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You are watching: What is the difference between aerobic and anaerobic bacteria

Baron S, editor. Clinical Microbiology. Fourth edition. Galveston (TX): university of Texas clinical Branch in ~ Galveston; 1996.


Clinical Manifestations

Symptoms are regarded the lack of oxygen indigenous the impacted area: hence, abscesses, devitalized tissue, and penetration of international matter bring about clinical infection.

Oxygen Toxicity

Low or undetectable level of superoxide dismutase and also catalase allow oxygen radicals to type in anaerobic bacteria and also to inactivate various other bacterial enzyme systems.

Pathogenic Anaerobes

Anaerobes are perhaps pathogenic as soon as displaced native normal atmospheres (human colon, soil) and implanted in dead or dice tissue; abscesses, pneumonias, and also oral and also pelvic infections result.

Processing of Clinical Specimens

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


The broad classification that bacteria together anaerobic, aerobic, or facultative is based upon the varieties of reaction they rental to generate power for growth and also other activities. In their metabolism of energy-containing compounds, aerobes require molecular oxygen as a terminal electron acceptor and cannot flourish in its absence (see thing 4). Anaerobes, ~ above the various other hand, cannot flourish in the presence of oxygen. Oxygen is toxicity for them, and they must as such depend on various other substances as electron acceptors. Your metabolism frequently is a fermentative type in which lock reduce easily accessible organic link to assorted end commodities such together organic acids and also alcohols. The facultative organisms are the most versatile. Castle preferentially make use of oxygen together a terminal electron acceptor, but additionally can metabolize in the absence of oxygen by reducing various other compounds. Much an ext usable energy, in the kind of high-energy phosphate, is obtained when a molecule of glucose is fully catabolized to carbon dioxide and water in the visibility of oxygen (38 molecule of ATP) than when it is just partially catabolized through a fermentative procedure in the absence of oxygen (2 molecule of ATP). The ability to make use of oxygen together a terminal electron acceptor gives organisms with very efficient mechanism for generating energy. Expertise the general features of anaerobiosis gives insight into just how anaerobic bacteria can proliferate in damaged tissue and why special treatment is necessary in processing clinical specimens that may contain them.

Oxygen Toxicity

Several studies suggest that aerobes have the right to survive in the existence of oxygen only by virtue of an intricate system the defenses. Without this defenses, vital enzyme equipment in the biology fail to duty and the biology die. Obligate anaerobes, i m sorry live just in the lack of oxygen, execute not possess the defenses the make aerobic life possible and because of this cannot endure in air.

During growth and also metabolism, oxygen reduction products are generated within microorganisms and secreted into the bordering medium. The superoxide anion, one oxygen reduction product, is developed by univalent reduction of oxygen:

O2e- → O2–

It is generated throughout the communication of molecule oxygen with miscellaneous cellular constituents, including reduced flavins, flavoproteins, quinones, thiols, and also iron-sulfur proteins. The exact procedure by i m sorry it causes intracellular damage is no known; however, that is capable of participating in a variety of destructive reactions perhaps lethal to the cell. Moreover, products of secondary reactions might amplify toxicity. Because that example, one theory holds that the superoxide anion reacts through hydrogen peroxide in the cell:

O2– + H2O2 → OH– + OH. + O2

This reaction, recognized as the Haber-Weiss reaction, generates a complimentary hydroxyl radical (OH·), which is the many potent biologic oxidant known. That can attack virtually any type of organic problem in the cell. A succeeding reaction between the superoxide anion and also the hydroxyl radical produces singlet oxygen (O2* ), i m sorry is also damaging to the cell:

O2– + five → five + O2*

The excited singlet oxygen molecule is really reactive. Therefore, superoxide must be eliminated for the cells to make it through in the existence of oxygen.

Most facultative and also aerobic organisms contain a high concentration of one enzyme dubbed superoxide dismutase. This enzyme switch the superoxide anion right into ground-state oxygen and also hydrogen peroxide, thus ridding the cabinet of terrible superoxide anions:

2O2– + 2H+Superoxide Dismutase O2 + H2 O2

The hydrogen peroxide created in this reaction is one oxidizing agent, however it go not damage the cabinet as much as the superoxide anion and also tends to diffuse out of the cell. Countless organisms own catalase or peroxidase or both to get rid of the H2O2. Catalase offers H2O2 as an oxidant (electron acceptor) and also a reductant (electron donor) to transform peroxide into water and ground-state oxygen:

H2O2 + H2O2Catalase 2H2O + O2

Peroxidase provides a reductant other than H2O2:

H2O2 + H2R Peroxidase 2H2O + R

One study verified that facultative and also aerobic organisms lacking superoxide dismutase possess high levels of catalase or peroxidase. High concentration of this enzymes may alleviate the require for superoxide dismutase, since they properly scavenge H2 O2 before it have the right to react through the superoxide anion to type the more active hydroxyl radical. However, most organisms show a positive correlation between the task of superoxide dismutase and resistance come the toxic results of oxygen.

