‘An army marches on its stomach’ said Napolean Bonapart.This indicates that food is important for any living organism and so also for microbes. Food is any substrate that can be metabolized to provide assimilable material or energy for the cell. Plants synthesize their own food requirements through photosynthesis. Animals ingest the presynthesized food from plants or by devouring other animals. All living organisms, from micro to macroorganisms require nutrients for growth and normal functioning. Animals ingest the food and digest them in their digestive system (Holozoic nutrition) to simpler nutrients which are absorbed by cells for synthesis of all cellular material and derive energy. Plants absorb the nutrients from soil solution (Holophytic nutrition) released by mineralisation of organic matter and grow. Microorganisms particularly fungi derive their nutrients from the extra corporeal digestion by secreting extracellular enzymes. The nutrients are absorbed and cellular materials are synthesized.
All organisms exhibit two universal requirements viz., water and elements. All organisms require energy which they derive from the chemical compounds or radiant energy like light. The elemental components are carbon, nitrogen, phosphorus, sulphur and potassium besides hydrogen and oxygen major ones for synthesizing cellular components. Metal ions like K, Ca, Mg and Fe are required for normal growth. Other metal ions like Zn, Cu, Mn, Mo , Ni, B, Co are often required in low quantities hence known as trace elements. Fe, Mg, Zn, Mo, Mn and Cu are cofactors/coenzymes or prosthetic group of various enzymes. Most bacteria do not require Na but certain marine bacteria, cyanobacteria and photosynthetic bacteria require it. Red extreme halophiles can-not grow with less than 12 to 15% NaCl which is required to maintain the integrity of cell walls and for the stability and activity of certain enzymes. Silicon is required for the growth of diatoms. Vitamins and vitamin like compounds are also present in living cells. These function either as coenzymes or as building blocks of coenzymes. Some bacteria synthesize their entire requirements of vitamins but some cannot grow unless supplied from external source.
Microorganisms are divided into several types based on the energy source or electron source and carbon assimilation. Those derive energy from the oxidation of chemical compounds are known as ‘chemotrophs’ and others utilizing radiant energy like light are known as ‘phototrophs’. Electrons are required for metabolism and based on the source from which bacteria derive electron they are grouped. Some organisms use reduced inorganic com pounds as electron donors and are termed as ‘lithotrophs’ literally meaning rock eating. Others use organic compounds are termed as ‘organotrophs’. Those organisms that derive energy from the chemical compounds (Chemotrophs) and uses inorganic compounds as e- donors (lithotrophs) are known as chemolithotrophs. Those that derive energy from light (phototrophs) and e- from inorganic compounds are photolithotrophs. Similarly those chemotrophs that use organic compounds, as e – donors are chemoorganotrophs and the phototrophs that utilize organic compounds as e- donors are photoorganotrophs. Chromatium okenii, a photosynthetic bacterium, uses radiant energy and H2S as electron donor oxidizing it to elemental sulphur. Some phototrophs use organic compounds such as fatty acids and alcohols as electron donors and hence called photoorganotrophs.
Rhodospirillum rubrum another phototrophic bacterium utilizes succinate as e- donor converting it to fumarate. A phototrophic bacterium can grow as chemotroph. In the anoxygenic environment (absence of O2) this bacterium grow as photoorganotroph but in the presence of oxygen and dark (absence of light) it grows as a chemoorganotroph. Among the chemotrophs some utilize inorganic compounds like NH 4 as e – donors and hence called chemolithotrophs. Nitrosomonas use ammonia for electrons and derive energy by oxidizing ammonia to nitrite. Certain chemotrophs use organic compounds like sugars and amino acids as e- donor and are called chemoorganotrophs. Some of the chemotrophs can grow either as chemolithotrophs or chemoorganotrophs. Pseudomonas pseudofulva can use glucose an organic compound (chemoorganotrophs) or inorganic compound H2 as e- source (chemolithotrophs)