Continous Culture

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Continous Culture

	Continous Culture - where growth occurs in a vessel of finite dimensions and eventually ceases. Is it not possible to devise an apparatus in which the cells are kept growing indefinitely by the continuous addition of fresh nutrient medium and the continuous removal of cells and their products. The work on these lines could be initiated only in 1940s and much of the development was the result of work on more efficient ways of producing cells for biological warfare (or as we are usually told, for defense against biological warfare). The method of growth was called continuous culture and the apparatus most commonly used was the chemostat, Fresh sterile medium is pumped into the growth vessel at a steady rate while a constant level device removes cells and their products at an equal rate.
A stirrer homogenizes the fresh medium with the culture and normally also acts as .an aeration device. The apparatus may be elaborated further by devices for sampling, pH control, oxygen control, temperature control, antifoam addition etc. If we inoculate a chemostat and supply fresh medium at a constant rate, the rate of change in biomass concentration will depend on the increase resulting from growth and the decrease due to removal of cells in the effluent.
The symbol D represents dilution rate which is equal to the rate of flow of fresh medium divided by the volume of medium in the culture vessel. If D is less than the maximum specific growth rate of the organism, a stead state is eventually established, in which the actual specific growth rate/ equals the rate of removal, i.e. p. = D. This occurs because specific growth rate becomes limited by the concentration of a single substrate and falls below its maximum value. The equation for relationship between substrate concentration (s) and specific growth rate is analogous to the Michaelis- Menten equation for enzyme kinetics. When the substrate is in excess, and equivalent to the specific growth rate during exponential growth in batch culture.
This equation is known as the Monad equation after the French microbiologist who first proposed it. The term Ks is the saturation constant for growth and is the substrate concentration at, which p is 1/2 pm The smaller the Ks value the greater the organism's growth rate at low substrate concentration. Bacteria generally have very low saturation constants, reflecting their ability to scavenge the low levels of nutrients often present in nature; this is one factor which determines competitive ability. The relationship between substrate concentration and specific growth rate as described by Monad equation can be expressed in the form of a curve in the figure.
	However, in practice the curve obtained in such figure will be slightly different. This is because the cells besides requiring energy for growth, must also maintain their structural integrity and carry out processes not directly associated with growth. The energy required for such processes is called maintenance energy and if we include it in the relationship the line in any figure designed will not pass through the origin, as even a non growing cell will require nutrients to supply energy for maintenance. Chemostats therefore operate at sub-maximal specific growth rates where growth is substrate limited.

In the steady state, biomass concentration, substrate concentration, and everything else in the vessel remain constant, and in theory can do so indefinitely Continuous culture has many advantages over batch culture, particularly in the research laboratory where reproducibility in the environment is more important than simplicity in technique and also in industry where large amounts of cells or products are required at economic costs. Some of the advantages are as follows:

(1) By altering the composition of the medium the substrate which is limiting growth may be changed and the specific roles of different nutrients can be studied.
(2) Environmental conditions and specific growth rate may be kept constant over extended periods and not just for the length of the exponential phase
(3) Growth at submaximal rates may be studied and growth rate may be changed to a new constant value merely by altering the setting on the medium pump.

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