Nutrition is a substance required in biosynthesis and energy production and for microbial growth.All organisms require a Source of energy. On the basis of energy source organisms are designated as:
1. Chemotrophs: The organism that relies on chemical compounds for their energy
2. Phototrophs: The organisms which can utilize light as an energy source are known as phototrophs.

All organisms require a Source of electron for their metabolism. On the basis of electron source organisms are designated as:

1. Lithotrophs: Some organisms can use reduced organic compounds as electron donors and are termed as Lithotrophs
They can be Chemolithotrophs and Photolithotrophs
2. Organotrophs: Some organisms can use organic compounds as electron donors and are termed as organotrophs.
Some can be Chemoorganotrophs and Photoorganotrophs.

All organisms require carbon in some form for use in synthesizing cell components. All organisms require at least 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).

(A) Autotrophic bacteria:
These bacteria synthesize all their food from inorganic substances (H2O, C02, H2S salts). The autotrophic bacteria are of two types:
(i) Photoautotrophs
(ii) Chemoautotrophs

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.

This 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.

(a) Sulphomonas (Sulphur bacteria): These bacteria obtain energy by oxidation of elemental sulphur or H2S, e.g., Thiobadllus, 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
Sulphuric acid

(b) Hydromonas (Hydrogen bacteria): These convert hydrogen into water, e.g., Bacillus pantotrophus, Hydrogenomonas.
2H2 + O2 → 2H2O + 55 kcal.
4H2 + CO2 → 2H2O + CH4 + Energy

(c) 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.

(d) Mcthanomonas (Methane bacteria): These bacteria get their energy by oxidation of methane into water and carbon dioxide.

(e) 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

(f) Carbon Bacteria:
These bacteria oxidizes CO into CO2 e.g., Bacillus oligocarbophillous, Oligotropha carboxydovorans
2CO + O2 → 2CO2 + Energy

(B) Heterotrophic bacteria:
The heterotrophic bacteria obtain their-ready made food from organic substances, living or dead. These 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 bacteia e.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 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, Ctostridium etc. Rhizobium spp.,B. radicicola and 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.