The following points highlight the five main types of autotrophic and heterotrophic nutrition in bacteria. The types are: 1. Photoautotrophic Bacteria 2. Chemoautotrophic Bacteria 3. Saprophytic Bacteria 4. Symbiotic Bacteria 5. Parasitic Bacteria.
Type # 1. Photoautotrophic Bacteria:
The bacteria possess photosynthetic pigments of two types, bacteriochlorophyll and bacteriophaeophytin (chlorobium chlorophyll). The two types of pigments respectively occur in purple bacteria (e.g., Thiopedia rosea, Rhodopseudomonas palustris) and green sulphur bacteria (e.g., Chlorobium limicola).
The pigments occur in the membranes of thylakoids. The photosynthetic bacteria are anaerobic. No oxygen is evolved in bacterial photosynthesis. Such type of photosynthesis is known as an oxygenic photosynthesis.
Water is not used as a source of reducing power. Instead, hydrogen is obtained either directly (some purple bacteria) or from various types of inorganic and organic compounds, e.g., H2S (green bacteria), aliphatic compounds (purple non-sulphur bacteria).
Hydrogen from the above mentioned sources is picked up by NAD+. In photochemical phase the photosynthetic pigments especially the bacteriochlorophyll synthesizes ATP. The energy and the reducing power are used in the synthesis of organic compounds.
Because the photoautotrophic bacteria do not require oxygen, these monerans live comfortably near the bottoms of ponds and lakes where reduced sulphur and other compounds are freely available and the oxygen content is very low.
Type # 2. Chemoautotrophic Bacteria:
They are bacteria which are able to manufacture their organic food from inorganic raw materials with the help of energy derived from exergonic chemical reactions involving oxidation of an inorganic substance present in the external medium.
The chemical energy obtained from oxidation reaction is trapped in ATP molecules. This energy is then used in carbon assimilation with the help of hydrogen brought from sources other than water. There are several types of chemoautotrophic bacteria but the well known examples are nitrifying bacteria, sulphur oxidizing bacteria, and iron bacteria.
(i) Nitrifying Bacteria:
Nitrosomonas and Nitrosococcus obtain energy by oxidizing ammonia to nitrite.
NH+4 + 2O2 → NO–2 + 2H2O + Energy
Nitrocystis and Nitrobacteroxidise nitrites to nitrates.
2 NO–2 + O2 → 2NO-3 + Energy
(ii) Sulphur Oxidising Bacteria:
Beggiatoa, a colourless sulphur bacterium, oxidises hydrogen sulphide to sulphur in order to obtain energy for chemosynthesis.
Thiobacillus thioxidans, another sulphur bacterium, oxidises sulphur to sulphate state.
(iii) Iron Bacteria:
Ferro bacillus ferro-oxidans obtains energy by oxidising ferrous compounds to ferric forms.
4FeCO3 + 6H2O + O2 → 4Fe (OH) 3 + 4CO2 + Energy
(iv) Other Chemosynthetic Bacteria:
Bacterium Methanomonas oxidises methane into CO2 and H2O.
CH4 + 2O2 → CO2 + 2H2O+ Energy
Carbon monoxide bacterium, Carboxydomonas, oxidises carbon monoxide to carbon dioxide for obtaining energy.
2CO+O2 → 2CO2 + Energy
There are a number of hydrogen bacteria which oxidise hydrogen (e.g., Thiothrix) with the help of either oxygen or oxidized salts for getting energy required in chemosynthesis.
2H2 + O2 → 2H2O + Energy
4H2 + H2SO4 → H2S + 4H2O + Energy
4H2 + CO2→ CH4 + 2H2O + Energy.
Type # 3. Saprophytic Bacteria:
They are free living bacteria which obtain their food from organic remains, e.g., corpses, animal excreta, fallen leaves, vegetables, fruits, meat, jams, jellies, bread and other products of plant and animal origin. Anaerobic breakdown of carbohydrates is often known as fermentation. Anaerobic breakdown of proteins is known as putrefaction. Aerobic breakdown of organic compounds is known as decay.
Saprophytic bacteria are both useful and harmful to human interests. The harmful activities include spoilage of food stuffs, food poisoning, de-nitrification of soils, desulphurification of soils and deterioration of a number of household articles like fibers, leather and wooden articles.
Saprophytic bacteria have been employed in several economically useful activities. In nature they along with fungi, are the decomposers of organic remains. This activity not only disposes off the dead bodies and organic wastes but also releases raw materials for reutilization.
Therefore, saprophytic bacteria are called nature’s scavengers. Saprophytic bacteria take part in disposal of sewage, production of manure, ensilage, preparation of curd and cheese, retting of fibres, cleaning of hides, curing of tea, tobacco, coffee and cocoa, industrial synthesis of alcohols and organic acids, vitamins, enzymes, antibiotics, etc.
The latest use of this type of bacteria is in the biodegradation of organic pollutants like petroleum spillage, e.g., Pseudomonas.
Type # 4. Symbiotic Bacteria:
These bacteria live in mutually beneficial association with other organisms. Enteric bacterium Escherichia coli, lives as a commensal in human intestine. It feeds on undigested matter, checks the growth of putrefying bacteria and produces large quantities of several В vitamins as well as vitamin K. Another common symbiotic bacterium is Rhizobium.
It has several species which form association with modulated roots of different legumes. The bacterium obtains shelter and food from the legume. It performs nitrogen fixation inside the nodules (not in the free state). There are several other examples of nitrogen fixing symbiotic bacteria, e.g., Frankie, Xanthomonas.
Type # 5. Parasitic Bacteria:
The bacteria live in contact with other living beings for obtaining nourishment or special organic compounds required for growth (growth factors). Parasitic bacteria may or may not cause disease. Disease producing forms are called pathogenic bacteria.
The disease is produced either due to breakdown of the host cells or due to liberation of toxins. Toxins are further of two types, endotoxins (e.g., Vibrio cholera— vibrio bacteria) and exotoxins (e.g., Clostridium tetani—bacillus bacteria).
Pathogenic bacteria produce 90% of the human diseases. In plants the bacterial diseases rank second to fungal diseases. Two common bacterial diseases of plants are crown galls (Agrobacterium tumefaciens) and Citrus canker (Xanthomonas citri).
Some examples of human diseases caused by bacteria are: