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8.2: Oxygen Requirements for Microbial Growth - Biology

8.2: Oxygen Requirements for Microbial Growth - Biology



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Learning Objectives

  • Interpret visual data demonstrating minimum, optimum, and maximum oxygen or carbon dioxide requirements for growth
  • Identify and describe different categories of microbes with requirements for growth with or without oxygen: obligate aerobe, obligate anaerobe, facultative anaerobe, aerotolerant anaerobe, microaerophile, and capnophile
  • Give examples of microorganisms for each category of growth requirements

Ask most people “What are the major requirements for life?” and the answers are likely to include water and oxygen. Few would argue about the need for water, but what about oxygen? Can there be life without oxygen?

The answer is that molecular oxygen (O2) is not always needed. The earliest signs of life are dated to a period when conditions on earth were highly reducing and free oxygen gas was essentially nonexistent. Only after cyanobacteria started releasing oxygen as a byproduct of photosynthesis and the capacity of iron in the oceans for taking up oxygen was exhausted did oxygen levels increase in the atmosphere. This event, often referred to as the Great Oxygenation Event or the Oxygen Revolution, caused a massive extinction. Most organisms could not survive the powerful oxidative properties of reactive oxygen species (ROS), highly unstable ions and molecules derived from partial reduction of oxygen that can damage virtually any macromolecule or structure with which they come in contact. Singlet oxygen (O2•), superoxide (O2−), peroxides (H2O2), hydroxyl radical (OH•), and hypochlorite ion (OCl), the active ingredient of household bleach, are all examples of ROS. The organisms that were able to detoxify reactive oxygen species harnessed the high electronegativity of oxygen to produce free energy for their metabolism and thrived in the new environment.

Oxygen Requirements of Microorganisms

Many ecosystems are still free of molecular oxygen. Some are found in extreme locations, such as deep in the ocean or in earth’s crust; others are part of our everyday landscape, such as marshes, bogs, and sewers. Within the bodies of humans and other animals, regions with little or no oxygen provide an anaerobic environment for microorganisms. (Figure (PageIndex{1})).

We can easily observe different requirements for molecular oxygen by growing bacteria in thioglycolate tube cultures. A test-tube culture starts with autoclaved thioglycolate medium containing a low percentage of agar to allow motile bacteria to move throughout the medium. Thioglycolate has strong reducing properties and autoclaving flushes out most of the oxygen. The tubes are inoculated with the bacterial cultures to be tested and incubated at an appropriate temperature. Over time, oxygen slowly diffuses throughout the thioglycolate tube culture from the top. Bacterial density increases in the area where oxygen concentration is best suited for the growth of that particular organism.

The growth of bacteria with varying oxygen requirements in thioglycolate tubes is illustrated in Figure (PageIndex{2}). In tube A, all the growth is seen at the top of the tube. The bacteria are obligate (strict) aerobes that cannot grow without an abundant supply of oxygen. Tube B looks like the opposite of tube A. Bacteria grow at the bottom of tube B. Those are obligate anaerobes, which are killed by oxygen. Tube C shows heavy growth at the top of the tube and growth throughout the tube, a typical result with facultative anaerobes. Facultative anaerobes are organisms that thrive in the presence of oxygen but also grow in its absence by relying on fermentation or anaerobic respiration, if there is a suitable electron acceptor other than oxygen and the organism is able to perform anaerobic respiration. The aerotolerant anaerobes in tube D are indifferent to the presence of oxygen. They do not use oxygen because they usually have a fermentative metabolism, but they are not harmed by the presence of oxygen as obligate anaerobes are. Tube E on the right shows a “Goldilocks” culture. The oxygen level has to be just right for growth, not too much and not too little. These microaerophiles are bacteria that require a minimum level of oxygen for growth, about 1%–10%, well below the 21% found in the atmosphere.

Examples of obligate aerobes are Mycobacterium tuberculosis, the causative agent of tuberculosis and Micrococcus luteus, a gram-positive bacterium that colonizes the skin. Neisseria meningitidis, the causative agent of severe bacterial meningitis, and N. gonorrheae, the causative agent of sexually transmitted gonorrhea, are also obligate aerobes.