In another study, facultative and aerobic organisms demonstrated high levels of superoxide dismutase. The enzyme to be present, generally at reduced levels, in few of the anaerobes studied, but was totally lacking in others. The most oxygen-sensitive anaerobes together a ascendancy contained small or no superoxide dismutase. In enhancement to the activity of superoxide dismutase, the rate at i m sorry an biology takes up and also reduces oxygen was established to it is in a factor in oxygen tolerance. Really sensitive anaerobes, which lessened relatively large quantities the oxygen and also exhibited no superoxide dismutase activity, were killed after brief exposure come oxygen. Much more tolerant organisms reduced very little oxygen or rather demonstrated high levels of superoxide dismutase activity.

The continuous spectrum that oxygen tolerance among bacteria shows up to it is in due partially to the tasks of superoxide dismutase, catalase, and also peroxidase in the cell and also partly come the price at i m sorry the cabinet takes increase oxygen (Fig. 17-1). Clearly, other determinants influence tolerance: the location of protective enzymes in the cell (surface versus cytoplasm), the price at which cells kind toxic oxygen commodities (e.g., the hydroxyl radical or singlet oxygen), and the sensitivities of key cellular components to the toxicity oxygen products.


Pathogenic Anaerobes

Anaerobic bacteria space widely spread in nature in oxygen-free habitats. Many members of the indigenous human being flora room anaerobic bacteria, consisting of spirochetes and Gram-positive and also Gram-negative cocci and rods. Because that example, the person colon, whereby oxygen tension is low, contains large populations of anaerobic bacteria, exceeding 1011 organisms/g of colon content. Anaerobes in this region frequently outnumber facultative organisms by a element of at the very least 100. Oxygen-sensitive organisms additionally are plenty of in other areas of the body, such as the gingival crevices, tonsillar crypts, nasal folds, hair follicles, the urethra and vagina, and also tooth surfaces.

Anaerobic native flora contents are potentially pathogenic if displaced from their regular habitat. Most anaerobic infections are endogenously got from members the the microflora, return Clostridium, discovered principally in the soil, also produces epidemic in humans. Proliferation of anaerobic bacteria in tissue depends on the absence of oxygen. Oxygen is excluded from the tissue once the neighborhood blood supply is impaired by trauma, obstruction, or surgical manipulation. Anaerobes multiply fine in dead tissue. Multiplication the aerobic or facultative organisms in association through anaerobes in infected tissue likewise diminishes oxygen concentration and develops a habitat that supports growth of anaerobic bacteria.

Infections created by anaerobic bacteria occur in all parts of the person body (Fig. 17-2). The infected tissues typically contain a mixture of several kinds that anaerobes and frequently likewise contain aerobic and facultative bacteria. The types of infections frequently produced through anaerobic bacteria are as follows:


Intra-abdominal infections.

Abscesses, postoperative wound infections, and also generalized peritonitis produced by anaerobes occur as a an effect of bowel perforation throughout surgery or injury.

Pulmonary infections.

Anaerobic lung infections may originate in the bronchi or the blood. Aspirations from the upper respiratory tract, i beg your pardon contain huge numbers of anaerobic bacteria, room responsible for initiating infection in the bronchi.

Pelvic infections.

Anaerobic epidemic of the vagina and uterus sometimes take place after gynecologic surgical procedure or in association with malignancy the pelvic organs.

Brain abscesses.

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

Skin and soft organization infections.

Combinations that anaerobes, aerobes, and also facultative organisms often act synergistically to create these infections.

Oral and also dental infections.

These local infections generally extend to the face and also neck and sometimes come other locations of the human body such as the brain.

Bacteremia and endocarditis.

Anaerobic bacteremia might follow disturbance in an area that the body where an developed flora or an infection exists. Endocarditis, an inflammation of the endothelial lining of the heart cavities, is occasionally brought about by anaerobic bacteria, specifically anaerobic streptococci.

