Nutritional Requirement

All forms of life, from microorganisms to human beings, share certain nutritional requirements for growth and normal functioning. The following observations substantiate this statement and also illustrate the great diversity of nutritional types found among bacteria.

Energy

• All organisms require a source of energy. 
• Some rely on chemical compounds for their energy and are designated as chemotrophs. Others can utilize radiant energy (light) and are called phototrophs. 
• Both chemotrophs and phototrophs exist among bacteria.

Electron

• All organisms require a source of electrons for their metabolism. 
• Some organisms can use reduced inorganic compounds as electron donors and are termed lithotrophs (some may be chemolithotrophs, others photolithotrophs). 
• Other organisms use organic compounds as electron donors and are called organography (some are chemoorganotrophs, others photoorganotrophs).

Carbon

• All organisms require carbon in some form for use in synthesizing cell components. 
• All organisms require at least small amounts of CO2. However, some can use CO2 as their major, or even sole, source of carbon; such organisms are termes autotrophs.
• Others require organic compounds as their carbon source and are termed heterotrophs.

Nitrogen

• All organisms require nitrogen in some form for cell components. 
• Bacteria are extremely versatile in this respect. Unlike eucaryotes, some bacteria can use atmospheric nitrogen. 
• Others thrive on inorganic nitrogen compounds such as nitrates, nitrites, or ammonium salts, and still others derive nitrogen from organic compounds such as amino acids.

Oxygen

• All organisms require oxygen, sulfur and phosphorus for cell components. 
• Oxygen is provided in various forms, such as water; component atoms of various nutrients: or molecular oxygen. 

Sulfur 

• Sulfur is needed for the synthesis of certain amino acids (cysteine, cystine, and methionine). 
• Some bacteria require organic sulfur compounds, some are capable of utilizing inorganic sulfur compounds, and some can even use elemental sulfur. 

Phosphorus 

• Phosphorus, usually supplied in the form of phosphate, is an essential component of nucleotides, nucleic acids, phospholipids, teichoic acids, and other compounds.

Metal Ions

• All living organisms require metal ions, such as K+, Ca2+, Mg2+, and Fe2+ for normal growth. 
• Other metal ions are also needed but usually only at very low concentrations, such as Zn2+, Cu2+, Mn2+, Mo**, Ni2+, B2+. and Co2+; these are often termed trace elements and often occur as contaminants of other components of culture media in amounts sufficient to support bacterial growth.

• Not all the biological functions of metal ions are known, but Fe2+, Mg2+, Zn2+, Mn2+, and Cu2+, are known to be cofactors for various enzymes.
• Most bacteria do not require Na2+, but certain marine bacteria, cyanobacteria, and photosynthetic bacteria do require it. For those members of the archaeobacteria known as the “red extreme halophiles,” the requirement is astonishing: they cannot grow with less than 12 to 15 percent NaCl. They require this high level of NaCl for maintenance of the integrity of their cell walls and for the stability and activity of certain of their enzymes. 

Vitamins and vitamin-like compounds

• All living organisms contain vitamins and vitamin-like compounds. 
• These function either as coenzymes for several enzymes or as the building blocks for coenzymes. 
• Some bacteria are capable of synthesizing their entire requirement of vitamins from other compounds in the culture medium, but others cannot do so and will not grow unless the required vitamins are supplied preformed to them in the medium. 

Water

• All living organisms require water, and in the case of bacteria, all nutrients must be in an aqueous solution before they can enter the cells. 
• Water is a highly polar compound that is unequaled in its ability to dissolve or disperse cellular components and to provide a suitable milieu for the various metabolic reactions of a cell. 
• Moreover, the high specific heat of water provides resistance to sudden, transient temperature changes in the environment. 
• Water is also a chemical reactant, being required for the many hydrolytic reactions carried out by a cell.

Classification of bacteria based on Nutritional Requirement

Microorganisms are classified on the basis of following nutritional requirement;

1. Classification of bacteria basis of Energy Source
2. Classification of bacteria basis of Electron Source
3. Classification of bacteria basis of Carbon Source

Classification of bacteria basis of Energy Source

Based on source of energy microorganisms are classified into two groups;

Type	Source of Energy
Phototrophs	Light is the main source of energy
Chemotrophs	Oxidation of organic or inorganic compounds

1. Phototrophs

• Those microorganisms that use light as their energy source are known as Phototrophs.
• Examples of phototrophs are Higher plants (maize plant, trees, grass etc), Euglena, Algae (Green algae etc), Bacteria (e.g. Cyanobacteria).