Many obligate anaerobes are found in the environment where anaerobic conditions exist, such as in deep sediments of soil, still waters, and at the bottom of the deep ocean where there is no photosynthetic life. Anaerobic conditions also exist naturally in the intestinal tract of animals. Obligate anaerobes, mainly Bacteroidetes, represent a large fraction of the microbes in the human gut. Transient anaerobic conditions exist when tissues are not supplied with blood circulation; they die and become an ideal breeding ground for obligate anaerobes. Another type of obligate anaerobe encountered in the human body is the gram-positive, rod-shaped Clostridium spp. Their ability to form endospores allows them to survive in the presence of oxygen. One of the major causes of health-acquired infections is C. difficile, known as C. diff. Prolonged use of antibiotics for other infections increases the probability of a patient developing a secondary C. difficile infection. Antibiotic treatment disrupts the balance of microorganisms in the intestine and allows the colonization of the gut by C. difficile, causing a significant inflammation of the colon.

Other clostridia responsible for serious infections include C. tetani, the agent of tetanus, and C. perfringens, which causes gas gangrene. In both cases, the infection starts in necrotic tissue (dead tissue that is not supplied with oxygen by blood circulation). This is the reason that deep puncture wounds are associated with tetanus. When tissue death is accompanied by lack of circulation, gangrene is always a danger.

The study of obligate anaerobes requires special equipment. Obligate anaerobic bacteria must be grown under conditions devoid of oxygen. The most common approach is culture in an anaerobic jar (Figure (PageIndex{3})). Anaerobic jars include chemical packs that remove oxygen and release carbon dioxide (CO2). An anaerobic chamber is an enclosed box from which all oxygen is removed. Gloves sealed to openings in the box allow handling of the cultures without exposing the culture to air (Figure (PageIndex{3})).

Staphylococci and Enterobacteriaceae are examples of facultative anaerobes. Staphylococci are found on the skin and upper respiratory tract. Enterobacteriaceae are found primarily in the gut and upper respiratory tract but can sometimes spread to the urinary tract, where they are capable of causing infections. It is not unusual to see mixed bacterial infections in which the facultative anaerobes use up the oxygen, creating an environment for the obligate anaerobes to flourish.

Examples of aerotolerant anaerobes include lactobacilli and streptococci, both found in the oral microbiota. Campylobacter jejuni, which causes gastrointestinal infections, is an example of a microaerophile and is grown under low-oxygen conditions.

The optimum oxygen concentration, as the name implies, is the ideal concentration of oxygen for a particular microorganism. The lowest concentration of oxygen that allows growth is called the minimum permissive oxygen concentration. The highest tolerated concentration of oxygen is the maximum permissive oxygen concentration. The organism will not grow outside the range of oxygen levels found between the minimum and maximum permissive oxygen concentrations.

Exercise (PageIndex{1})

  1. Would you expect the oldest bacterial lineages to be aerobic or anaerobic?
  2. Which bacteria grow at the top of a thioglycolate tube, and which grow at the bottom of the tube?

An Unwelcome Anaerobe

Charles is a retired bus driver who developed type 2 diabetes over 10 years ago. Since his retirement, his lifestyle has become very sedentary and he has put on a substantial amount of weight. Although he has felt tingling and numbness in his left foot for a while, he has not been worried because he thought his foot was simply “falling asleep.” Recently, a scratch on his foot does not seem to be healing and is becoming increasingly ugly. Because the sore did not bother him much, Charles figured it could not be serious until his daughter noticed a purplish discoloration spreading on the skin and oozing (Figure). When he was finally seen by his physician, Charles was rushed to the operating room. His open sore, or ulcer, is the result of a diabetic foot.

The concern here is that gas gangrene may have taken hold in the dead tissue. The most likely agent of gas gangrene is Clostridium perfringens, an endospore-forming, gram-positive bacterium. It is an obligate anaerobe that grows in tissue devoid of oxygen. Since dead tissue is no longer supplied with oxygen by the circulatory system, the dead tissue provides pockets of ideal environment for the growth of C. perfringens.

