabolism of free energy-containing compounds, aerobes require molecular oxygen as a terminal electron acceptor and cannot grow in its absence (meet Affiliate 4). Anaerobes, on the other hand, cannot grow in the presence of oxygen. Oxygen is toxic for them, and they must therefore depend on other substances equally electron acceptors. Their metabolism frequently is a fermentative type in which they reduce available organic compounds to various cease products such as organic acids and alcohols. The facultative organisms are the most versatile. They preferentially utilize oxygen as a terminal electron acceptor, but besides tin metabolize in the absenteeism of oxygen by reducing other compounds. Much more usable energy, in the grade of loftier-energy phosphate, is obtained when a molecule of glucose is completely catabolized to carbon dioxide and water in the presence of oxygen (38 molecules of ATP) than when it is only partially catabolized by a fermentative process in the absence of oxygen (two molecules of ATP). The ability to utilize oxygen as a terminal electron acceptor provides organisms with an extremely efficient mechanism for generating energy. Understanding the general characteristics of anaerobiosis provides insight into how anaerobic leaner can proliferate in damaged tissue and why special care is needed in processing clinical specimens that may contain them.

Oxygen Toxicity

Several studies betoken that aerobes tin can survive in the presence of oxygen just by virtue of an elaborate arrangement of defenses. Without these defenses, key enzyme systems in the organisms fail to office and the organisms dice. Obligate anaerobes, which live simply in the absence of oxygen, exercise not possess the defenses that make aerobic life possible and therefore cannot survive in air.

During growth and metabolism, oxygen reduction products are generated within microorganisms and secreted into the surrounding medium. The superoxide anion, one oxygen reduction production, is produced by univalent reduction of oxygen:

O₂ ^e- → O₂ ^–

Information technology is generated during the interaction of molecular oxygen with diverse cellular constituents, including reduced flavins, flavoproteins, quinones, thiols, and iron-sulfur proteins. The exact process by which it causes intracellular damage is not known; yet, it is capable of participating in a number of subversive reactions potentially lethal to the cell. Moreover, products of secondary reactions may amplify toxicity. For case, one hypothesis holds that the superoxide anion reacts with hydrogen peroxide in the cell:

O₂ ^– + H_iiO₂ → OH^– + OH^. + O₂

This reaction, known as the Haber-Weiss reaction, generates a costless hydroxyl radical (OH·), which is the almost potent biologic oxidant known. It tin can assault almost any organic substance in the cell. A subsequent reaction between the superoxide anion and the hydroxyl radical produces singlet oxygen (O₂* ), which is too damaging to the cell:

O_ii ^– + OH → OH + O_ii*

The excited singlet oxygen molecule is very reactive. Therefore, superoxide must be removed for the cells to survive in the presence of oxygen.

Nearly facultative and aerobic organisms contain a loftier concentration of an enzyme called superoxide dismutase. This enzyme converts the superoxide anion into basis-state oxygen and hydrogen peroxide, thus ridding the cell of destructive superoxide anions:

2O_ii ^– + 2H^{+ Superoxide Dismutase} O₂ + H₂ O_two

The hydrogen peroxide generated in this reaction is an oxidizing agent, just it does not damage the jail cell as much as the superoxide anion and tends to diffuse out of the cell. Many organisms possess catalase or peroxidase or both to eliminate the H₂O₂. Catalase uses H_twoO₂ as an oxidant (electron acceptor) and a reductant (electron donor) to convert peroxide into h2o and ground-state oxygen:

H₂O₂ + H_twoO₂ ^Catalase 2H₂O + O₂

Peroxidase uses a reductant other than H₂O₂:

H₂O_two + H₂R ^Peroxidase 2H₂O + R

I report showed that facultative and aerobic organisms lacking superoxide dismutase possess high levels of catalase or peroxidase. Loftier concentrations of these enzymes may convalesce the need for superoxide dismutase, considering they finer scavenge H₂ O_ii earlier it can react with the superoxide anion to form the more active hydroxyl radical. Even so, most organisms show a positive correlation between the activity of superoxide dismutase and resistance to the toxic furnishings of oxygen.

In another study, facultative and aerobic organisms demonstrated high levels of superoxide dismutase. The enzyme was nowadays, generally at lower levels, in some of the anaerobes studied, but was totally absent-minded in others. The nigh oxygen-sensitive anaerobes as a rule independent little or no superoxide dismutase. In addition to the action of superoxide dismutase, the rate at which an organism takes up and reduces oxygen was adamant to be a factor in oxygen tolerance. Very sensitive anaerobes, which reduced relatively large quantities of oxygen and exhibited no superoxide dismutase activity, were killed after curt exposure to oxygen. More than tolerant organisms reduced very little oxygen or else demonstrated loftier levels of superoxide dismutase activeness.

