A chemostat is a specialized laboratory device designed to cultivate microorganisms or cells continuously under highly controlled environmental conditions. It functions as an open system where a constant volume of fresh nutrient medium is supplied to the culture vessel while an equal volume of spent medium and cells is simultaneously removed. This continuous flow allows researchers to maintain a culture in a state of sustained growth, providing a steady supply of cells with uniform characteristics. The apparatus is specifically engineered to keep all physical and chemical growth parameters constant over extended periods.
What is a Chemostat?
A chemostat setup fundamentally consists of a sterile nutrient reservoir, a precisely controlled pump, and a culture vessel. The vessel is typically a closed, well-mixed container where the microorganisms are grown, equipped with sensors to monitor parameters like temperature, pH, and oxygen levels. The pump delivers fresh medium from the reservoir into the vessel at a constant, defined rate, defining the continuous nature of the culture.
To maintain a constant volume, an overflow system extracts the culture liquid at exactly the same rate the fresh medium is introduced. The most distinguishing feature of a chemostat is the limitation of a single nutrient, such as glucose, phosphate, or nitrogen. This limiting nutrient is present in the inflowing medium at a specific, low concentration, ensuring the growth rate of the microbial population is restricted and controlled by its availability. All other nutrients are supplied in surplus, forcing the cells to adapt their metabolism to the scarcity of the limiting factor.
Achieving the Steady State
The primary goal of a chemostat is to establish and maintain a “steady state,” a condition where all measurable culture parameters remain constant over time. In this state, the cell density, nutrient concentrations, and metabolic activity within the vessel do not change. This stability is achieved by balancing the rate at which new cells are created through growth with the rate at which cells are physically removed from the system.
The core control mechanism is the dilution rate ($D$), which is the ratio of the medium flow rate to the culture volume. This dilution rate directly dictates the specific growth rate ($\mu$) of the microorganisms in the steady state, meaning $D$ equals $\mu$. For example, if $D$ is set to 0.1 per hour, the cell population grows just fast enough to replace the 10% of the culture volume being removed hourly.
Any temporary increase in the cell population quickly consumes the limiting nutrient, slowing growth and restoring the balance. Conversely, if the growth rate lags, the limiting nutrient concentration temporarily increases, stimulating faster growth until the removal rate is matched again. This negative feedback loop inherently stabilizes the chemostat system.
If the dilution rate exceeds the maximum possible growth rate of the organism, the cells cannot reproduce fast enough to compensate for their removal, leading to a complete washout of the culture. By controlling the input pump’s flow rate, the experimenter precisely sets the growth rate, while the concentration of the limiting nutrient controls the final cell density.
Why Continuous Culture Matters
Maintaining a culture in a steady state provides scientific and industrial advantages not possible with traditional batch cultures, where conditions constantly change. Researchers can systematically study how a single parameter, such as temperature or pH, affects the organism’s physiology without the confounding effects of changing growth rates. This is valuable for generating large, uniform populations of cells with stable properties for studying cellular processes, producing recombinant proteins, or creating mathematical models of microbial metabolism.
In industrial biotechnology, chemostats are used to optimize production processes, such as the manufacturing of biofuels or therapeutic proteins. By operating at a steady state, manufacturers maximize the yield of a product over an extended period, leading to higher productivity than processes that must be constantly stopped and restarted. For ecological and evolutionary studies, the chemostat serves as a powerful microcosm. It simulates the slow, nutrient-limited conditions typical of many natural environments, allowing scientists to observe how microorganisms adapt and evolve over hundreds of generations under constant environmental pressure.