A surgeon examines the ulcer and radiographs of Charles’s foot and determines that the bone is not yet infected. The wound will have to be surgically debrided (debridement refers to the removal of dead and infected tissue) and a sample sent for microbiological lab analysis, but Charles will not have to have his foot amputated. Many diabetic patients are not so lucky. In 2008, nearly 70,000 diabetic patients in the United States lost a foot or limb to amputation, according to statistics from the Centers for Disease Control and Prevention.

Exercise (PageIndex{2})

Which growth conditions would you recommend for the detection of C. perfringens?

Detoxification of Reactive Oxygen Species

Aerobic respiration constantly generates reactive oxygen species (ROS), byproducts that must be detoxified. Even organisms that do not use aerobic respiration need some way to break down some of the ROS that may form from atmospheric oxygen. Three main enzymes break down those toxic byproducts: superoxide dismutase, peroxidase, and catalase. Each one catalyzes a different reaction. Reactions of type seen in Reaction 1 are catalyzed by peroxidases.

[X-(2H^+)+H_2O_2 ightarrow ext{oxidized}-X+2H_2O]

In these reactions, an electron donor (reduced compound; e.g., reduced nicotinamide adenine dinucleotide [NADH]) oxidizes hydrogen peroxide, or other peroxides, to water. The enzymes play an important role by limiting the damage caused by peroxidation of membrane lipids. Reaction 2 is mediated by the enzyme superoxide dismutase (SOD) and breaks down the powerful superoxide anions generated by aerobic metabolism:

[2O^{2-} + 2H^+ ightarrow H_2O_2+O_2]

The enzyme catalase converts hydrogen peroxide to water and oxygen as shown in Reaction 3.

[2H_2O_2 ightarrow 2H_2O+O_2]

Obligate anaerobes usually lack all three enzymes. Aerotolerant anaerobes do have SOD but no catalase. Reaction 3, shown occurring in Figure (PageIndex{5}), is the basis of a useful and rapid test to distinguish streptococci, which are aerotolerant and do not possess catalase, from staphylococci, which are facultative anaerobes. A sample of culture rapidly mixed in a drop of 3% hydrogen peroxide will release bubbles if the culture is catalase positive.

Bacteria that grow best in a higher concentration of CO2 and a lower concentration of oxygen than present in the atmosphere are called capnophiles. One common approach to grow capnophiles is to use a candle jar. A candle jar consists of a jar with a tight-fitting lid that can accommodate the cultures and a candle. After the cultures are added to the jar, the candle is lit and the lid closed. As the candle burns, it consumes most of the oxygen present and releases CO2.

Exercise (PageIndex{3})

  1. What substance is added to a sample to detect catalase?
  2. What is the function of the candle in a candle jar?

Clinical Focus: part 2

The health-care provider who saw Jeni was concerned primarily because of her pregnancy. Her condition enhances the risk for infections and makes her more vulnerable to those infections. The immune system is downregulated during pregnancy, and pathogens that cross the placenta can be very dangerous for the fetus. A note on the provider’s order to the microbiology lab mentions a suspicion of infection by Listeria monocytogenes, based on the signs and symptoms exhibited by the patient.

Jeni’s blood samples are streaked directly on sheep blood agar, a medium containing tryptic soy agar enriched with 5% sheep blood. (Blood is considered sterile; therefore, competing microorganisms are not expected in the medium.) The inoculated plates are incubated at 37 °C for 24 to 48 hours. Small grayish colonies surrounded by a clear zone emerge. Such colonies are typical of Listeria and other pathogens such as streptococci; the clear zone surrounding the colonies indicates complete lysis of blood in the medium, referred to as beta-hemolysis (Figure (PageIndex{6})). When tested for the presence of catalase, the colonies give a positive response, eliminating Streptococcus as a possible cause. Furthermore, a Gram stain shows short gram-positive bacilli. Cells from a broth culture grown at room temperature displayed the tumbling motility characteristic of Listeria (Figure (PageIndex{6})). All of these clues lead the lab to positively confirm the presence of Listeria in Jeni’s blood samples.

