Chemostat

Chemostat

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A chemostat is a bioreactor
Bioreactor
A bioreactor may refer to any manufactured or engineered device or system that supports a biologically active environment. In one case, a bioreactor is a vessel in which a chemical process is carried out which involves organisms or biochemically active substances derived from such organisms. This...

 to which fresh medium is continuously added, while culture liquid is continuously removed to keep the culture volume constant. By changing the rate with which medium is added to the bioreactor the growth rate of the microorganism
Microorganism
A microorganism or microbe is a microscopic organism that comprises either a single cell , cell clusters, or no cell at all...

 can be easily controlled.

Steady State


One of the most important features of chemostats is that micro-organisms can be grown in a physiological steady state. In steady state, growth occurs at a constant rate and all culture parameters remain constant (culture volume, dissolved oxygen concentration, nutrient and product concentrations, pH, cell density, etc.). In addition environmental conditions can be controlled by the experimenter. Micro-organisms grown in chemostats naturally strive to steady state: if a low amount of cells are present in the bioreactor, the cells can grow at growth rates higher than the dilution rate, as growth isn't limited by the addition of the limiting nutrient. The limiting nutrient is a nutrient essential for growth, present in the media at a limiting concentration (all other nutrients are usually supplied in surplus). However, if the cell concentration becomes too high, the amount of cells that are removed from the reactor cannot be replenished by growth as the addition of the limiting nutrient is insufficient. This results in a steady state (not to be mistaken as equilibrium).nm

Dilution Rate


At steady state the specific growth rate
Specific growth rate
Specific growth rate is defined as the increase in cell mass per unit time, e.g., grams cells per gram cells per hour:...

 (μ) of the micro-organism is equal to the dilution rate (D). The dilution rate is defined as the rate of flow of medium over the volume of culture in the bioreactor


Maximal growth rate


Each microorganism growing on a particular substrate has a maximum specific growth rate (μmax) (the rate of growth observed if none of the nutrients are limiting). If a dilution rate is chosen that is higher than μmax, the culture will not be able to sustain itself in the bioreactor, and will wash out. Even though maximum rates can be obtained, the reactors may become very large. This is especially true in E. coli fatty acid production in a glucose medium.

Research


Chemostats in research are used for investigations in cell biology, as a source for large volumes of uniform cells or protein. The chemostat is often used to gather steady state data about an organism in order to generate a mathematical model relating to its metabolic processes. Chemostats are also used as microcosms
Microcosm: Model / experimental ecosystem
Microcosms are artificial, simplified ecosystems that are used to simulate and predict the behaviour of natural ecosystems under controlled conditions. Open or closed microcosms provide an experimental area for ecologists to study natural ecological processes. Microcosm studies can be very useful...

 in ecology and evolutionary biology. In the one case, mutation/selection is a nuisance, in the other case, it is the desired process under study. Chemostats can also be used to enrich
Enrichment culture
An enrichment culture is a medium with specific and known qualities that favors the growth of a particular microorganism. The enrichment culture's environment will support the growth of a selected microorganism, while inhibiting the growth of others...

 for specific types of bacterial mutants in culture such as auxotrophs or those that are resistant to antibiotic
Antibiotic
An antibacterial is a compound or substance that kills or slows down the growth of bacteria.The term is often used synonymously with the term antibiotic; today, however, with increased knowledge of the causative agents of various infectious diseases, antibiotic has come to denote a broader range of...

s or bacteriophage
Bacteriophage
A bacteriophage is any one of a number of viruses that infect bacteria. They do this by injecting genetic material, which they carry enclosed in an outer protein capsid...

s for further scientific study.

Competition for single and multiple resources, the evolution of resource acquisition and utilization pathways, cross-feeding/symbiosis, antagonism, predation, and competition among predators have all been studied in ecology
Ecology
Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...

 and evolutionary biology using chemostats.

Industry


Chemostats are frequently used in the industrial manufacturing of ethanol
Ethanol
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...

. In this case, several chemostats are used in series, each maintained at decreasing sugar concentrations.

