Table of Contents
Introduction to Classification of Bacteria on the basis of Nutrition
- Nutrients are substances used in biosynthesis and energy production and are therefore essential for all living organisms.
- Bacteria, like all living cells, require energy and nutrients to synthesize proteins, build structural membranes, and carry out biochemical processes.
- Bacteria need sources of carbon, nitrogen, phosphorus, iron, and many other molecules for growth and survival.
- Among these requirements, carbon, nitrogen, and water are needed in the greatest quantities.
- The nutritional requirements of bacteria are commonly classified according to their sources of carbon and energy.
- Some bacteria must obtain energy by consuming preformed organic molecules.
- Other bacteria are capable of generating their own energy from inorganic sources.
- Differences in carbon and energy sources form the basis for the nutritional classification of bacteria.
Nutritional Types of Bacteria
On the Basis of Energy Source, Organisms are Designated as:
Phototrophs
- Organisms that utilize light as their source of energy are called phototrophs.
- These bacteria obtain energy directly from light.
Chemotrophs
- Chemotrophs obtain energy from chemical compounds.
- They are unable to perform photosynthesis.
On the Basis of the Electron Source, Organisms are Designated as:
Lithotrophs
- Lithotrophs use reduced inorganic compounds as electron donors.
- These organisms obtain electrons from inorganic substances.
- Lithotrophs can be classified as:
- Chemolithotrophs
- Photolithotrophs
Organotrophs
- Organotrophs use organic compounds as electron donors.
- These organisms obtain electrons from organic substances.
- Organotrophs can be classified as:
- Chemoorganotrophs
- Photoorganotrophs
Thus, Bacteria May Be Either:
Photo-lithotrophs
- Gain energy from light.
- Use reduced inorganic compounds such as hydrogen sulfide (H₂S) as electron sources.
- Example: Chromatium okeinii
Photo-organotrophs
- Gain energy from light.
- Use organic compounds such as succinate as electron sources.
- Example: Rhodospirillum
Chemo-lithotrophs
- Gain energy from reduced inorganic compounds.
- Use inorganic compounds such as ammonia (NH₃) as electron sources.
- Example: Nitrosomonas
Chemo-organotrophs
- Gain energy from organic compounds.
- Use organic compounds such as glucose and amino acids as electron sources.
- Example: Pseudomonas pseudoflava
Metabolic Flexibility
- Some bacteria can live either as chemo-lithotrophs or chemo-organotrophs.
- For example, Pseudomonas pseudoflava can utilize either glucose or H₂S as an electron source depending on environmental conditions.
On the Basis of the Carbon Source, Bacteria May Be:
- All organisms require carbon in some form for the synthesis of cellular components.
- All organisms require at least a small amount of carbon dioxide (CO₂).
- Some organisms use CO₂ as their major or sole source of carbon.
Autotrophs (Autotrophic Bacteria)
- Use carbon dioxide (CO₂) as their primary or only carbon source.
- Synthesize organic compounds from inorganic carbon.
Heterotrophs (Heterotrophic Bacteria)
- Require organic compounds as their carbon source.
- Obtain carbon from preformed organic molecules produced by other organisms.
Autotrophic Bacteria
- Autotrophic bacteria synthesize all of their food from inorganic substances such as water (H₂O), carbon dioxide (CO₂), hydrogen sulfide (H₂S), and mineral salts.
- These bacteria are capable of producing organic compounds from inorganic materials.
The Autotrophic Bacteria are of Two Types:
(i) Photoautotrophs
- Photoautotrophic bacteria capture energy from sunlight and convert it into chemical energy.
- During this process, carbon dioxide (CO₂) is reduced to form carbohydrates.
- Water acts as the hydrogen donor in oxygenic photosynthesis.
- Free oxygen is produced as a by-product of the process.
- Photoautotrophs contain chlorophyll pigments that capture sunlight for photosynthesis.
- Example: Cyanobacteria
- Some photoautotrophic bacteria are anaerobic and contain bacteriochlorophyll and bacterioviridin pigments instead of chlorophyll.
Purple Sulphur Bacteria
- Purple sulphur bacteria contain the pigment bacteriochlorophyll.
- This pigment is located on intracytoplasmic membranes known as thylakoids.
- These bacteria obtain energy using sulfur compounds.
- Examples:
- Chromatium
- Thiopedia rosea
- Thiospirillum
Green Sulphur Bacteria
- Green sulphur bacteria use hydrogen sulfide (H₂S) as the hydrogen donor.
- Photosynthetic reactions occur in the presence of light and pigments such as:
- Bacterioviridin
- Bacteriopheophytin
- Chlorobium chlorophyll
- Examples:
- Chlorobium limicola
- Chlorobacterium
- These bacteria obtain hydrogen from inorganic compounds such as sulfides and thiosulfates.
- Because they use light as an energy source and inorganic compounds as electron donors, they are also known as photolithotrophs.
ii) Chemoautotrophs
- Chemoautotrophic bacteria do not require light for nutrition.
- They lack the light-dependent phase of photosynthesis but possess the dark phase, allowing them to synthesize organic compounds.
- These bacteria do not contain photosynthetic pigments.
- They obtain energy by oxidizing inorganic substances in the presence of atmospheric oxygen.
- The oxidation reactions are exothermic and release energy.
- The released energy is utilized to drive the biosynthetic and metabolic processes of the cell.
Sulphomonas (Sulphur Bacteria)
- Sulphur bacteria obtain energy by oxidizing elemental sulfur or hydrogen sulfide (H₂S).