Exercise (PageIndex{4})

How serious is Jeni’s condition and what is the appropriate treatment?

Key Concepts and Summary

  • Aerobic and anaerobic environments can be found in diverse niches throughout nature, including different sites within and on the human body.
  • Microorganisms vary in their requirements for molecular oxygen. Obligate aerobes depend on aerobic respiration and use oxygen as a terminal electron acceptor. They cannot grow without oxygen.
  • Obligate anaerobes cannot grow in the presence of oxygen. They depend on fermentation and anaerobic respiration using a final electron acceptor other than oxygen.
  • Facultative anaerobes show better growth in the presence of oxygen but will also grow without it.
  • Although aerotolerant anaerobes do not perform aerobic respiration, they can grow in the presence of oxygen. Most aerotolerant anaerobes test negative for the enzyme catalase.
  • Microaerophiles need oxygen to grow, albeit at a lower concentration than 21% oxygen in air.
  • Optimum oxygen concentration for an organism is the oxygen level that promotes the fastest growth rate. The minimum permissive oxygen concentration and the maximum permissive oxygen concentration are, respectively, the lowest and the highest oxygen levels that the organism will tolerate.
  • Peroxidase, superoxide dismutase, and catalase are the main enzymes involved in the detoxification of the reactive oxygen species. Superoxide dismutase is usually present in a cell that can tolerate oxygen. All three enzymes are usually detectable in cells that perform aerobic respiration and produce more ROS.
  • A capnophile is an organism that requires a higher than atmospheric concentration of CO2 to grow.

Oxygen Requirements of Microorganisms

Many ecosystems are still free of molecular oxygen. Some are found in extreme locations, such as deep in the ocean or in earth’s crust others are part of our everyday landscape, such as marshes, bogs, and sewers. Within the bodies of humans and other animals, regions with little or no oxygen provide an anaerobic environment for microorganisms. (Figure 7.9).

Figure 7.9 Anaerobic environments are still common on earth. They include environments like (a) a bog where undisturbed dense sediments are virtually devoid of oxygen, and (b) the rumen (the first compartment of a cow’s stomach), which provides an oxygen-free incubator for methanogens and other obligate anaerobic bacteria. (credit a: modification of work by National Park Service credit b: modification of work by US Department of Agriculture)

We can easily observe different requirements for molecular oxygen by growing bacteria in thioglycolate tube cultures. A test-tube culture starts with autoclaved thioglycolate medium containing a low percentage of agar to allow motile bacteria to move throughout the medium. Thioglycolate has strong reducing properties and autoclaving flushes out most of the oxygen. The tubes are inoculated with the bacterial cultures to be tested and incubated at an appropriate temperature. Over time, oxygen slowly diffuses throughout the thioglycolate tube culture from the top. Bacterial density increases in the area where oxygen concentration is best suited for the growth of that particular organism.

The growth of bacteria with varying oxygen requirements in thioglycolate tubes is illustrated in Figure 7.10. In tube A, all the growth is seen at the top of the tube. The bacteria are obligate (strict) aerobes that cannot grow without an abundant supply of oxygen. Tube B looks like the opposite of tube A. Bacteria grow at the bottom of tube B. Those are obligate anaerobes, which are killed by oxygen. Tube C shows heavy growth at the top of the tube and growth throughout the tube, a typical result with facultative anaerobes. Facultative anaerobes are organisms that thrive in the presence of oxygen but also grow in its absence by relying on fermentation or anaerobic respiration, if there is a suitable electron acceptor other than oxygen and the organism is able to perform anaerobic respiration. The aerotolerant anaerobes in tube D are indifferent to the presence of oxygen. They do not use oxygen because they usually have a fermentative metabolism, but they are not harmed by the presence of oxygen as obligate anaerobes are. Tube E on the right shows a “Goldilocks” culture. The oxygen level has to be just right for growth, not too much and not too little. These microaerophiles are bacteria that require a minimum level of oxygen for growth, about 1%–10%, well below the 21% found in the atmosphere.

