Bacterial Nutrition is classified into two categories -
(A) Autotrophic bacteria or "self feeders"
(B) Heterotrophic bacteria or "other feeders"
(A) Autotrophic bacteria
Autotrophic bacteria are capable of synthesizing their food from organic or inorganic substances are called autotrophic. The autotrophic further divided into two subgroups based on their source of energy -
(1) Photoautotrophic bacteria
(2) Chemoautotrophic bacteria
(1) Photoautotrophic bacteria
(2) Chemoautotrophic bacteria
(1) Photosynthetic bacteria - Also known as photoautotrophic or photolithotrophic bacteria. Like higher plants, they capable of converting radiant energy into chemical energy. They use light as a source of energy to produce their own food. Photosynthetic bacteria possess bacteriochlorophyll. In the presence of sunlight they can produce carbohydrates. using CO2 as the source of carbon, electron donors are H2S, elemental S and other sulphur containing inbrgani compounds. Bacterial photosynthesis is an anaerobic process not involving the photolysis of water with the evolution of oxygen (O2). This process is called anoxygenic (without producing oxygen) photosynthesis.
Photosynthetic bacteria may be classified in the following two main types -
(a) Green sulphur bacteria - These are small non-motile, rod-shaped bacteria which are strictly anaerobic photoautotrophs. Their photosynthetic pigment is chlorobium chlorophyll located in the invaginations of cytoplasmic membrane. These bacteria use H2S or other reduced inorganic sulphur compounds as electron donor. Elemental sulphur formed by product in this process is deposited extracellularly. Chlorobium, Prosthecochloris, Pelodictyon, and Clathrochloris are four well known examples of green sulphur bacteria.
6CO2 + 12H₂S →C6H12O6+ 6H2O + 12S
(b) Purple Ssulphur bacteria - These are also autotrophs and their photosynthetic pigments are bacteriochlorophyll a and b. In this case, in place of HS and elemental sulphur, the electron donors are some other sulphur containing organic or inorganic compounds but the sulphur is not the byproduct, though light is utilized, example Chromatium.
6CO₂ + 15H2O + 3Na2S2O3 → in presence of light and Bacteriochlorophyll → C6H12O6 + 6H₂O + 6NaHSO4
(c) Non-Sulphur Bacteria - These are motile bacteria found in mud and stagnant water. These also have bacteriochlorophyll as photosynthetic pigment. In their metabolism organic compounds are utilized as hydrogen donor which replace sulphur. Thus sulphur is never accumulated within the cell. The extent of reduction depends upon the organic substrate. For example -
2C3H6O2 + H2O + CO₂ → 7(CH2O) Carbohydrate
(Propionic acid)
C4H6O2 → 3(CH2O) + CO₂ (Carbohydrate)
(Malic acid)
(2) Chemosynthetic bacteria - These bacteria are without photosynthetic pigments. For the synthesis of their own organic food, (carbohydrates), they obtain carbon from CO2 of the atmosphere necessary energy from oxidation of inorganic or organic compounds. such as hydrogen sulphide (H2S), ferrous compounds (Fe²+), molecular (H2), ammonia (NH3) and nitrites (NO2-).
They are following types-
(a) Sulphur bacteria - These bacteria occur in sulphur containing terrestrial and aquatic environments. They derive energy by the oxidation of reduced sulphur compounds. The sulphur bacterium Beggiatoa, oxidises hydrogen sulphide to elemental sulphur.
H2S + 1/2 O2 → S + H2O
Another sulphur bacterium, Thiobacillus, thiooxidans, utilizes free sulphur and product sulphuric acid.
S + H2O + 1/2 O2 → H2SO4 + Energy
(b) Hydrogen bacteria (methanogens) - These chemosynthetic bacteria produce methane (CH4) and water from hydrogen gas and CO2 and liberating energy for the manufacture of their organic food. Example- Hydrogenomonas.
H2 + 1/2 O2 → 2H2O + energy
(c) Iron bacteria - The iron bacteria live in the stream that run over iron containing rocks and derive their energy from the oxidation of bivalent iron salts of ferrous compound oxidize into ferric form and released. e.g., Ferrobacillus.
2Fe(HCO3)2 + 1/2 O2 + H2O → 2Fe(OH)3 + 4CO2 + energy
Ferrous bicarbonate
(d) Nitrifying bacteria - The nitrifying bacteria are found in the soil. Through metabolic activities they convert ammonia (NH3) or ammonium ion (NH4+), which is formed by the break down of animal and plant proteins. During decay to nitrates (NO3-). Formation of NO3- from NH3 or NH4+ is called nitrification. This process in two step:
(i) First step Nitrosomonas is most common organism found in soil that oxidizes ammonia into nitrite ion. (-NO₂-).
2NH3 + 3O2 → 2NO2- + 2H+ + 2H2O + E
(i) Second step Nitrobacter oxidizes nitrite to nitrates ion (-NO3-).
2NO2- + O2 → 2NO₂- + Energy
Nitrition Nitration
(B) Heterotrophic bacteria
(d) Nitrifying bacteria - The nitrifying bacteria are found in the soil. Through metabolic activities they convert ammonia (NH3) or ammonium ion (NH4+), which is formed by the break down of animal and plant proteins. During decay to nitrates (NO3-). Formation of NO3- from NH3 or NH4+ is called nitrification. This process in two step:
(i) First step Nitrosomonas is most common organism found in soil that oxidizes ammonia into nitrite ion. (-NO₂-).
2NH3 + 3O2 → 2NO2- + 2H+ + 2H2O + E
(i) Second step Nitrobacter oxidizes nitrite to nitrates ion (-NO3-).
2NO2- + O2 → 2NO₂- + Energy
Nitrition Nitration
(B) Heterotrophic bacteria
Heterotrophic bacteria obtain readymade food from any organic source. They can be distinguished into three major nutritional categories.
1. Parasitic bacteria - Parasitic bacteria feed on living organisms are called as parasitic bacteria. The organism from which parasitic bacteria obtain their food is known as host. Some parasitic bacteria are known to cause diseases in plants (example- citrus canker) and animals (example- pneumonia, typhoid), and such bacteria are called pathogenic bacteria. Example- Xanthomonas citri, Salmonella typhii, etc.
2. Saprophytic bacteria - Saprophytic bacteria grow on dead and decaying organic matter. They obtain their food by decomposing complex organic molecules into simple inorganic constituents. The decomposition of carbohydrates and proteins by saprophytic bacteria is technically known as fermentation and putrifaction;
1. Parasitic bacteria - Parasitic bacteria feed on living organisms are called as parasitic bacteria. The organism from which parasitic bacteria obtain their food is known as host. Some parasitic bacteria are known to cause diseases in plants (example- citrus canker) and animals (example- pneumonia, typhoid), and such bacteria are called pathogenic bacteria. Example- Xanthomonas citri, Salmonella typhii, etc.
2. Saprophytic bacteria - Saprophytic bacteria grow on dead and decaying organic matter. They obtain their food by decomposing complex organic molecules into simple inorganic constituents. The decomposition of carbohydrates and proteins by saprophytic bacteria is technically known as fermentation and putrifaction;
- Fermentation - Breakdown of sugars by yeast is a common example of fermentation. In this process CO2 is released. Similarly, lactic acid fermentation of milk is carried out by lactic acid bacteria (Lactobacillus).
- Putrifaction - In putrifaction reduction of protein to peptone, polypeptide, peptides and amino acids takes place by enzymes secreted by anaerobic bacteria. This process does not require oxygen, but many saprophytic bacteria which breakdown amino acids into methane, CO2, NH3, H2 and N2 do require oxygen.
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