- Examples:
- Thiobacillus
- Beggiatoa
Elemental Sulphur Oxidizing Bacteria
- Denitrifying sulfur bacteria oxidize elemental sulfur to sulfuric acid.
- Example:
- Thiobacillus denitrificans
- Reaction:
Sulphide Oxidizing Bacteria
- These bacteria oxidize hydrogen sulfide (H₂S) and release elemental sulfur.
- Example:
- Beggiatoa
- Reactions
2H₂S + 4O₂ → 2H₂O + 2S + 141.8 kcal
Hydromonas (Hydrogen Bacteria)
- Hydrogen bacteria obtain energy by oxidizing hydrogen to water.
- Examples:
- Bacillus pantotrophus
- Hydrogenomonas
- Reactions:
2H₂ + O₂ → 2H₂O + 55 kcal
4H₂ + CO₂ → 2H₂O + CH₄ + Energy
Ferromonas (Iron Bacteria)
- Iron bacteria are commonly found in aquatic environments.
- They obtain energy by oxidizing ferrous compounds into ferric compounds.
- Examples:
- Thiobacillus ferrooxidans
- Ferrobacillus
- Leptothrix
- Reaction:
- 4FeCO₃ + 6H₂O + O₂ → 4Fe(OH)₃ + 4CO₂ + 81 kcal
Methanomonas (Methane Bacteria)
- These bacteria obtain energy by oxidizing methane.
- The oxidation process converts methane into carbon dioxide and water.
- Nitrifying bacteria obtain energy by oxidizing ammonia and other nitrogen compounds into nitrates.
- Nitrosomonas oxidizes ammonia (NH₃) into nitrite (NO₂⁻).
- Reaction:
Nitrite Oxidation
- Nitrobacter converts nitrite into nitrate (NO₃⁻).
- Reaction:
NO₂⁻ + ½O₂ → NO₃⁻ + Energy
Carbon Bacteria
- Carbon bacteria obtain energy by oxidizing carbon monoxide (CO) into carbon dioxide (CO₂).
- Examples:
- Bacillus oligocarbophilus
- Oligotropha carboxidovorans
- Reaction:
2CO + O₂ → 2CO₂ + Energy
Heterotrophic Bacteria
- Heterotrophic bacteria obtain ready-made food from organic substances derived from living or dead organisms.
- Most pathogenic bacteria of humans, plants, and animals are heterotrophs.
- Some heterotrophs have simple nutritional requirements, whereas others require large amounts of vitamins and growth-promoting substances.
- Such bacteria with complex nutritional requirements are known as fastidious heterotrophs.
a. Photoheterotrophs
- Photoheterotrophic bacteria can utilize light as an energy source.
- They cannot use carbon dioxide (CO₂) as their sole source of carbon.
- They obtain carbon and electrons from organic compounds.
- These bacteria contain the pigment bacteriochlorophyll.
- Examples of purple non-sulphur bacteria include:
- Rhodospirillum
- Rhodomicrobium
- Rhodopseudomonas palustris
b. Chemoheterotrophs
- Chemoheterotrophs obtain both carbon and energy from organic compounds.
- Common organic nutrient sources include carbohydrates, lipids, and proteins.
General Reaction
- Glucose or monosaccharides [(CH₂O)â‚™] + O₂ → CO₂ + H₂O + Energy
There are Three Main Categories that Differ in How Chemoheterotrophs Obtain Their Organic Nutrients:
(i) Saprophytic Bacteria
- Saprophytic bacteria obtain food from dead and decaying organic matter.
- Common sources include:
- Dead leaves
- Fruits
- Vegetables
- Meat
- Animal feces
- Leather
- Humus
- These bacteria secrete enzymes onto the organic material to digest it externally.
- The enzymes break down complex compounds such as carbohydrates and proteins into simpler soluble compounds.
- The simpler compounds are then absorbed by the bacteria.
- Examples include:
- Bacillus mycoides
- Bacillus ramosus
- Acetobacter
(ii) Parasitic Bacteria
- Parasitic bacteria obtain nutrition from the tissues of their host organisms.
- They grow on or within living hosts.
- Some may be harmless, while others cause serious diseases.
- Disease-causing parasitic bacteria are known as pathogens.
- Examples include:
- Salmonella Typhi
- Bacillus anthracis
- Clostridium tetani
- Corynebacterium diphtheriae
- Mycobacterium tuberculosis
- Klebsiella pneumoniae
- Vibrio cholerae
- Pseudomonas citri
(iii) Symbiotic Bacteria
- Symbiotic bacteria live in close association with other organisms as symbionts.
- Both the bacteria and their host benefit from the relationship.
- Many symbiotic bacteria are involved in nitrogen fixation.
- Common examples include:
- Rhizobium
- Azotobacter
- Clostridium
- Bacillus radicicola
- These bacteria commonly inhabit the roots of leguminous plants.
- They convert free atmospheric nitrogen into nitrogenous compounds that can be utilized by plants.
- In return, the plants provide nutrients, shelter, and protection to the bacteria.
References
- Greenwood, R. S. (2002). Medical microbiology. Churchill Livingstone.
- Pelczar, M. J. (1993). Microbiology. Tata McGraw-Hill.
- Orbit Biotech. (n.d.). Nutrition in bacteria: Autotrophs, phototrophs, chemotrophs, lithotrophs, and heterotrophs. Retrieved from Orbit Biotech
- Kullabs. (n.d.). Nutrition in bacteria notes. Retrieved from Kullabs
- Biology Discussion. (n.d.). Nutrition in bacteria (with diagram) | Microbiology. Retrieved from Biology Discussion
- Sciencing. (n.d.). Nutritional types of bacteria. Retrieved from Sciencing