Figure 7.10 Diagram of bacterial cell distribution in thioglycolate tubes.


2. Solutes and Water

The microorganism is separated from the environment by the selectively semi permeable membrane. The membrane also plays a vital role maintaining the internal solute and water content. When the environment is changed in terms of water or solute concentration in the outside environment it affects the internal content due to the semi permeability nature of membrane. The surrounding environment, medium or solution can be hypertonic, hypotonic, and isotonic. There are few channel gates which allow the flow of water across the membrane.

In hypertonic media, the concentration of solute in higher in media than the cell due to which water from cell goes out and causes cell shrinkage.

In Isotonic media, there is balance of concentration of solutes and ions outside and inside the membrane of cell. This is the optimum physical environment of solute and water content for the Bacterial growth.

In hypotonic medium, the solute and ions concentration is less than it is present inside the cell and hence the water molecules moves in and causes swelling of the cell. This may also lead to bursting of the cell.


Nutritional Requirements of Microorganisms

The microbial nutrients can be classified as macro (major) nutrients, and micro (minor) nutrients or trace elements on the basis of their amount required.

1. Macro or Major Mineral Nutrients:

The microbial cells contain water accounting for some 80-90% of their total weight and, therefore, the water is always the major essential nutrient in quantitative terms.

The solid matter of cells contain, in addition to oxygen and hydrogen (derivable metabolically from water), the other macro (major) elements, namely, carbon, nitrogen, phosphorus, sulphur, potassium, magnesium, sodium, calcium and iron in order of decreasing abundance.

About 95% of cellular dry weight of microbial cells is accounted for only six macro (major) elements (O, H, C, N, P and S). However, approximate percentage of dry weight and general physiological functions of major mineral nutrients are given in Table 18.1.

Carbon assumes great importance as the main constituent of all organic cell materials and represents about 50% of cell’s dry weight. CO2 is the most oxidized form of carbon and the photo-synthetic microorganisms reduce CO2 to organic cell constituents. On the other hand, all the non-photosynthetic microorganisms obtain their carbon requirement mainly from organic nutrients which contain reduced carbon compounds.

These organic compounds not only provide the carbon for synthesis but also meet the energy requirement by entering into energy yielding metabolic pathways and are eventually oxidised to CO2.

Some microbes have the ability to synthesize all their cellular components using a single organic carbon source while others, in addition to this one major carbon source, also need other complex carbon containing components which they cannot synthesize.

These components are called growth factors and include vitamins. Some microbes can utilize more than one carbon compound and exhibit a great degree of versatility. The others, however, are specialized in this regard.

Sulphur and nitrogen are taken up by most organisms and are subsequently reduced within the cell and utilized in other biosynthetic processes. The sulphur and nitrogen requirements of most organisms can also be met with organic nutrients that contain these two elements in reduced organic combinations such as amino acids. A few microorganisms are capable of reducing elemental nitrogen to ammonia and this process of nitrogen assimilation is known as biological nitrogen fixation.

Most of the microorganisms need molecular oxygen for respiration. In these, the oxygen serves as terminal electron acceptor, and such organisms are referred to as ‘obligate aerobes’.

As opposed to this there are a few organisms which do not use molecular oxygen as terminal electron acceptor. We recall that oxygen is a component of the cellular material of all the microorganisms. These microbes are called ‘obligate anaerobes’.

In fact, molecular oxygen is toxic to these organisms. Aerobes which can grow in the absence of oxygen are called ‘facultative anaerobes’ and the anaerobes which can grow in the presence of oxygen are referred to as ‘facultative aerobes’. In addition to these major classes, there are organisms which grow best at reduced oxygen pressure but are obligate aerobes and these are called ‘Microaerophilic’.

2. Micro or Minor Mineral Nutrients or Trace Elements:

The microorganisms, in general do not use only macro (major) elements but also others like cobalt, copper, manganese, molybdenum, nickel, selenium, tungsten, vanadium and zinc which are required in residual fraction by nearly all microorganisms.

These elements are often referred to as minor (micro) nutrients or trace elements. The micronutrients or trace elements are nevertheless just as critical to cell function as are the macronutrients.

