MICROBIAL NUTRITION AND GROWTH

BY: SHREELAKSHMI S NIAR

Microorganisms require the supply of raw materials or elements to construct new cellular components, these raw materials are called nutrients. These nutrients are provided by the culture medium where the microbes grow. These medium provides the nutrients which is required for the energy, building of cell substances and biosynthesis of fermentation products. On the basis of requirement nutrition can be classified as

Macronutrients: Carbon, oxygen, hydrogen, nitrogen, sulphur, phosphorous, potassium, calcium, magnesium and iron are included in macronutrients. They are required by the microbes in large amount and constitutes over95% of cell dry weight.Potassium,calcium,magnesium,iron,iron exist in a cell which act as cofactor for enzyme whereas carbon, oxygen, hydrogen, nitrogen, sulphur, phosphorous are the major components of biomolecules like carbohydrtes,proteins,lipids and nucleic acid.

Micronutrients: they are also called as trace elements. They require only in a small amount for microbes. It includes manganese, zinc, cobalt, nickel, molybdenum and copper. They mainly form a part of enzyme and help in catalysis of reaction. Contamination present in water, glasswares are sufficient to provide these nutrients.

Culture media components include inorganic nutrients, nitrogen supplements, carbon sources and growth factors. Inorganic Nutrients include Potassium Phosphate, Magnesium sulphate,Ammonium sulphate  or phosphate,Calcium carbonate.,Cobalt,  Copper, Iron, Mn, Mo, Zn.

NUTRITIONAL TYPES OF MICROORGANISMS

ON THE BASIS OF SOURCES OF CARBON

1. Autotrophs: They majorly use carbon dioxide and hence they can carry out photosynthesis.
2. Heterotrophs: They use reduced, performed organic molecules from other sources.

ON THE BASIS OF ENERGY SOURCE

1. Lithotrophs: They reduced inorganic molecules as a source for electron.
2. Organotrophs: They extract electrons from organic molecules.

ON THE BASIS OF THEIR PRIMARY SOURCE OF CARBON AND ENERGY

1. Photolithoautotrophy: They use light as energy source and carbon dioxide source.E.G. Algae, cyanobacteria.
2. Photoorganoheterotrophy: They use organic carbon and light as there source.
3. Chemolithoautotrophy: They use inorganic compounds like iron, sulphur, nitrogen and carbon dioxide as the source.
4. Chemoorganoheterotrophy: They generally use Organic molecule and carbon as there source.

MICROBIAL GROWTH

Growth is the orderly increase in all the components of an organism such as size or population number. The microbes are grown in batch culture or closed system. Because of the limited increase in cell size and frequency of cell division, growth in microorganisms is measured by increase in cell number. Bacteria multiply by binary fission, the process in which parent cell splits into two daughter cells. Bacterial cells first elongate, then followed by the formation of transverse membrane and new cell wall. The new membrane and cell wall will grow inward from the outer layers. Cell divides into two daughter cells. The growth of microorganisms reproducing by binary fission in culture can is plotted as the logarithm of the number of viable cells on Y-axis and the incubation time on X-axis. The resulting curve is called as standard bacterial growth curve that have four phases of growth namely lag phase, log phase, stationary phase and death phase.

Characteristics of Each Phase

1. The Lag Phase: when the microorganisms are inoculate into fresh medium they do not increase significantly in number and thus this phase is called lag phase. But, the microbes are metabolically active and thus they increase in size. Enzymes, essential cofactors are formed and accumulate until they are present in concentrations that permit growth. They also produce quantities of energy in the form of ATP.it shows that they are preparing for replication. The length of lag phase depends on the condition of microorganisms and nature of medium. Refrigerated culture or microbes inoculated from chemically different media takes more time to adapt
2. The Log or Exponential phase: The organisms divide at their most rapid rate .The population of organism’s doubles in this phase. The cell division depends upon the composition of growth and conditions of incubation. It provides the optimal conditions growth. Exponential growth can be balanced or unbalanced. When all the cellular constituents are synthesized at the constant rates relative to each other, it result in balanced growth and when rate of synthesis vary relative to each other, it is unbalanced growth. The bacteria’s at this stages are suitable for biochemical and morphological identification, drug sensitivity test.
3.  The Stationary Phase: The cell division decreases to the point that new are produced at the same rate as old cells die, the total number of viable cells remains constant. The culture is said to be in stationary phase is represented by a horizontal straight line in the curve. At this phase, population may simply cease to divide but is metabolically active. This phase may result due to depletion of essential nutrients, lack of biological space, accumulation of toxic waste products.
4. The Death Phase: As the condition of medium become more detrimental, population reaches in death phase, in which cells lose their ability to divide and thus, they die. In the death or decline phase ,the number of viable cells decreases exponentially, i.e., a constant proportion of cells dies every hour. However, sometimes death is not logarithmic and can vary with both environmental conditions and microorganisms involved. The duration of this phase depends on the genetic characteristics of the organisms.