Nutritional types of bacteria
Bacteria differ in their nutritional requirements. The differences among the bacteria are specific. A systematic approach for the nutritional classification of bacteria was first made by Monod. Amongst various requirements for nutrition, characteristic differences exist for the requirement of sources of energy, electron donor and carbon. Therefore, bacteria are classified in various categories on the basis of these requirements.
1]. Based on source of energy :
a) Phototrophs
b) Chemotrophs
c) Hypotrophs
2]. Basesd on source of electron donor :
a) Lithotrophs
b) Organotrophs
3]. Based on source of carbon :
a) Autotrophs
b) Heterotrophs
c) Paratrophs
1]. Classification on the basis of requirements for sources of energy
Based on requirements for sources of energy bacteria can be classified in to three major categories,
1. Phototrophs
2. Chernotrophs
3. Hypotrophe
Phototrophs
These are the bacteria which obtain energy by using radiant energy i.e. light. These bacteria possess photosynthetic pigments and photosynthetic apparatus. With its help, they capture radiant energy and transform it into biologically utilizable form of energy i.e. ATP. Bacteria belonging to this category Include1. Cyanobacteria,
2. Green sulfur bacteria,
3. Purple sulfur bacteria and
4. Purple non sulfur bacteria.
Chemotrophs
These are the bacteria which obtain energy by oxidising chemicals. Upon oxidation of chemicals, chemical energy is released. It is trapped in ATP molecules during biochemical reactions and is made available for cellular processes. A wide variety of chemical compounds can be used by Chemotrophs as source of energy. They include both inorganic and organic.- Thiobacillus and other sulfur oxidizers obtain energy from oxidation of reduced sulfur compounds.
- Nitrifying bacteria obtain energy from oxidation of ammonia and ammonium compounds.
- Iron bacteria obtain energy by oxidizing Iron from Fe++ to Fe+++ form.
- Hydrogen bacteria can oxidize molecular hydrogen to obtain energy.
Organic compounds, which are attacked by the bacteria for the purpose of obtaining energy range from most simple substance like methane to complex substance like paraffin. eg. E. coli, Bacillus and various other bacteria obtain energy from oxidation of organic carbon compounds.
Hypotrophs
These are the organisms, which cannot utilize any external source of energy. This is because of their inability to synthesis ATP. They require ready made ATP for growth. It may be obtained from other living host cells. Thus these organisms grow as parasites. e.g. Viruses and rickettsiae.2]. Classification on the basis of source of electron donor
Bacteria show characteristic differences in the substances used as electron donor. On the basis of type of electron donor utilized, bacteria can be classified in to two categories
1. Lithotrophs
2. Organotrophs
Lithotrophs
These are the bacteria, which utilize Inorganic substances as electron donor. They oxidize selective Inorganic substances and generate necessary reducing power required for biosynthesis. Inorganic substances used as electron donor by organisms include reduced sulfur compounds, ferrous salts, ammonia, ammonium compounds and molecular hydrogen.Organotrophs
These bacteria generate their reducing power from oxidation of various organic compounds.3]. Classification on the
basis of carbon sources
Bacteria also possess characteristic difference in their requirement of substances used as source of carbon in their nutrition. Accordingly, they can be classified in to three categories
1. Autotrophs
2. Heterotrophs
3. Paratrophs
Autotrophs
The bacteria, which use inorganic carbon compounds as the carbon source in their nutrition are called autotrophs. They mostly use CO2. They fix CO2 and reduce it into cellular organic matter. Usually, they - fix CO2 by Calvin cycle or reductive TCA cycle. Some bacteria can also assimilate carbonates and carbon monoxide.Heterotrophs
These bacteria obtain their carbon from various organic substances, ranging from methane to paraffin. Majority of bacteria fall in this category.Paratrophs
Certain bacteria lack biosynthetic abilities and are not able to use non cellular organic or Inorganic carbon compounds. Instead, they need ready made supply of organic molecules, which can be assimilated directly during their cellular synthesis, such as amino acids, nucleotides etc. These organisms are classified as paratrophs.e.g. viruses and rickettsiae.
Thus it is clear that bacteria differ in their nutritional categories drastically. Even a bacterium belonging to one category for its requirements for energy may show a marked difference in its requirements for sources of electron donor as well as carbon.