Classification of Phototrophs

The Phototrophs are classified into the following groups

a. Photolithotrophs

• Among the phototrophic bacteria are species that use inorganic compounds as their source of electrons are known as Photolithotrophs.
• For example, Chromatium okenii uses H2S as its electron donor, oxidizing it to elemental sulfur: H2S = S +2e + 2H

b. Photoorganotrophs

• Some phototrophic bacteria use organic compounds such as fatty acids and alcohols as electron donors and are therefore photoorganotrophs. 
• For example, Rhodospirillum rubrum can use succinate as an electron donor: Succinate = fumarate + 2e + 2H

2. Chemotrophs

• Those organisms that obtain their energy from the oxidation of chemical compounds (either organic or inorganic) are known as Chemotrophs.
• Examples of Chemotrophs are sulphur oxidizing proteobacteria (obtain energy from sulphur), neutrophilic iron-oxidizing bacteria etc.

Classification of Chemotrophs

The Chemotrophs are classified into the following groups;

a. Chemolithotrophs

• Among the chemotrophic bacteria are species that use inorganic compounds as their source of electrons, they are known as Chemolithotrophs.
• For example, bacteria of the genus Nitrosomonas use ammonia as their electron source, obtaining energy by oxidizing ammonia to nitrite: NH4(+) + ½ O2 + H2O = NO2(-) + 2H3O(-) 
• This reaction involves a net transfer of 6 electrons, causing a valence change of the nitrogen atom from – 3 to +3.

b. Chemoorganotrophs

• Many other chemotrophic bacteria use organic compounds, such as sugars and amino acids, as electron donors and are therefore chemoorganotrophs.
• Certain bacteria can grow as either chemolithotrophs or chemoorganotrophs. For example, Pseudomonas pseudoflava can use either the organic compound glucose or the inorganic compound H2 as its source of electrons: C6H12O6 + 6H2O = 6CO2  + 24H(+) +2e (-)

Classification of bacteria basis of Electron Source

Based on electron source microorganisms are classified into two groups such as;

Type	Source of Electron
Lithotrophs	Reduced inorganic molecules
Organotrophs	Organic molecules

1. Lithotrophs

• Those microorganisms use reduced inorganic substances as their electron source are known as Lithotrophs.
• Examples of lithotrophs include iron-oxidizing bacteria that metabolize reduced iron to oxidized iron, purple sulfur bacteria that transform sulfide into sulfur, nitrifying bacteria that use ammonia and convert it into nitrite or use nitrite to produce nitrate, hydrogen bacteria that oxidize hydrogen to water.

2. Organotrophs

• Those microorganisms extract electrons from reduced organic compounds are known as Organotrophs.
• Some examples of organotrophs are animals, bacteria, and heterotrophs. 
• They can be either anaerobic or aerobic.

Classification of bacteria basis of Carbon Source

Based on carbon source microorganisms are classified into two groups such as;

Type	Source of Carbon
Autotrophs	CO2 sole or principal biosynthetic carbon Source
Heterotrophs	Reduced, preformed, organic molecules from other organisms

1. Heterotrophs

• Organisms that use reduced, preformed organic molecules as their carbon source for growth are known as Heterotrophs.
• All the species of Heterotrophs cause diseases of human beings, other animals, and plants, as well as those that constitute the greater part of the population in our immediate environment. 
• Examples of heterotrophic bacteria are Agrobacterium, Xanthomonas, Pseudomonas, Salmonella, Escherichia, Rhizobium, etc.

Classification of Heterotrophs

The Heterotrophs are classified into the following groups;

a. Photoheterotrophs

• Photoheterotrophs use light energy but cannot use CO2 as their sole source of carbon.
• They get energy from organic compounds to full fill their carbon and electron requirements.
• Examples of Photoheterotrophs are; Purple non-sulphur bacteria such as Rhodospirillum, Rhodomicrobium, Rhodopseudomonas palustris.

