Classification of Bacteria on the basis of Nutrition

• Nutrition is substances used in biosynthesis and energy production and therefore are required for all living things.
• Bacteria, like all living cells, require energy and nutrients to build proteins and structural membranes and drive biochemical processes.
• Bacteria require sources of carbon, nitrogen, phosphorous, iron and a large number of other molecules.
• Carbon, nitrogen, and water are used in the highest quantities.
• The nutritional requirements for bacteria can be grouped according to the carbon source and the energy source.
• Some types of bacteria must consume pre-formed organic molecules to obtain energy, while other bacteria can generate their own energy from inorganic sources.

Nutritional Types of Bacteria

On the basis of energy source organisms are designated as:

Phototrophs:

• The organisms which can utilize light as an energy source are known as phototrophs. These bacteria gain energy from light.

Chemotrophs:

• These bacteria gain energy from chemical compounds. They cannot carry out photosynthesis.

On the basis of electron source organisms are designated as:

Lithotrophs: 

• Some organisms can use reduced organic compounds as electron donors and are termed as Lithotrophs.
• They can be Chemolithotrophs and Photolithotrophs

Organotrophs: 

• Some organisms can use organic compounds as electron donors and are termed as organotrophs.
• Some can be Chemoorganotrophs and Photoorganotrophs.

Thus, bacteria may be either:

• Photo-lithotrops: These bacteria gain energy from light and use reduced inorganic compounds such as H₂S as a source of electrons. eg: Chromatium okeinii.
• Photo-organotrophs: These bacteria gain energy from light an d use organic compounds such as Succinate as a source of electrons.eg; Rhodospirillum.
• Chemo-lithotrophs: These bacteria gain energy from reduced inorganic compounds such as NH3 as a source of electron eg; Nitrosomonas.
• Chemo-organotrophs: These bacteria gain energy from organic compounds such as glucose and ammino acids as a source of electrons.eg; Pseudomonas pseudoflora.
• Some bacteria can live ether chemo-lithotrophs or chemo-organotrophs like Pseudomonas pseudoflora as they can use either glucose or H2S as electron source.

On the basis of carbon source bacteria may be:

• All organisms require carbon in some form for use in synthesizing cell components.
• All organisms require at least a small amount of CO2.
• However, some can use CO2 as their major or even sole source of carbon; such organisms are termed as Autotrophs (Autotrophic bacteria).
• Others require organic compounds as their carbon source and are known as Heterotrophs (Heterotrophic bacteria).

Autotrophic Bacteria

These bacteria synthesize all their food from inorganic substances (H2O, C02, H2S salts).

The autotrophic bacteria are of two types:

(i)  Photoautotrophs

• These bacteria capture the energy of sunlight and transform it into the chemical energy.
• In this process, CO2 is reduced to carbohydrates.
• The hydrogen donor is water and the process produce free oxygen.
• Photoautotroph has Chlorophyll pigment in the cell and its main function is to capture sunlight e.g., Cyanobacteria.
• Some photoautotrophic bacteria are anaerobes and have bacteriochlorophyll and bacteriovirdin pigments respectively.

Purple Sulphur Bacteria:

These bacteria have the pigment bacteriochlorophyll located on the intracytoplasmic membrane i.e., thylakoids. These bacteria obtain energy from sulfur compounds e.g., Chromatiiun. Theopedia rosea, Thiospirilium.

Green Sulphur Bacteria:

These bacteria use hydrogen sulfide (H2S) as hydrogen donor. The reaction takes place in the presence of light and pigment termed as bacteriovirdin or bacteriopheophytin or chlorobium chlorophyll e.g., Chlorobium limicola, Chlorobacterium etc.

These bacteria take hydrogen from inorganic sources like sulphides and thiosulphates. Therefore, these bacteria are also known as photolithographs.

(ii) Chemoautotrophs

• These bacteria do not require light (lack the light phase but have the dark phase of photosynthesis) and pigment for their nutrition.
• These bacteria oxidize certain inorganic substances with the help of atmospheric oxygen.
• This reaction releases the energy (exothermic) which is used to drive the synthetic processes of the cell.

Sulphomonas (Sulphur bacteria):

These bacteria obtain energy by oxidation of elemental sulphur or H2S, e.g., Thiobacillus, Beggiatoa.

