Continuous Culture - Definition, Types of method, instrumentation and applications

The culture in which organisms can grow continuously for indefinite period of time is called continuous culture

  Generally in batch culture, the organisms cannot grow for long. This is due to the development of unfavourably conditions In the growth medium, as a result of depletion of nutrients, accumulation of toxic metabolites, Increased cell density etc.

  However if the environment conditions are maintained favorable continuous culture is possible. Based on this principle, continuous culture is obtained. Continuous culture system is also referred to as pen culture system.

Principle of obtaining continuous culture

  If the environment conditions, optimal for growth can be maintained in the culture medium, continuous growth is possible. In the growing culture, this can be achieved by
  1. Continuous removal of toxic waste metabolites from medium produced during the growth.
  2. Continuous feeding of the growth medium with fresh nutrients at a rate, they are being utilized.
  3. Removal of excess of cells such that the desirable cell density is maintained in the growth medium.
The methods used for obtaining continuous culture are based on these principles.

Methods of obtaining continuous culture

Two methods are used for obtaining continuous culture.    
  1). Turbidostat
  2). Chemostat

Turbidostat :

  In turbidostat, the basic approach for obtaining continuous culture is by maintaining uniform turbidity or cell density in the growth tube.  
   The turbidity of the growth medium is measured by an optical device. The turbidity in the growth medium is maintained by replacing spent medium with fresh medium.

Method of Turbidostat

  The device consists of a growth tube, facility for removal of spent medium by an overflow device through a siphon,  reservoir of fresh medium and optical sensing device for measurement of turbidity in growth tube.

  In the growth tube, when the turbidity exceeds beyond a set limit, an electronic signal is obtained from optical sensing device which measures turbidity.

  This signal allows a flow of fresh medium into the growth tube. Simultaneously, the spent medium, along with growth, is allowed to leave the growth tube by overflow device through a siphon.

  This continues till the culture gets diluted and turbidity drops to a set low level. Thus, In the turbidostat, number of cells in the growth tube controls the flow rate. The rate of growth in the culture adjusts to this flow rate.

Apparatus for continuous cultivation of Bacteria. The system consists of (a) Fresh culture medium reservoir, (b) Flow rate regulator, (c) growth tube with over flow device to remove excess cell mass and spent medium and (d) spent culture bottle

Chemostat :

 In Chemostat, the basic approach for obtaining continuous culture is by monitoring uniform chemical composition in the growth medium. The method involves replacement of the spent medium by fresh one at a rate; the nutrients are being consumed from the growth medium.

Method of Chemostat

  The device and assembly for chemostat is almost similar to removal of spent medium by a siphon device and reservoir for the fresh medium.
  In chemostat, fresh medium is allowed to enter the growth tube at a specified flow rate. The spent medium is removed from the growth tube by an over flow device using siphon. Thus the device allows 
  1. Continuous removal of toxic waste metabolites and excess of cell mass from the growth tube as well as
  2. Supply of the fresh nutrients at a rate, they are being consumed.
  Thus, uniform chemical environment for growth of organisms is maintained continuously in the growth tube, which allows continuous cultivation. The fresh medium contains some nutrients ingrowth limiting concentration (usually vitamin or growth factor).
  Their concentration decides the growth rate of the culture in chemostat and the growth rate adjusts to the flow rate of fresh medium.

Growth rate and flow rate of fresh medium in the chemostat

  In continuous culture, the growth rate of the organisms adjusts to the flow rate of the fresh medium. For example, In chemostat, rate of entry of fresh medium is changed, growth rate adjusts to match the new rate of loss of cells through overflow and maintain new level of cell density. This can be explained as under.

  1. The rate of growth of organisms in continuous culture is always limited by the concentration of nutrients. Initially the organisms grow at a maximum growth rate.
  2. As the culture density increases, the rate of utilization of nutrients will increase. This continues till level of one of the nutrient depleted. The depleting of this nutrient will limit the growth rate.
  3. As long as the growth rate exceeds the rate of loss ol cells by siphon, cell density will continue to increase In the growth tube and concentration of nutrient in the growth tube will decrease.
  4. As a consequence to this, the growth rate decreases till the rate of Increase in cells is equal to the rate of loss of cells and a balance between the growth rate and loss of cells is achieved. Thus. the continuous culture system is a self regulatory system and it can be decided as - (Rate of production of cells  through growth) = (Rate of loss of cell through overflow)

However, if the flow rate for addition of fresh nutrients exceeds far beyond the rate of new cell formation. It will result Into washing out of the culture from the growth tube.

Advantages of continuous culture system:

  1. Isolation of an organism capable of growth on a simpler medium make basic of cheaper commercial process and the process should be more resistant to contamination.
  2. Use of high temperature during isolation results in the isolation of thermophilic strain minimising cooling problems in the subsequent process.

Limitation of continuous culture system:

Use of continuous enrichment process may lead to washout of the inoculum before an adapted culture is established.

Application of continuous culture system

The continuous culture system has two Important features:
  1. Cells in culture always occur in exponential phase of growth
  2. Cells are allowed to grow continuously at extremely low concentration of substrate or nutrients.
This allows studies on the regulation of metabolism of limiting growth substance.
  Continuous culture systems has also a potential application in the fermentation Industries, which allows processing of large bulk of medium by installation of relatively smaller capacities of reservoir for fresh medium, fermentation vessel and product recovery plants.