The continuous spectrum of oxygen tolerance among leaner appears to exist due partly to the activities of superoxide dismutase, catalase, and peroxidase in the prison cell and partly to the rate at which the cell takes up oxygen (Fig. 17-i). Clearly, other factors influence tolerance: the location of protective enzymes in the cell (surface versus cytoplasm), the charge per unit at which cells grade toxic oxygen products (due east.g., the hydroxyl radical or singlet oxygen), and the sensitivities of primal cellular components to the toxic oxygen products.

Figure 17-1

Furnishings of oxygen on aerobic, anaerobic, and facultative anaerobic leaner.

Pathogenic Anaerobes

Anaerobic bacteria are widely distributed in nature in oxygen-gratis habitats. Many members of the ethnic man flora are anaerobic bacteria, including spirochetes and Gram-positive and Gram-negative cocci and rods. For example, the human colon, where oxygen tension is low, contains large populations of anaerobic bacteria, exceeding 10¹¹ organisms/k of colon content. Anaerobes in this region frequently outnumber facultative organisms past a cistron of at least 100. Oxygen-sensitive organisms also are numerous in other areas of the body, such equally the gingival crevices, tonsillar crypts, nasal folds, hair follicles, the urethra and vagina, and molar surfaces.

Anaerobic indigenous flora components are potentially pathogenic if displaced from their normal habitat. Most anaerobic infections are endogenously acquired from members of the microflora, although Clostridium, establish principally in the soil, also produces infections in humans. Proliferation of anaerobic bacteria in tissue depends on the absence of oxygen. Oxygen is excluded from the tissue when the local claret supply is dumb past trauma, obstacle, or surgical manipulation. Anaerobes multiply well in dead tissue. Multiplication of aerobic or facultative organisms in association with anaerobes in infected tissue likewise diminishes oxygen concentration and develops a habitat that supports growth of anaerobic bacteria.

Infections produced by anaerobic bacteria occur in all parts of the human torso (Fig. 17-ii). The infected tissues ordinarily comprise a mixture of several kinds of anaerobes and frequently also contain aerobic and facultative bacteria. The types of infections unremarkably produced by anaerobic leaner are every bit follows:

Figure 17-2

Types of infection commonly produced past anaerobic bacteria.

Intra-abdominal infections.

Abscesses, postoperative wound infections, and generalized peritonitis produced by anaerobes occur as a consequence of bowel perforation during surgery or injury.

Pulmonary infections.

Anaerobic lung infections may originate in the bronchi or the claret. Aspirations from the upper respiratory tract, which contain big numbers of anaerobic bacteria, are responsible for initiating infection in the bronchi.

Pelvic infections.

Anaerobic infections of the vagina and uterus sometimes occur after gynecologic surgery or in association with malignancy of pelvic organs.

Brain abscesses.

Anaerobes infrequently produce meningitis, just are a common cause of brain abscesses. The infecting organisms usually originate in the upper respiratory tract.

Skin and soft tissue infections.

Combinations of anaerobes, aerobes, and facultative organisms oftentimes act synergistically to produce these infections.

Oral and dental infections.

These local infections oftentimes extend to the face and cervix and sometimes to other areas of the torso such as the brain.

Bacteremia and endocarditis.

Anaerobic bacteremia may follow disturbance in an surface area of the body where an established flora or an infection exists. Endocarditis, an inflammation of the endothelial lining of the heart cavities, is occasionally caused by anaerobic bacteria, especially anaerobic streptococci.

With the exception of the clostridia, which have been studied extensively, the mechanisms past which anaerobes cause infections in humans are not well understood. Clostridium species produce diverse toxins that destroy tissue cells, and 2 species, C botulinum and C tetani, release the neurotoxins responsible for botulism and tetanus, respectively. Enzymes excreted by other anaerobic bacteria, including proteases, lipases, hyaluronidase, chondroitin sulfatase, and neuraminidase, may play a role in infection by causing tissue prison cell destruction, and ß-lactamase may act every bit a virulence cistron by inactivating antibiotics that possess a ß-lactam ring, such as the penicillins and cephalosporins. In addition, the capsules surrounding some anaerobic bacteria probably interfere with phagocytosis and act equally a barrier against penetration by antimicrobial agents.

Processing of Clinical Specimens

When collecting specimens from patients for isolation and identification of anaerobic bacteria associated with infections, precautions must be taken to exclude air (Fig. 17-3). Materials for anaerobic culture are best obtained with a needle and syringe. Unless the specimen can be sent to the laboratory immediately, it is placed in an anaerobic transport tube containing oxygen-free carbon dioxide or nitrogen. The specimen is injected through the rubber stopper in the transport tube and remains in the anaerobic environment of the tube until processed in the bacteriology laboratory. If the specimen is nerveless with a swab, simply a special commercially available anaerobic swab transport system is used.