Nutritional subgroups of organisms
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| Subgroups of nutritional types of bacteria on the basis of their diversity in the requirements for different sources of nutrients. |
Photolithotrophs
These are the bacteria, which obtain energy from light and use inorganic substances as electron donor. These Bacteria usually utilize H₂S or other reduced inorganic sulfur as electron donor and oxidize it to elemental sulfur.H₂S ➞ S + 2e+ + 2H+
- Green sulfur bacteria and purple sulfur bacteria belong to this category.
Photoorganotrophs
These bacteria obtain energy from light and generate reducing power from oxidation of organic compounds. They use substances like pyruvate, succinate, formate as electron donor and generate reducing power on oxidation of these substances. e.g.Succinate ➞ Fumarate + 2e–+ 2H+
- Purple non sulfur bacteria belong to this category.
Photoautotrophs
These are the phototrophic bacteria, which obtain their energy from light and carbon from CO₂ They are more related to photolithotrophs.Photoheterotrophs
These bacteria obtain their energy from light and carbon from organic substances. Nutritionally they are close to photoorganotrophs. They can use same organic compound, both as electron donor and source of carbon.Chemolithotrophs
These bacteria can use the same inorganic chemical substances as the sources of energy and electron donor. They oxidize selected Inorganic chemical compounds for the purpose. The chemicals used by these organisms include:1). H₂S and other reduced sulfur compounds
Organisms oxdize these reduced sulfur compounds, finally to SO₂ during their metabolism.
e.g. Sulfur oxidizers (Thiobacillus).
2). Ferrous compounds
Organisms oxidize ferrous iron (Fe++) compounds to ferric (Fe+++) form and generate both energy and reducing power, e.g. Iron oxidizers (Ferrobacillus).
3). Ammonia and ammonium compounds
The organisms oxidize ammonia and ammonium compounds finaly to NO3 through the phenomenon of nitrosification and nitrification, e.g. Nitrobacter
4). Molecular hydrogen
Many bacteria can oxidize molecular hydrogen to obtain both energy and reducing power, e.g. Hydrogenbacteria.
Chemoorganotrophs
These are the bacteria, which obtain their energy and reducing power through oxidation of same organic compound. They can oxidize a wide variety of organic substances for the purpose. Most bacteria belong to this category. Nutritionally these bacteria are more related to Chemoheterotrophs.Chemoautotrophs
These are the bacteria which obtain energy from oxidation of chemical compounds (usually inorganic), and carbon from CO₂. Nutritionally, they are more close to chemolithotrophs.Chemoheterotrophs
These are the bacteria, which obtain their energy from oxidation or chemicals (usually organic) and carbon from organic compounds Usually, they use same organic compounds for getting energy reducing power and carbon.Mixotrophs
Mixotrophs are the organisms which have ability to utilize either categories of substances (organic or Inorganic), as source of energy, electron donor as well as carbon.Certain phototrophs are able to grow even as chemotrophs under special circumstances. For example, Rhodospirillum rubrum can grow as phototroph and obtain energy from light under anaerobic conditions. But the same organisms can grow as chemotroph under aerobic condition and obtain energy from oxidation of organic compounds.
Similarly, all chemolithotrophs usually utilize CO₂ as source of carbon and grow autotrophically. But there are some organisms which derive their energy and reducing power from oxidation of Inorganic substances, but carbon from organic substrates. These organisms are also categorized as mixotrophs.
e.g. Desulfoubrio desulfuricans obtains reducing power from axidation of hydrogen. Yet It can also use organic carbon compounds as source of both electron donor as well as carbon.
Parasites
Parasites are the organisms, which require living host cells for their growth. This is because of the lack of their biosynthetic capabilities for ATP and other cellular molecules or their complex for nutritional requirements for growth. They are, therefore, nutritionally related to both paratrophs and hypotrophs. These organisms may be grouped in to two categories:a. Obligate parasites
b. Facultative parasites
Obligate parasites are the organisms. which essentially require host for growth. They can not grow on artificial nonliving media, e.g. Treponema pellidum, Chlamydiae, Rickettsine, Bdelloribrio etc.
Facultative parasites are the organisms. which can grow even on artificial non living media. They may be fastidious for growth factors in their nutrition. They may also be called us fastidious heterotrophs.