They are metals playing the role of cell’s catalysts and many of them are play a structural role in various enzymes. Table 18.2 summarizes the major micronutrients of living systems and gives examples of enzymes in which each plays a role. Some microorganisms, however, need additional specific mineral nutrients, for example, diatoms and some microalgae require silica, supplied as silicate, to impregnate their cell walls.

Growth Factors:

Besides the mineral nutrients, the microorganisms need some organic compounds. Most of the microorganisms are capable of synthesizing these organic compounds from simpler carbon resources, others cannot and need their supply from outside for their proper growth and development.

Organic nutrients of this type are known collectively as growth factors (essential metabolites) and can be categorized into three groups (amino acids, purines and pyrimidines and vitamins) on the basis of their chemical structure and metabolic function.

Amino acids and purines and pyrimidines are the constituents of proteins and nucleic acids, respectively. Vitamins, however, are the most commonly needed growth factor and form parts of the prosthetic groups or active centres of certain enzymes. Some important vitamins and their functions are summarized in Table 18.3.

Since the growth factors fulfill specific needs in biosynthesis of certain molecules, they are needed in very small amounts the vitamins even in less smaller quantities, because of the various coenzymes of which they are precursors, have catalytic roles and consequently are present at levels of a few parts per million in the microbial cell.


Temperature

Bacteria have adapted to a wide range of temperatures. Bacteria that grow at temperatures of less than about 15 °C (59 °F) are psychrophiles. The ability of bacteria to grow at low temperatures is not unexpected, since the average subsurface temperature of soil in the temperate zone is about 12 °C (54 °F) and 90 percent of the oceans measure 5 °C (41 °F) or colder. Obligate psychrophiles, which have been isolated from Arctic and Antarctic ocean waters and sediments, have optimum growth temperatures of about 10 °C (50 °F) and do not survive if exposed to 20 °C (68 °F). The majority of psychrophilic bacteria are in the gram-negative genera Pseudomonas, Flavobacterium, Achromobacter, and Alcaligenes. Mesophilic bacteria are those in which optimum growth occurs between 20 and 45 °C (68 and 113 °F), although they usually can survive and grow in temperatures between 10 and 50 °C (50 and 122 °F). Animal pathogens are mesophiles.

Thermophilic prokaryotes can grow at temperatures higher than 60 °C (140 °F). These temperatures are encountered in rotting compost piles, hot springs, and oceanic geothermal vents. In the runoff of a hot spring, thermophiles such as the bacterium Thermus aquaticus (optimum temperature for growth, 70 °C [158 °F] maximum temperature, 79 °C [174 °F]) are found near the source where the temperature has fallen to about 70 °C. Thick mats of the cyanobacterium Synechococcus and the phototrophic gliding bacterium Chloroflexus develop in somewhat cooler portions of the runoff. The archaeon Sulfolobus acidocaldarius has a high tolerance for acidic conditions, which allows growth in a pH range of about 1.0 to 6.0 and a temperature optimum of 80 °C (176 °F). Numerous bacteria and archaea are adapted to the temperature range of 50 to 70 °C (122 to 158 °F), including some members of the genera Bacillus, Thermoactinomyces, Methanobacterium, Methylococcus, and Sulfolobus. Most striking was the discovery in the mid-1980s of bacteria and archaea in nutrient-rich, extremely hot hydrothermal vents on the deep seafloor. The archaea in the genus Pyrodictium thrive in the temperature range of 80 to 110 °C (176 to 230 °F), temperatures at which the water remains liquid only because of the extremely high pressures.

Most bacteria grow in the range of neutral pH values (between 5 and 8), although some species have adapted to life at more acidic or alkaline extremes. An example of an acidophilic bacterium is A. ferrooxidans. When coal seams are exposed to air through mining operations, the pyritic ferrous sulfide deposits are attacked by A. ferrooxidans to generate sulfuric acid, which lowers the pH to 2.0 or even 0.7. However, acid tolerance of A. ferrooxidans applies only to sulfuric acid, since these bacteria die when exposed to equivalent concentrations of other acids such as hydrochloric acid. Many bacteria cannot tolerate acidic environments, especially under anaerobic conditions, and, as a result, plant polymers degrade slowly in acidic (pH between 3.7 and 5.5) bogs, pine forests, and lakes. In contrast to acidophilic bacteria, alkalophilic bacteria are able to grow in alkaline concentrations as great as pH 10 to 11. Alkalophiles have been isolated from soils, and most are species of the gram-positive genus Bacillus.