Specimens should be free of contaminating bacteria. Material from sites that are unremarkably sterile, such as blood, spinal fluid, or pleural fluid, poses no problem provided the usual precautions are taken to decontaminate the skin properly before puncturing it to obtain the specimen. Fecal specimens, sputum specimens, or vaginal secretions cannot be cultured routinely for pathogenic anaerobes because they ordinarily contain other anaerobic organisms. Aspirates from abscesses or the specific sites of infections must exist obtained in these cases to avert undue contagion with indigenous flora components.

Although several techniques are available for maintaining an oxygen-costless environment during the processing of specimens for anaerobic civilisation, the anaerobic jar is the well-nigh common. It is a medium-sized glass or plastic jar with a tightly fitting chapeau containing palladium-coated alumina particles, which serve every bit a goad. It tin can be set past two methods. The easiest uses a commercially available hydrogen and carbon dioxide generator envelope (GasPak) that is placed in the jar along with the culture plates. The generator is activated with water. Oxygen within the jar and the hydrogen that is generated are converted to water in the presence of the catalyst, thus producing anaerobic weather condition. Carbon dioxide, which is also generated, is required for growth by some anaerobes and stimulates the growth of others. An culling method for achieving anaerobiosis in the jar consists of evacuation and replacement. Air is evacuated from the sealed jar containing the civilisation plates and is replaced with an oxygen-complimentary mixture of 80 pct nitrogen, ten percent hydrogen, and 10 percentage carbon dioxide.

More sophisticated procedures are used to isolate extremely oxygen-sensitive microorganisms that cannot be recovered by using the anaerobic jar. One, the roll tube method, consists of a stoppered test tube containing oxygen-complimentary gas and a sparse layer of prereduced agar medium on its inside surface. The medium in the tube is inoculated with a loop while the tube is rotated. This produces a screw track on the agar surface. The tube is flushed with a stream of carbon dioxide to prevent entry of air while it is open during inoculation.

The anaerobic glove box isolator is some other innovation developed for isolating anaerobic leaner. Information technology is essentially a large articulate-vinyl sleeping accommodation, with attached gloves, containing a mixture of fourscore per centum nitrogen, 10 per centum hydrogen, and 10 percent carbon dioxide. A lock at one end of the chamber is fitted with two hatches, i leading to the outside and the other to the inside of the chamber. Specimens are placed in the lock, the outside hatch is airtight, and the air in the lock is evacuated and replaced with the gas mixture. The within hatch is and so opened to innovate the specimen into the chamber. Conventional bacteriologic procedures are employed to process the specimen in the oxygen-free atmosphere.

Although these complex systems are needed to isolate anaerobic flora components, studies accept shown that the anaerobic jar is adequate to recover clinically meaning anaerobes. The extremely oxygen-sensitive bacteria of the microflora obviously are not associated with infectious processes.

Procedures for cultivation and identification of anaerobic leaner are well established (Fig. 17-3). A diversity of selective and nonselective media is available for cultivation of anaerobes. A reliable, nonselective medium consists of Brucella agar supplemented with sheep claret, hemin, cysteine, sodium carbonate, and menadione. Usual bacteriologic procedures are used to identify anaerobes. These are based on Gram-staining reactions, cellular and colony morphology, antibiotic sensitivity patterns, carbohydrate fermentation reactions, and other biochemical tests. Analysis of metabolic end products, especially organic acids, provides additional information useful in classifying these organisms.

References

1. Balows A, DeHaan RM, Dowell VR, Guze LB (eds): Anaerobic Leaner. Charles C Thomas, Springfield, IL, 1974 .

2. Finegold SM: Anaerobic Leaner in Homo Illness. Bookish Press, San Diego, 1977 .

3. Finegold SM, George WL (eds): Anaerobic Infections in Humans. Academic Press, San Diego, 1989 .

4. Holdeman LV, Cato EP, Moore WEC (eds): Anaerobe Laboratory Manual. 4th Ed. Virginia Polytechnic Establish and Land University Anaerobe Laboratory, Blacksburg, VA, 1977 .

5. Lennette EH, Spaulding EH, Truant JP (eds): Transmission of Clinical Microbiology. 2nd Ed. American Society for Microbiology, Washington, D.C., 1974 .

6. Morris JG. The physiology of obligate anaerobiosis. Adv Microb Physiol. 1975;12:169–246.

7. Sutter VL, Citron DM, Edelstein MAC, Finegold SM: Wadsworth Anaerobic Bacteriology Manual, quaternary Ed. Star Publishing, Belmont, CA, 1985 .

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Source: https://www.ncbi.nlm.nih.gov/books/NBK7638/

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