b. Chemoheterotrophs

• They get both carbon and energy from organic compounds such as carbohydrates, lipids and proteins. Glucose or Monosaccharide [(CH2O)n] + O2 → CO2 + H2O + Energy
• There are three main categories that differ in how chemohetrotrophs obtain their organic nutrients, such as Saprophytic bacteria, Parasitic bacteria, and Symbiotic bacteria.
• Saprophytic bacteria obtain their food from the dead and organic decaying matter such as leaves, fruits, vegetables, meat, animal feces, leather, humus etc. They release enzymes to digest the food and absorb it. Some Examples are Bacillus mycoides, B. ramosus, Acetobacter etc.
• Parasitic bacteria, obtain their nutrition from the tissues of the hosts on which they grow. They cause various diseases in plants and animals are known as pathogens, e.g., Bacillus typhosus, B. anthracis, B.tetani. B.diplheriae, B.tuberculosis, B. pneumoniae, Vibrio cholerae, Pseudomonas citri etc.
• Symbiotic bacteria, live in close association with other organisms as symbionts. They are beneficial to the organisms. Some examples are the nitrogen-fixing bacteria, e.g., Bacillus radicicola, B. azotobacter, Rhizobium, Clostridium, Rhizobium spp., B. radicicolaand B. azotobacter.

2. Autotrophs

• Organisms that use carbon dioxide (CO2) as their sole or principal source of carbon are known as Autotrophs.
• Examples of Autotrophs are land plants (e.g. dicots, grass, gymnosperms, bryophytes, ferns, etc.), lichen, photosynthetic algae (e.g. chlorophytes, charophytes, dinoflagellates, diatoms, etc.).

Classification of Autotrophs

Autotrophs are classified into following groups;

• Photoautotrophs
• Chemoautotrophs

a. Photoautotrophs

• They get the energy from sunlight and transform it into chemical energy. During this process the CO2 is reduced to carbohydrates.
• Water acts as a hydrogen donor and the process leads to the formation of free oxygen.
• They contain Chlorophyll pigment in the cell which helps to capture sunlight e.g., Cyanobacteria.
• Some of them are anaerobes and have bacteriochlorophyll and bacteriovirdin pigments respectively.
• Examples of Photoautotrophs are; Purple Sulphur Bacteria, Green Sulphur Bacteria.
• Purple Sulphur Bacteria contain bacteriochlorophyll which is located on the intracytoplasmic membrane i.e., thylakoids. They get their energy from sulfur compounds e.g., Chromatiiun. Theopedia rosea, Thiospirilium.
• Green Sulphur Bacteria use hydrogen sulfide (H2S) as hydrogen donor. The reaction occurs in the presence of light and pigment termed as bacteriovirdin or bacteriopheophytin or chlorobium chlorophyll e.g., Chlorobium limicola, Chlorobacterium etc. They take hydrogen from inorganic sources like sulphides and thiosulphates.

b. Chemoautotrophs

• Chemoautotrophs don’t require any light source(lack the light phase but have the dark phase of photosynthesis) and pigment for their nutrition.
• They oxidize certain inorganic substances by using atmospheric oxygen.
• Examples of Chemoautotrophs are; Sulphomonas (Sulphur bacteria), Hydromonas (Hydrogen bacteria), Ferromonas (Iron bacteria), Methanomonas (Methane bacteria), Nitrosomonas (Nitrifying bacteria).
• Sulphomonas obtain energy by oxidation of elemental sulphur or H2S, e.g., Thiobacillus, Beggiatoa.
• Hydromonas convert hydrogen into water, e.g., Bacillus pantotrophus, Hydrogenomonas. 2H2 + O2 → 2H2O + 55 kcal, 4H2 + CO2 → 2H2O + CH4 + Energy
• Ferromonas  inhabit water and obtain energy by oxidation of ferrous compounds into ferric forms. e.g., Thiobacillus ferroxidans, Ferro bacillus, Leptothrix. 4FeCo3 + 6H2O + O2 → 4Fe (OH)3 + 4CO2 + 81 kcal.
• Methanomonas get their energy by oxidation of methane into water and carbon dioxide.
• Nitrosomonas get their energy by oxidation of ammonia and nitrogen compounds into nitrates. Nitrosomonas oxidises NH3 to nitrites. (NH3 + ½O2 = H2O + HNO2 + Energy) Then, Nitrobacter converts nitrites to nitrates. (NO2 + ½O2 = NO2 + Energy)

Other Types of Bacteria

Mixotrophs

• A few chemolithotrophs are now recognized as being chemolithotrophic heterotrophs (mixotrophs): i.e., they obtain energy by utilizing inorganic electron donors, but obtain most of their carbon from organic compounds.
• One such organism is Desulfovibrio desulfuricans, which uses electrons from H2 for the reduction of sulfate, yet derives most of its carbon from organic compounds in the culture medium.

Facultative autotrophs

• Some autotrophs are facultative autotrophs: i.e., they can either live as autotrophs, deriving their carbon from CO2, or they can live as heterotrophs, deriving their carbon from organic compounds.
• For example, P. pseudoflava can live as a heterotroph, using glucose as a source of carbon for assimilation (and also as its source of electrons, as mentioned above): however, if H2 is provided as the electron source, then it can use CO2 as its sole carbon source and can grow as an autotroph.

