Classification of Bacteria According to Their Respiration Type
Bacteria can be classified into five major groups based on their requirements for oxygen. Usually the bacteria are supplied with air that contains about 20% of oxygen .
- Obligate (or strict) aerobes. These microorganisms have an absolute requirement for oxygen because they metabolize sugars through aerobic respiration. Since oxygen is not soluble enough in watery solutions, aerobes grow best in cultural vessels with continuous stirring or shake that enhances oxygen circulation. Members of this group pertain to the genera Bacillus, Pseudomonas and to many other microbial taxa (brucellae, meningococci, mycobacteria, etc.).
- Obligate (or strict) anaerobes. The representatives of this group can’t propagate even at small amounts of oxygen. Some of them are literally killed by the contact with traces of oxygen as they are unable to detoxify the active forms of oxygen generated in metabolism. Moreover, some of their enzymes are inactivated by oxygen. The anaerobic bacteria encompass many microbial genera, such as the sporeforming genus Clostridium (causative agents of tetanus, botulism, etc.), or the numerous group of non-sporeforming gram-negative anaerobic bacteria that comprises bacteroids, prevotellae, porphyromonads, fusobacteria, and others.
- Facultatively anaerobic bacteria. These microbial agents can utilize oxygen as terminal oxidant if available, but can also propagate in its absence. It is obvious, that their metabolism is more rapid under oxygen consumption – more ATP molecules are synthesized by aerobic respiration. Typical examples of facultative anaerobes are Escherichia coli and Saccharomyces yeasts.
- Microaerophilic bacteria. They need small or moderate amounts of oxygen, (2% to 10%), as higher concentrations are deleterious. Not so many pathogenic microorganisms pertain to this group, e.g., Helicobacter pylori, the agent of gastritis and gastroduodenal ulcer. Inside its natural habitation site, mucous gastric epithelium, only the low levels of oxygen are available. In addition, microaerophiles usually grow more actively in the presence of elevated concentrations of carbon dioxide (capnophiles). To support this, microaerophiles can be cultured in anaerobic jars with controlled supply of CO2 to final concentrations of 3-5%. At the same time the concentration of remaining oxygen is enough to prevent the growth of strict anaerobes.
- Aerotolerant bacteria. These microbials are indifferent to the presence or absence of oxygen, capable of growing in both conditions. However, they don’t use oxygen for biological oxidation. Many aerotolerant bacteria also grow more actively under the increased concentrations of carbon dioxide (capnophilic bacteria). Therefore, they are better cultured in conditions of high humidity within the special chambers – CO2-incubators with microprocessor gas control.
As an example, Streptococcus pyogenes, the causative agent of streptococcal sore throat, is an aerotolerant microorganism.