• Elemental Sulphur Oxidising Bacteria: Denitrifying sulphur bacteria oxidize elemental sulphur to sulphuric acid e.g., Thiobacillus denitrificans
  2S + 2H2O + 3O2 → 2H2SO4 + 126 kcal.
• Sulphide Oxidizing Bacteria: These bacteria oxidizes H2S and release the sulphur e.g., Beggiatoa.
  2H2S +4O2 → 2H2O + 2S + 141.8 cal

Hydromonas (Hydrogen bacteria)

• These convert hydrogen into water, e.g., Bacillus pantotrophus, Hydrogenomonas.
  2H2 + O2 → 2H2O + 55 kcal.
  4H2 + CO2 → 2H2O + CH4 + Energy

Ferromonas (Iron bacteria):

• These bacteria 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 (Methane bacteria):

• These bacteria get their energy by oxidation of methane into water and carbon dioxide.

Nitrosomonas (Nitrifying bacteria):

• These bacteria get their energy by oxidation of ammonia and nitrogen compounds into nitrates.
• Nitrosomonas oxidises NH3 to nitrites. NH3 + ½O2 ® H2O + HNO2 + Energy
• Nitrobacter converts nitrites to nitrates. NO2 + ½O2 ® NO2 + Energy

Carbon Bacteria:

These bacteria oxidizes CO into CO2 e.g., Bacillus oligocarbophillous, Oligotropha carboxydovorans
2CO + O2 → 2CO2 + Energy

Heterotrophic Bacteria

• The heterotrophic bacteria obtain their-ready made food from organic substances, living or dead.
• Most of pathogenic bacteria of human beings, other plants and animals are heterotrophs.
• Some heterotrops have simple nutritional requirement while some of them require large amount of vitamin and other growth promoting substance. Such organisms are called fastidious heterotrophs.
• Heterotrophic bacteria are of three types:

a. Photoheterotrophs

• These bacteria can utilize light energy but cannot use CO2 as their sole source of carbon.
• They obtain energy from organic compounds to satisfy their carbon and electron requirements. Bacteriochlorophyll pigment is found in these bacteria.
• g., Purple non-sulphur bacteria (Rhodospirillum, Rhodomicrobium, Rhodopseudomonas palustris).

b. Chemoheterotrophs

• Chemoheterotrophs obtain 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:(i) Saprophytic bacteria.
(ii) Parasitic bacteria.
(iii) Symbiotic bacteria.

i) Saprophytic bacteria

• Saprophytic bacteria obtain their food from the dead and organic decaying matter such as leaves, fruits, vegetables, meat, animal feces, leather, humus etc.
• These bacteria secrete enzymes to digest the food and absorb it.
• The enzymes secreted to break down the complex compounds such as carbohydrate and protein, into simpler soluble compounds, which are easily absorbed.
• Examples are Bacillus mycoides, B. ramosus, Acetobacter etc.

ii) Parasitic bacteria

• These bacteria obtain their nutrition from the tissues of the hosts on which they grow.
• They may be harmless or may cause serious diseases.
• Parasitic bacteria which 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.

iii) Symbiotic bacteria

• Symbiotic bacteria live in close association with other organisms as symbionts.
• They are beneficial to the organisms.
• The common examples are the nitrogen-fixing bacteria, e.g., Bacillus radicicola, B. azotobacter, Rhizobium, Clostridium, Rhizobium spp., B. radicicolaand B. azotobacter.
• These bacteria live inside the roots of leguminous plants.
• These bacteria fix free atmospheric nitrogen into nitrogenous compounds which are utilized by the plants. In return, the plant provides nutrients and protection to the bacteria.

References

1. https://orbitbiotech.com/nutrition-bacteria-autotrophs-phototrophs-chemotrophs-lithotrophs-heterotrophs/
2. https://www.kullabs.com/classes/subjects/units/lessons/notes/note-detail/3363
3. http://www.biologydiscussion.com/bacteria/nutrition-in-bacteria-with-diagram-microbiology/63897
4. Greemwood, R. S. (2002). Medical Microbiology. London: Churchill Livingstone.
5. J.Pelezar. (1993). Microbiology. Tata McGraw hill.
6. https://sciencing.com/nutritional-types-bacteria-2515.html