Major Nutritional Types of Microorganisms

By combining the roots of these terms, most organisms can be placed in one of five nutritional types based on their primary sources of carbon, energy, and electrons.

1. Photolithoautotrophs

• They are often known as photoautotrophs, including photosynthetic protists (algae) and cyanobacteria.
• These microorganisms are plantlike in that they employ water as the electron donor and release oxygen.
• Other photolithoautotrophs, such as the purple sulfur bacteria and the green sulfur bacteria, cannot oxidize water but extract electrons from inorganic donors such as hydrogen, hydrogen sulfide, and elemental sulfur.
• Examples of Photolithoautotrophs are purple sulfur bacteria and the green sulfur bacteria, diatoms, cyanobacteria.

2. Chemolithoautotrophs

• They oxidize reduced inorganic compounds such as iron-, nitrogen-, or sulfur containing molecules to derive both energy and electrons for biosynthesis, using carbon dioxide as the carbon source.
• Chemolithoautotrophs contribute greatly to the chemical transformations of elements (e.g., the conversion of ammonia to nitrate or sulfur to sulfate) that continually occur in ecosystems. 
• Photoautotrophs and chemolithoautotrophs also are important primary producers in ecosystems. 
• They fix CO2, making reduced organic molecules that sustain the chemoorganoheterotrophs that share their habitats.
• Example of Chemolithoautotrophs are; Sulfur-oxidizing bacteria, hydrogen-oxidizing bacteria, methanogens, nitrifying bacteria, iron-oxidizing bacteria.

3. Chemoorganoheterotrophs

• Chemoorganoheterotrophs are sometimes known as chemoheterotrophs or chemoorganotrophs.
• They use reduced organic compounds as sources of energy, electrons, and carbon. 
• Many chemoorganotrophs are used industrially to make foods (e.g., yogurt, pickles, cheese), medical products (e.g., antibiotics), and beverages (e.g., beer and wine). Nearly all pathogenic microorganisms are chemoorganoheterotrophs.
• Example of Chemoorganoheterotrophs: Most non photosynthetic microbes, including most pathogens, fungi and many protists.

4. Photoorganoheterotrophs

• Some phototrophic bacteria (e.g., purple nonsulfur and green bacteria) use organic matter as their electron donor and carbon source. 
• These photoorganoheterotrophs are common inhabitants of polluted lakes and streams.
• Examples of Photoorganoheterotrophs are; purple nonsulfur bacteria, green nonsulfur bacteria.

5. Chemolithoheterotrophs

• Chemolithoheterotrophs use reduced inorganic molecules as their energy and electron source but derive their carbon from organic sources.
• Example of Chemolithoheterotrophs; some sulfur oxidizing bacteria.

Nutritional Type	Source of Nutrition	Example
Photolithoautotroph	Carbon Source: CO2; Energy Source: Light; Electron Source: Inorganic electron donor	Purple and green sulfur bacteria, cyanobacteria, diatoms
Photoorganoheterotroph	Carbon Source: Organic carbon; Energy Source: Light; Electron Source: Organic electron donor	Purple nonsulfur bacteria, green nonsulfur bacteria
Chemolithoautotroph	Carbon Source: Co2; Energy Source: Inorganic chemicals; Electron Source: Inorganic electron donor	Sulfur-oxidizing bacteria, hydrogenoxidizing bacteria, methanogens, nitrifying bacteria, iron-oxidizing bacteria
Chemolithoheterotroph	Carbon Source: Organic carbon; Energy Source: Inorganic chemicals; Electron Source: Inorganic electron donor	Some sulfur-oxidizing bacteria le.g.. Beggiatod spp.)
Chemoorganoheterotroph	Carbon Source: Organic carbon; Energy Source: Organic chemicals, often same as C source; Electron Source: Organic electron donor, often same as C source;	Most nonphotosynthetic microbes, including most pathogens, fungi, and many protists

References

• Microbiology by Michael J. Pelczar, JR., E.C.S. Chan, Noel R. Krieg
• Prescott’s Microbiology
• https://en.wikipedia.org/wiki/Organotroph
• https://link.springer.com/referenceworkentry/10.1007%2F978-3-642-11274-4_903
• https://www.javatpoint.com/phototrophs-vs-chemotrophs
• https://www.biologyonline.com/dictionary/autotroph