# Chemostat

Stirred bioreactor operated as a chemostat, with a continuous inflow (the feed) and outflow (the effluent). The rate of medium flow is controlled to keep the culture volume constant.

A chemostat (from Chemical environment is static) is a bioreactor to which fresh medium is continuously added, while culture liquid is continuously removed to keep the culture volume constant.[1][2] By changing the rate with which medium is added to the bioreactor the growth rate of the microorganism can be easily controlled.

## Operation

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.[3] 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.

### Dilution rate

At steady state the specific growth rate (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

$D = \dfrac{\mbox{Medium flow rate}}{\mbox{Culture volume}} = \dfrac{\mbox{F}}{\mbox{V}}$

### 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.

## Applications

### 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 in ecology[4][5] and evolutionary biology.[6][7][8][9] 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 for specific types of bacterial mutants in culture such as auxotrophs or those that are resistant to antibiotics or bacteriophages for further scientific study.[10]

Competition for single and multiple resources, the evolution of resource acquisition and utilization pathways, cross-feeding/symbiosis,[11][12] antagonism, predation, and competition among predators have all been studied in ecology and evolutionary biology using chemostats.[13][14][15]

### 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.[citation needed]

## Concerns

• Foaming results in overflow with the volume of liquid not exactly constant.
• Some very fragile cells are ruptured during agitation and aeration.
• Cells may grow on the walls or adhere to other surfaces,[16] which is easily overcome by treating the glass walls of the vessel with a silane 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[17][18]
• Various applications use paddles, bubbling, or other mechanisms for mixing[19]
• 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, the auxostat 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.

Batch culture

## References

1. ^ Novick A, Szilard L (1950). "Description of the Chemostat". Science 112 (2920): 715–6. doi:10.1126/science.112.2920.715. PMID 14787503.
2. ^ James TW (1961). "Continuous Culture of Microorganisms". Annual Review of Microbiology 15: 27–46. doi:10.1146/annurev.mi.15.100161.000331.
3. ^ D Herbert, R Elsworth Telling RC (1956). "The continuous culture of bacteria;a Theoretical and Experimental study". J. Gen. Microbiol 14 (3): 601–622. doi:10.1099/00221287-14-3-601. PMID 13346021.
4. ^ Becks L, Hilker FM, Malchow H, Jürgens K, Arndt H (2005). "Experimental demonstration of chaos in a microbial food web". Nature 435 (7046): 1226–9. doi:10.1038/nature03627. PMID 15988524.
5. ^ Pavlou S, Kevrekidis IG (1992). "Microbial predation in a periodically operated chemostat: a global study of the interaction between natural and externally imposed frequencies". Math Biosci 108 (1): 1–55. doi:10.1016/0025-5564(92)90002-E. PMID 1550993.
6. ^ Wichman HA, Millstein J, Bull JJ (2005). "Adaptive Molecular Evolution for 13,000 Phage Generations: A Possible Arms Race". Genetics 170 (1): 19–31. doi:10.1534/genetics.104.034488. PMC 1449705. PMID 15687276.
7. ^ Dykhuizen DE, Dean AM (2004). "Evolution of specialists in an experimental microcosm". Genetics 167 (4): 2015–26. doi:10.1534/genetics.103.025205. PMC 1470984. PMID 15342537.
8. ^ Wick LM, Weilenmann H, Egli T (2002). "The apparent clock-like evolution of Escherichia coli in glucose-limited chemostats is reproducible at large but not at small population sizes and can be explained with Monod kinetics". Microbiology (Reading, Engl.) 148 (Pt 9): 2889–902. PMID 12213934.
9. ^ Jones LE, Ellner SP (2007). "Effects of rapid prey evolution on predator-prey cycles". J Math Biol 55 (4): 541–73. doi:10.1007/s00285-007-0094-6. PMID 17483952.
10. ^ Schlegel HG, Jannasch HW (1967). "Enrichment cultures". Annu. Rev. Microbiol. 21: 49–70. doi:10.1146/annurev.mi.21.100167.000405. PMID 4860267.
11. ^ Daughton CG, Hsieh DP (1977). "Parathion utilization by bacterial symbionts in a chemostat". Appl. Environ. Microbiol. 34 (2): 175–84. PMC 242618. PMID 410368.
12. ^ Pfeiffer T, Bonhoeffer S (2004). "Evolution of cross-feeding in microbial populations". Am. Nat. 163 (6): E126–35. doi:10.1086/383593. PMID 15266392.
13. ^ G. J. Butler and G. S. K. Wolkowicz. (July 1986). "Predator-mediated competition in the chemostat" (PDF). J Math Biol. 24 (2): 67–191. doi:10.1007/BF00275997.
14. ^ Dykhuizen DE, Hartl DL (June 1983). "Selection in chemostats". Microbiol. Rev. 47 (2): 150–68. PMC 281569. PMID 6308409.
15. ^ Dykhuizen DE, Hartl DL (May 1981). "Evolution of Competitive Ability in Escherichia coli". Evolution (Evolution, Vol. 35, No. 3) 35 (3): 581–94. doi:10.2307/2408204. JSTOR 2408204.
16. ^ Bonomi A, Fredrickson AG (1976). "Protozoan feeding and bacterial wall growth". Biotechnol. Bioeng. 18 (2): 239–52. doi:10.1002/bit.260180209. PMID 1267931.
17. ^ de Crécy E, Metzgar D, Allen C, Pénicaud M, Lyons B, Hansen CJ, de Crécy-Lagard V (2007). "Development of a novel continuous culture device for experimental evolution of bacterial populations". Appl. Microbiol. Biotechnol. 77 (2): 489–96. doi:10.1007/s00253-007-1168-5. PMID 17896105.
18. ^ Zhang Z, Boccazzi P, Choi HG, Perozziello G, Sinskey AJ, Jensen KF (2006). "Microchemostat-microbial continuous culture in a polymer-based, instrumented microbioreactor". Lab Chip 6 (7): 906–13. doi:10.1039/b518396k. PMID 16804595.
19. ^ Van Hulle SW, Van Den Broeck S, Maertens J, Villez K, Schelstraete G, Volcke EI, Vanrolleghem PA (2003). "Practical experiences with start-up and operation of a continuously aerated lab-scale SHARON reactor". Commun. Agric. Appl. Biol. Sci. 68 (2 Pt A): 77–84. PMID 15296140.