Concerns

  • Foaming results in overflow with the volume of liquid not exactly constant.
  • Some very fragile cells are ruptured during agitation and aeration
    Aeration
    Aeration is the process by which air is circulated through, mixed with or dissolved in a liquid or substance.-Aeration of liquids:-Methods:Aeration of liquids is achieved by:...

    .
  • Cells may grow on the walls or adhere to other surfaces, which is easily overcome by treating the glass walls of the vessel with a silane
    Silane
    Silane is a toxic, extremely flammable chemical compound with chemical formula SiH4. In 1857, the German chemists and Friedrich Woehler discovered silane among the products formed by the action of hydrochloric acid on aluminum silicide, which they had previously prepared...

     to render them hydrophobic.
  • Mixing may not truly be uniform, upsetting the "static" property of the chemostat.
  • Dripping the media into the chamber actually results in small pulses of nutrients and thus oscillations in concentrations, again upsetting the "static" property of the chemostat.
  • Bacteria travel upstream quite easily. They will reach the reservoir of sterile medium quickly unless the liquid path is interrupted by an air break in which the medium falls in drops through air.


Continuous efforts to remedy each defect lead to variations on the basic chemostat quite regularly. Examples in the literature are numerous.
  • Antifoaming agents are used to suppress foaming.
  • Agitation and aeration can be done gently.
  • Many approaches have been taken to reduce wall growth
  • Various applications use paddles, bubbling, or other mechanisms for mixing
  • Dripping can be made less drastic with smaller droplets and larger vessel volumes
  • Many improvements target the threat of contamination

Variations


Fermentation setups closely related to the chemostats are the turbidostat
Turbidostat
A turbidostat is a continuous culture device, similar to a chemostat or an auxostat, which has feedback between the turbidity of the culture vessel and the dilution rate. The theoretical relationship between growth in a chemostat and growth in a turbidostat is somewhat complex, in part because it...

, the auxostat
Auxostat
An auxostat is a continuous culture device which, while in operation, uses feedback from a measurement taken on the growth chamber to control the media flow rate, maintaining the measurement at a constant. Auxo was the Greek goddess of spring growth, and as a prefix represents nutrients...

 and the retentostat. In retentostats culture liquid is also removed from the bioreactor, but a filter retains the biomass. In this case, the biomass concentration increases until the nutrient requirement for biomass maintenance has become equal to the amount of limiting nutrient that can be consumed.

See also

  • Fed-batch
    Fed-batch
    A fed-batch is a biotechnological batch process which is based on feeding of a growth limiting nutrient substrate to a culture.The fed-batch strategy is typically used in bio-industrial processes to reach a high cell density in the bioreactor....

  • Biochemical engineering
    Biochemical engineering
    Biochemical engineering is a branch of chemical engineering or biological engineering that mainly deals with the design and construction of unit processes that involve biological organisms or molecules, such as bioreactors...

  • Continuous stirred-tank reactor (CSTR)
    Continuous stirred-tank reactor model
    The continuous stirred-tank reactor , also known as vat- or backmix reactor, is a common ideal reactor type in chemical engineering. A CSTR often refers to a model used to estimate the key unit operation variables when using a continuous agitated-tank reactor to reach a specified output...

  • Bacterial growth
    Bacterial growth
    250px|right|thumb|Growth is shown as L = log where numbers is the number of colony forming units per ml, versus T Bacterial growth is the division of one bacterium into two daughter cells in a process called binary fission. Providing no mutational event occurs the resulting daughter cells are...

  • E. coli long-term evolution experiment
    E. coli long-term evolution experiment
    The E. coli long-term evolution experiment is an ongoing study in experimental evolution led by Richard Lenski that has been tracking genetic changes in 12 initially identical populations of asexual Escherichia coli bacteria since 24 February 1988...


Batch culture

External links

  1. http://www.midgard.liu.se/~b00perst/chemostat.pdf
  2. http://www.rpi.edu/dept/chem-eng/Biotech-Environ/Contin/chemosta.htm
  3. A final thesis including mathematical models of the chemostat and other bioreactors
  4. A page about one laboratory chemostat design