CHEMOSTAT: CHARACTERISTICS, HISTORY AND USES - BIOLOGY - 2024

The chemostat is a device or apparatus used for the cultivation of cells and microorganisms. It is also called a bioreactor and has the ability to experimentally reproduce aquatic environments such as lakes, sedimentation or treatment ponds, among others.

It is generally described as a container (the size will depend on whether the use is industrial or laboratory) with an inlet so that sterile material enters, and an outlet through which the material resulting from the process will exit, which are generally nutrients. waste, sterile material, microorganisms among others.

Diagram of a chemostat. Taken and edited from: CGraham2332.

It was discovered and presented independently and almost simultaneously by scientists Jacques Monod, Aaron Novick and Leo Szilard in 1950. Monod worked alone and called it a bactogen, while Novick and Szilard worked together and called it a chemostat, a name that lasts to this day..

Chemostat Features

The chemostat is characterized by the constant addition of a medium containing a single nutrient that limits growth and simultaneously removes part of the culture, such as excess production, metabolites, and other substances. This removal is constantly replaced by new material, thus achieving a stable balance.

Under these conditions, the rate at which the microorganism culture develops is equal to the rate at which it is diluted. This is key compared to other cultivation methods, as a stable state can be reached in a constant and defined environment.

Another important feature is that with the chemostat the operator can control physical, chemical and biological variables such as volume of individuals in the culture, dissolved oxygen, amount of nutrients, pH, etc.

Method principle

The method consists of a population of microorganisms that grows from the beginning in a similar way to that of discontinuous or batch cultures (the simplest liquid culture). When populations grow it is necessary to simultaneously withdraw a volume of culture similar to that added, whether the withdrawn culture has been used or not.

In this way, in the chemostat a dilution is carried out using continuous addition of fresh medium and the elimination of the culture as described in part in the previous paragraph. A single nutrient is responsible for limiting growth in the container while the rest is present in excess.

This single growth-limiting nutrient is predetermined by the person developing the experiment, it can be any nutrient and in many cases it will depend on the species in culture.

History

Batch cultures of microorganisms date back centuries (brewing of beers and other beverages). However, continuous crops are something relatively more modern. Some microbiologists attribute the beginnings of continuous cultivation to the famous Russian microbiologist Sergey Vinogradsky.

Vinogradski studied the growth of sulforeductive bacteria in a device of his own design (Vinogradski column). During his studies, he fed drops of hydrogen sulfide to the column as food for these bacteria.

When talking about continuous cultivation it is mandatory to speak of 3 characters: Jacques Monod, Aaron Novick and Leo Szilard. Monod was a renowned biologist and winner of the Nobel Prize in 1965.

This researcher (Monod), while part of the Pasteur Institute, developed many tests, calculations and analyzes between 1931 and 1950. During this time he created the mathematical model of microorganism growth that would later be called the Monod Equation.

In 1950, based on the equation that bears his name, he designed a model of apparatus that allowed a culture of microorganisms continuously and called it a bactogen.

On the other hand, scientists Novick (physicist) and Szilard (chemist) met while working on the Manhattan project (the atomic bomb) in 1943; years later they would begin to show interest in bacterial growth and in 1947 they partnered to work together and take advantage of this.

After multiple tests and analyzes, Novick and Szilard, based on Monod's calculations (Monod's equation), also devised in 1950 a model of continuous culture of microscopic organisms which they called chemostat, and it is the name that has been maintained to date.. But all three are credited with the invention.

Applications

Adaptive Biology and Evolution

The tools offered by this system of continuous microorganism culture are used by ecologists and evolutionists to study how the growth rate affects cellular processes and metabolism, and how it controls selection pressure and gene expression.

This is made possible by evaluating and maintaining tens to hundreds of generations in the chemostat under controlled conditions.

Two chemostats, used in the analysis of ammonium toxicity in yeasts. Taken and edited from: (Image: Maitreya Dunham).

Cell biology

Virtually all studies related to the chemostat are related to cell biology, even molecular, evolutionary, etc.

However, specifically, the use of the chemostat for this branch of biology provides valuable information that allows the development of mathematical models necessary to understand the metabolic processes in the study population.

Molecular biology

In the last 10 or more years, interest in the use of chemostat in molecular analysis of microbial genes has grown. The culture method facilitates obtaining information for comprehensive or systemic analysis of microorganism cultures.

Chemostat studies in this field allow DNA transcription analysis throughout the genome, as well as quantifying gene expression or identifying mutations in specific genes of organisms such as the yeast Saccharomyces cerevisiae, for example.

Enriched cultures

These studies have been carried out using discontinuous systems since the end of the 19th century with the works of Beijerinck and Vinogradski, while in the 60s of the last century they began to be carried out in continuous cultures using the chemostat.

These studies consist of enriching culture media to harvest different types of microbes (bacteria in general), it is also used to determine the absence of certain species or detect the presence of some whose proportion is very low or almost impossible to observe in the medium. natural.

Enriched cultures in open continuous systems (chemostats) are also used to develop mutant bacteria cultures, mainly auxotrophs or those that may become resistant to drugs such as antibiotics.

Ethanol production

From an industrial point of view, the use and production of biofuels is increasingly common. In this case it is the production of ethanol from the Gram negative bacterium Zymomonas mobilis.

In the process, several large serial chemostats are used, maintained at constant concentrations of glucose and other sugars, to be converted into ethanol under anaerobic conditions.

References

1. The Chemostat: the ideal continuous stirred tank reactor. Recovered from: biorreactores.tripod.
2. Chemostat. Recovered from: en.wikipedia.org.
3. N. Ziv, NJ Brandt, & D. Gresham (2013). The Use of Chemostats in Microbial Systems Biology. Journal of visualized experiments.
4. A. Novick & L. Szilard (1950). Description of the chemostat. Science.
5. J. Monod (1949). The growth of bacterial culturesAnnual Review of Microbiology.
6. D. Gresham & J. Hong (2015). The functional basis of adaptive evolution in chemostats. FEMS microbiology reviews.
7. HG Schlegel, & HW Jannasch (1967). Enrichment Cultures. Annual Review of Microbiology.
8. J. Thierie (2016). Introduction to polyphasic dispersed systems theory. (eds) Springer Nature. 210 pp.