As all living organisms, microorganisms also require a combination of various physical and chemical factors for their growth and multiplication. Although individual cells approximately double in size during their lifetime this change is not very significant. Microbial growth actually refers to increase in numbers of the cells. The requirements for microbial growth can be divided into two main categories: physical and chemical.
- Physical aspects include temperature, pH, and osmotic pressure.
- Chemical requirements macromolecules (carbon, nitrogen, hydrogen, sulfur, phosphorus, oxygen) and micro molecules (trace elements and organic growth factors as magnesium, potassium, sodium, calcium and iron in their ionised forms)
Temperature: Microorganisms are classified into three primary groups on the basis of their preferred range of temperature:
- psychrophiles (cold- loving microbes),
- mesophiles (moderate-temperature-loving microbes),
- thermophiles (heat-loving microbes).
pH: pH 6.5-7 (neutral range) is best suited for bacterial growth. However some bacteria can grow at an acidic pH below about pH 4, are called acidophiles. While some bacteria prefer an alkaline pH (8-9), are called alkalophiles. Molds and yeasts (fungus) require an optimum pH of about 5 to 6 for growth.
Osmotic Pressure: Microorganisms require water for growth as they obtain almost all their nutrients in solution from the surrounding water. Some organisms adapt to the extreme high salt concentrations are called extreme halophiles, while some microorganisms require an optimum salt concentration for their growth, are called as obligate halophiles. Some microorganisms do not require salt for growth but are able to grow in salt concentrations upto 2%, (a concentration that inhibits the growth of many other organisms). These are called facultative halophiles.
- Carbon forms the skeleton or backbone of all organic molecules and hence, is the central component of the biological macromolecules as it. Hydrogen is also an important molecule that participates in energy generation processes in most microorganisms. Oxygen is of central importance to the respiration of many microorganisms while nitrogen is needed for the synthesis of proteins and nucleic acids, as well as for important molecules such as ATP.
In addition to the need for carbon, hydrogen and oxygen, microorganisms also require sources of energy and electrons for growth to take place. They can be grouped into classes on the basis of their nutritional requirements.
- Autotrophs and Hetertrophs: Microorganisms can be classified as either heterotrophs or autotrophs with respect to their preferred source of carbon. Autotrophs can use \(CO_2\) as their sole or principal source of carbon. Organisms that use reduced, preformed organic molecules as carbon sources are heterotrophs
- Phototrophs and Chemotrophs: Phototrophs use light as their energy source, while chemotrophs obtain energy from the oxidation of chemical compounds (either or- ganic or inorganic). Microorganisms also have only two sources forelectrons.
- Lithotrophs and Organotrophs : Lithotrophs (i.e., “rock-eaters”) use reduced inorganic substances as their electron source, whereas organotrophs extract electrons from organic compounds.
All microorganisms come into one of four nutritional classes based on their primary sources of carbon, energy, and electrons, as demonstrated in the following table
|Major Nutritional types||Source of Energy, Hydrogen/Electron, Carbon||Examples|
|Inorganic hydrogen/electron donor||Purple and Green sulphur bacteria|
|Carbon source-\( CO_2\)||Cyanobacteria|
|Photoorganoheterotrophy||Light Energy||Purple nonsulphur bacteria|
|Organic hydrogen/electron donor||Green nonsulphur|
|Organic carbon source|
|Chemolithoautotrophy||Chemical (organic) energy source||Sulphur oxidising bacteria|
|Inorganic hydrogen/electron donor||Hydrogen bacteria|
|Carbon source-\( CO_2\)||Nitrifying, Iron oxidising bacteria|
|Chemoorganoheterotrophy||Chemical (organic) energy source||Protozoa|
|Organic hydrogen/electron donor||Fungi|
|Organic carbon source||Non photosynthetic bacteria|
Points to Remember:
- Microorganisms require optimum physical conditions and a combination of various chemical factors for their growth and multiplication.
- Microorganisms can be classified and grouped into 4 major categories on the basis of source of energy they utilize, hydrogen/electron donor used and source of carbon.
- Autotrophs use \( CO_2\) as their primary or sole carbon source; heterotrophs employ organic molecules.
- Phototrophs use light energy, and chemotrophs obtain energy from the oxidation of chemical compounds.
- Electrons are extracted from reduced inorganic substances by lithotrophs and from organic compounds by organotrophs