8.2: Oxygen Requirements for Microbial Growth - Biology

An aerobic organism or aerobe is an organism that can survive and grow in an oxygenated environment. Several varietis of aerobes exist. Obligate aerobes require oxygen for aerobic cellular respiration. In a process known as cellular respiration, these organisms use oxygen to oxidize substrates (for example sugars and fats) in order to obtain energy. Facultative anaerobes can use oxygen, but also have anaerobic (i.e. not requiring oxygen) methods of energy production. Microaerophiles are organisms that may use oxygen, but only at low concentrations. Aerotolerant organisms can survive in the presence of oxygen, but they are anaerobic because they do not use it as a terminal electron acceptor.

Identity of aerobic and anaerobic bacteria: Aerobically different bacteria behave differently when grown in liquid culture: 1) Obligate aerobic bacteria gather at the top of the test tube in order to absorb maximal amount of oxygen. 2) Obligate anaerobic bacteria gather at the bottom to avoid oxygen. 3) Facultative bacteria gather mostly at the top, since aerobic respiration is advantageous (ie, energetically favorable) but as lack of oxygen does not hurt them, they can be found all along the test tube. 4) Microaerophiles gather at the upper part of the test tube but not at the top. They require oxygen, but at a lower concentration. 5) Aerotolerant bacteria are not affected at all by oxygen, and they are evenly spread along the test tube.

An anaerobic organism or anaerobe is any organism that does not require oxygen for growth. It could possibly react negatively and may even die if oxygen is present. For practical purposes there are three categories: obligate anaerobes, which cannot use oxygen for growth and are even harmed by it. Aerotolerant organisms, which cannot use oxygen for growth, but tolerate the presence of it. And finally, facultative anaerobes, which can grow without oxygen but can utilize oxygen if it is present.

Since normal microbial culturing occurs in atmospheric air, which is an aerobic environment, the culturing of anaerobes poses a problem. Therefore, a number of techniques are employed by microbiologists when culturing anaerobic organisms, for example, handling the bacteria in a glovebox filled with nitrogen or the use of other specially-sealed containers.

Glovebox: Terra Universal 100 Glovebox

The GasPak System is an isolated container that achieves an anaerobic environment by the reaction of water with sodium borohydride and sodium bicarbonate tablets to produce hydrogen gas and carbon dioxide. Hydrogen then reacts with oxygen gas on a palladium catalyst to produce more water, thereby removing oxygen gas.


Summary

Several studies indicate that aerobes can survive in the presence of oxygen only by virtue of an elaborate system of defenses. Without these defenses, key enzyme systems in the organisms fail to function and the organisms die.
Obligate anaerobes, which live only in the absence of oxygen, do not possess the defenses that make aerobic life possible and therefore can not survive in air.

The tolerance to oxygen is related to the ability of the bacterium to detoxify superoxide and hydrogen peroxide, produced as a byproduct of aerobic respiration.

The assimilation of glucose in aerobic conditions results in the terminal generation of free radical superoxide (O2 – ). The superoxide is reduced by the enzyme superoxide dismutase to oxygen gas and hydrogen peroxide (H2O2). Subsequently, the toxic hydrogen peroxide generated in this reaction is converted to water and oxygen by the enzyme catalase, which is found in aerobic and facultative bacteria, or by various peroxidases which are found in several aerotolerant anaerobes.

Obligate aerobes and most facultative anaerobes have both superoxide dismutase and catalase. Some facultative and aerotolerant anaerobes have superoxide dismutase but lack catalase. Most obligate anaerobes lack both enzymes.


Watch the video: Bacterial Growth Requirements (August 2022).