Anaerobic Respiration
Unlike classical aerobic metabolism, anaerobic respiration utilizes inorganic molecules as the terminal electron acceptors.
- Nitrate respiration. Nitrate serves as the acceptor. Using the enzyme nitrate reductase, bacteria reduce it via two pathways: ammonification (to ammonia) or denitrification (to gaseous molecular nitrogen or nitrous oxide). This type of respiration is common among facultative anaerobes.
- Sulfate respiration. Sulfate is used as the terminal acceptor, while molecular hydrogen serves as the electron donor. This process is typical of obligate chemolithotrophic anaerobes (genera Desulfovibrio, Desulfotomaculum). They inhabit hydrogen sulfide-rich sludge, can be found in the human body (in feces or the oral cavity), and are major producers of hydrogen sulfide in nature.
Classification of Bacteria Based on Oxygen Requirements
Microorganisms are divided into three basic groups depending on their requirement for free atmospheric oxygen (which makes up about 21% of ambient air):
- Obligate aerobes. Grow exclusively in an oxygen-rich environment. Their metabolism is oxidative, with oxygen acting as the terminal electron acceptor and cytochrome oxidase serving as a key enzyme. They include strict aerobes (requiring high atmospheric oxygen tension) and microaerophiles (growing at reduced oxygen content because their enzymes, such as hydrogenase, are inactivated by strong oxidizers).
- Obligate anaerobes. Generate energy without oxygen, primarily through fermentation. Strict anaerobes (e.g., clostridia C. botulinum, C. tetani, and bacteroides) die upon exposure to oxygen and derive energy via butyric acid fermentation. Aerotolerant bacteria (lactic acid bacteria) survive in air but do not use oxygen, utilizing heterofermentative lactic acid fermentation.
- Facultative anaerobes. Versatile bacteria with mixed energy-yielding pathways. In the presence of air, they respire using oxygen; in its absence, they switch to fermentation or nitrate respiration.
Biochemical Basis of Oxygen Toxicity and Cellular Defense
During the oxidation of flavoproteins in the presence of oxygen, toxic compounds are formed: hydrogen peroxide and the superoxide radical, which contains an unpaired electron. These trigger lipid peroxidation of fatty acids, disrupting membranes, and oxidize sulfhydryl groups in proteins, inactivating enzymes. Microorganism survival depends on enzymatic defense mechanisms.
- Aerobes and facultative anaerobes possess a two-step system. First, superoxide dismutase (SOD) converts the radical into hydrogen peroxide and molecular oxygen. Then, catalase breaks down the peroxide into water and harmless oxygen.
- Aerotolerant microbes lack SOD. They are protected from superoxide radicals by high concentrations of manganese ions, which chemically bind the radicals. Peroxidase handles hydrogen peroxide by breaking it down during organic oxidation reactions.
- Strict anaerobes completely lack catalase and peroxidase. Some species 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).
Cultivation Atmosphere for Microorganisms
The type of respiration dictates the conditions used to grow microorganisms 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 a liquid column requires forced aeration (constant shaking or agitation on orbital shakers). Facultative anaerobes are cultivated in air because oxidative metabolism yields more energy than fermentation.
- Microaerophiles require an atmosphere with reduced partial oxygen pressure and an increased concentration of carbon dioxide (1–5%). This is achieved using $CO_2$ incubators or candle jars.
- Obligate anaerobes require complete exclusion of air contact. Chemical methods involve adding reducing agents to the medium to bind oxygen (thioglycolic or ascorbic acids, cysteine, sulfides). Physical methods include boiling liquid media to drive off dissolved oxygen, followed by sealing with rubber stoppers. Specialized equipment such as anaerobic jars and anaerobic chambers are also utilized.