Oxygen-free (anaerobic) respiration
Unlike classical aerobic metabolism, anaerobic respiration uses inorganic molecules as the final electron acceptors.
- Nitrate respiration. Nitrate acts as the acceptor. Using the enzyme nitrate reductase, bacteria reduce it via two pathways: ammonification (to ammonia) or denitrification (to molecular nitrogen gas or its nitrous oxide). This type of respiration is common among facultative anaerobes.
- Sulfate respiration. Sulfate is used as the terminal acceptor, and molecular hydrogen serves as the electron donor. The process is characteristic of strict anaerobic chemolithotrophs (genera Desulfovibrio, Desulfotomaculum). They inhabit hydrogen sulfide-rich sludge, can be detected in the human body (in feces or the oral cavity), and are the primary producers of hydrogen sulfide in nature.
Classification of bacteria by oxygen demand
Microorganisms are divided into three basic groups depending on their requirement for free atmospheric oxygen (which makes up about 21% of air):
- Obligate aerobes. They grow exclusively in an oxygen environment. Their metabolism is oxidative, with oxygen acting as the terminal electron acceptor, and cytochrome c oxidase serving as the key enzyme. These include strict aerobes (requiring high atmospheric oxygen pressure) and microaerophiles (which grow at reduced oxygen levels, as their enzymes, such as hydrogenase, are inactivated by strong oxidizing agents).
- Obligate anaerobes. Obtain energy without oxygen, primarily through fermentation. Strict anaerobes (e.g., clostridia C. botulinum, C. tetani, and bacteroides) perish upon contact with oxygen and generate energy via butyric acid fermentation. Aerotolerant bacteria (lactic acid bacteria) survive in air but do not use oxygen, employing heterofermentative lactic acid fermentation.
- Facultative anaerobes. Versatile bacteria with mixed energy-yielding pathways. In the presence of air, they respire oxygen, and without it, they switch to fermentation or nitrate respiration.
Biochemical principles of oxygen toxicity and cell defense
During the oxidation of flavoproteins in the presence of oxygen, toxic compounds are formed: hydrogen peroxide and the superoxide radical, which possesses an unpaired electron. They trigger lipid peroxidation of fatty acids, destroying membranes, and oxidize sulfhydryl groups of proteins, inactivating enzymes. Microbial survival depends on the presence of enzymatic defense.
- In aerobes and facultative anaerobes, a two-step system operates. First, superoxide dismutase (SOD) converts the radical into hydrogen peroxide and molecular oxygen. Then, catalase breaks down the peroxide into water and harmless oxygen.
- In aerotolerant microbes, SOD is absent. They are protected from the superoxide radical by high concentrations of manganese ions, which chemically bind it. Peroxidase combats the peroxide by destroying it during organic oxidation reactions.
- In strict anaerobes, catalase and peroxidase are completely absent. Some representatives possess SOD, and its presence directly correlates with the degree of bacterial resistance to accidental exposure to an oxygen atmosphere (e.g., certain species of the genus Clostridium).
Microbial cultivation atmosphere
The type of respiration determines the conditions under which microorganisms are grown in the laboratory.
- Strict aerobes and facultative anaerobes grow on the surface of solid media or in the upper layer of liquid media. Deep growth within the liquid bulk requires forced aeration (constant shaking or mixing on shakers). Facultative anaerobes are cultivated in air because oxidative metabolism yields more energy than fermentation.
- Microaerophiles require an atmosphere with a reduced partial pressure of oxygen and an increased concentration of carbon dioxide (1–5%). $CO_2$ incubators or desiccators with a burning candle are used for this purpose.
- Obligate anaerobes require the complete exclusion of contact with air. The chemical method involves adding oxygen-scavenging reducing agents to the medium (thioglycolic or ascorbic acids, cysteine, sulfides). The physical method involves boiling liquid media to remove dissolved oxygen, followed by sealing with rubber stoppers. Special apparatuses—gas generator jars (anaerostats) and anaerobic chambers—are also used.
Briefly: answers to topical questions
Which enzymes and electron carriers form the respiratory chain on the cytoplasmic membrane in obligate aerobes?
The bacterial respiratory chain involves:
- Cytochromes are components of the chain; their composition may vary depending on growth conditions.
- Quinones — bacteria may use naphthoquinones instead of ubiquinones.
- Cytochrome oxidase is a complex of cytochromes $a+a_3$ that transfers electrons directly to oxygen.
- In some bacteria, cytochromes are absent; in this case, hydrogen transfer to oxygen is carried out by flavoproteins, and the end product is hydrogen peroxide.
What biological methods exist for creating anaerobic conditions for cultivating microorganisms?
The biological method for creating anaerobic conditions is the Fortner method: co-cultivating the studied microorganisms with aerobes that consume oxygen.
Which bacteria belong to obligate non-spore-forming anaerobes?
Obligate non-spore-forming anaerobes include:
- Gram-negative rods — bacteroides (Bacteroides), prevotellae (Prevotella), porphyromonads (Porphyromonas), fusobacteria (Fusobacterium), leptotrichiae (Leptotrichia).
- Gram-positive non-spore-forming rods — eubacteria (Eubacterium).
- Gram-positive anaerobic cocci — peptococci (Peptococcus), peptostreptococci (Peptostreptococcus), ruminococci, gemellae.
- Gram-negative anaerobic cocci — veillonellae (Veillonella).
These microorganisms are part of the normal microflora of the intestine and oral cavity; Gram-negative anaerobic rods are also found in the vaginal microflora and are capable of causing purulent-inflammatory diseases.
SOC: Superoxide dismutase, Oxidase (cytochrome oxidase), Catalase — the three main enzymes that allow bacteria to "befriend" oxygen.
Class and exam questions
Why do strict anaerobes die in the presence of oxygen?
They lack protective enzymes (catalase and peroxidase) capable of neutralizing toxic oxidation products—hydrogen peroxide and the superoxide radical.
How do facultative anaerobes behave in the presence of air?
Upon access to oxygen, they switch from fermentation to oxidative metabolism (oxygen respiration) because this pathway is energetically more favorable.
Why is a burning candle used when cultivating microaerophiles?
A candle burning in a closed desiccator burns out excess oxygen and accumulates carbon dioxide, creating an ideal gaseous environment (reduced O2 level and 1–5% CO2) for the growth of these bacteria.
What else to study on this topic
- Butyric acid fermentation of strict anaerobes (clostridia)
- Heterofermentative lactic acid fermentation
- Design and operating principle of anaerostats and anaerobic chambers
- The role of cytochrome oxidase in the respiratory chain of obligate aerobes
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