A chemostat is a technique used to grow microorganisms or cells continually in a specific phase of growth. In this technique a bioreactor is provided with fresh medium which is continuously added, and culture liquid containing left over nutrients, metabolic end products and to keep the culture volume constant microorganisms are continuously removed at the same rate.
The growth rate of the microorganisms in bioreactor can be changed by changing the rate with which medium is added hence growth can be easily controlled within limits.
In continous culture the Bacterial cultures can be stabled at state of exponential growth phase over long periods of time. The process is designed in such a way to relieve the conditions that stop exponential growth in batch cultures.

Steady state
• The main feature of this technique is that microbial growth in a continuous culture takes place under steady state conditions i.e. growth occurs at a constant rate and in a constant environment.
• All culture parameters like dissolved oxygen concentration, culture volume, nutrient and cell density , product concentrations, pH, etc.remain constant in the process.

• The flow of media in the vessel is related to volume of vessel in the form of dilution factor .
D = F/V where
F= flow rate
V= volume of vessel
D= dilution factor

• The environmental conditions can be controlled by the experimenter.
• Microorganisms cultured in the chemostat bioreactor usually grow at a steady state because of a negative feedback between growth rate and nutrient consumption: if the number of cells present in the bioreactor is low, the cells growth rates will be higher than the dilution rate as they consume little nutrient so growth is less limited by the addition of limiting nutrient with the inflowing fresh medium. The limiting nutrient is a nutrient component required for growth, present in the medium at a limiting concentration. However, if the number of cells becomes higher, the more nutrient is consumed, lowering the concentration of the limiting nutrient. hence this will reduce the specific growth rate of the cells which will lead to a decline in the number of cells as they keep being removed from the system with the outflow. This results in a steady state.
• As the process is regulated by the experimenter , there is more stability of the steady state. This helps the experimenter to control the specific growth rate of the microorganisms which can be done by changing the speed of the pump feeding fresh medium into the vessel.
Maintainence of purity of culture:
In the chemostat bioreactor there is chances of the contaimination . the containmination of continous clture with the foreign organisms and mutation of parent organism is sufficient to alter the basic characterstics of the culture. To find out the occurences the sample taken dirrectly from the chemostat bioreactor must be repeatedly examined, microscopically, by plating out and examination of colonies, by regular subculturing and examination by biochemical tests of typical colonies .

Applications
Research
A continous culture are used for investigations in cell biology, as they form are the source of large volumes of uniform cells or protein. In order to generate a mathematical model relating to its metabolic processes, The chemostat is often used to gather steady state data about an organism. Continous culture are also works as microcosms in ecology and evolutionary biology. In the one case, mutation is a problem, in the other case, it is the desired process under study. Chemostats can also be used to enrich for specific types of bacterial mutants in culture such as auxotrophs or those that are resistant to antibiotics or bacteriophages for further scientific study. Variations in the dilution rate permit the study of the metabolic strategies pursued by the organisms at different growth rates.[12][13] Competition for single and multiple resources, the evolution of resource acquisition and utilization pathways, cross-feeding/symbiosis,[14][15] antagonism, predation, and competition among predators have all been studied in ecology and evolutionary biology using chemostats.

Industry
Chemostats are frequently used in the industrial manufacturing of ethanol. In this case, several chemostats are used in series, each maintained at decreasing sugar concentrations. The chemostat also serves as an experimental model of continuous cell cultures in the biotechnological industry

Advantages and disadvantages of continuous cultures

Advantages
• Bacterial Growth rate is higher due to the continuous addtion of nutrients to the fermentation tank
• Effeciency of Continuous cultures are high as the fermenter operates continuously
• Very useful for techique for the production of primary metabolites.
Disadvantages
• The fermentor Set up is highly difficult, the maintenance of required growing conditions are also difficult to achieve
• There is more chances of contaimination and If contamination occurs, large volumes of product may be lost.