With the exception of the clostridia, which have actually been studied extensively, the mechanisms whereby anaerobes reason infections in people are no well understood. Clostridium species produce assorted toxins that destroy tissue cells, and two species, C botulinum and also C tetani, release the neurotoxins responsible because that botulism and also tetanus, respectively. Enzymes excreted by various other anaerobic bacteria, including proteases, lipases, hyaluronidase, chondroitin sulfatase, and also neuraminidase, may play a role in infection by leading to tissue cell destruction, and also ß-lactamase might act together a virulence variable by inactivating antibiotics that possess a ß-lactam ring, such together the penicillins and cephalosporins. In addition, the capsules neighboring some anaerobic bacteria probably interfere v phagocytosis and also act as a barrier versus penetration through antimicrobial agents.

Processing the Clinical Specimens

When collecting specimens indigenous patients for isolation and identification of anaerobic bacteria connected with infections, precautions need to be taken to exclude waiting (Fig. 17-3). Products for anaerobic culture are ideal obtained v a needle and also syringe. Uneven the specimen can be sent to the activities immediately, that is put in an anaerobic transport tube containing oxygen-free carbon dioxide or nitrogen. The specimen is injected v the rubber stopper in the deliver tube and also remains in the anaerobic environment of the tube till processed in the bacteriology laboratory. If the specimen is built up with a swab, just a special commercially available anaerobic swab transport mechanism is used.


Specimens have to be complimentary of contaminating bacteria. Material from sites that are typically sterile, such as blood, spinal fluid, or pleural fluid, poses no problem noted the usual precautions room taken come decontaminate the skin properly prior to puncturing it to attain the specimen. Fecal specimens, sputum specimens, or vaginal secretions cannot be cultured on regular basis for pathogenic anaerobes because they usually contain various other anaerobic organisms. Aspirates native abscesses or the details sites the infections must be acquired in these instances to prevent undue pollution with aboriginal flora components.

Although several methods are obtainable for keeping an oxygen-free environment during the handling of specimens for anaerobic culture, the anaerobic jar is the most common. The is a medium-sized glass or plastic jar through a tightly installation lid comprise palladium-coated alumina particles, which serve as a catalyst. It can be collection up by two methods. The easiest provides a commercially easily accessible hydrogen and also carbon dioxide generator envelope (GasPak) that is inserted in the jar together with the society plates. The generator is activated through water. Oxygen within the jar and the hydrogen that is created are converted to water in the visibility of the catalyst, thus creating anaerobic conditions. Carbon dioxide, i beg your pardon is also generated, is forced for development by part anaerobes and stimulates the growth of others. One alternative an approach for afford anaerobiosis in the jar consists of evacuation and also replacement. Wait is evacuated indigenous the sealed jug containing the culture plates and also is changed with an oxygen-free mixture that 80 percent nitrogen, 10 percent hydrogen, and 10 percent carbon dioxide.

More sophisticated procedures are provided to isolate incredibly oxygen-sensitive microorganisms the cannot it is in recovered by utilizing the anaerobic jar. One, the roll tube method, consists of a stoppered test tube containing oxygen-free gas and also a slim layer the prereduced agar tool on its inside surface. The medium in the tube is inoculated through a loop when the pipe is rotated. This produce a spiral monitor on the agar surface. The pipe is flushed with a present of carbon dioxide to prevent entry of air while that is open throughout inoculation.

The anaerobic gloves box isolator is one more innovation developed for isolating anaerobic bacteria. The is essentially a big clear-vinyl chamber, with attached gloves, include a mixture of 80 percent nitrogen, 10 percent hydrogen, and also 10 percent carbon dioxide. A lock at one finish of the room is fitted with two hatches, one causing the outside and also the various other to the within of the chamber. Specimens are put in the lock, the external hatch is closed, and also the air in the lock is evacuated and also replaced v the gas mixture. The within hatch is then opened up to present the specimen right into the chamber. Traditional bacteriologic actions are to work to process the specimen in the oxygen-free atmosphere.

Although these complicated systems are essential to isolate anaerobic flora components, studies have presented that the anaerobic seasoned is adequate to recoup clinically significant anaerobes. The very oxygen-sensitive bacteria the the microflora apparently are not associated with contagious processes.

Procedures for cultivation and identification the anaerobic bacteria room well created (Fig. 17-3). A selection of selective and also nonselective media is easily accessible for growing of anaerobes. A reliable, nonselective medium consists of Brucella agar supplemented with sheep blood, hemin, cysteine, sodium carbonate, and also menadione. Normal bacteriologic measures are offered to recognize anaerobes. This are based on Gram-staining reactions, cellular and colony morphology, antibiotic sensitivity patterns, carbohydrate fermentation reactions, and other biochemical tests. Analysis of metabolic finish products, specifically organic acids, provides added information helpful in classifying these organisms.

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