chemostats

Chemostats are a type of continuous culture device used in microbiology and biotechnology to maintain microbial cultures in a steady state. In a chemostat, fresh nutrient medium is continuously added to a bioreactor while an equal volume of culture is simultaneously removed, keeping the culture volume constant. The growth of microorganisms is limited by a limiting nutrient supplied at a fixed concentration in the inflowing medium, which fixes the specific growth rate through the dilution rate.

The system consists of a reactor, a feed reservoir, an effluent outlet, and a pump to drive inflow. The culture is usually well mixed so substrate and cells are approximately uniform. The dilution rate D is defined as F/V, where F is the inflow rate and V is the culture volume. At steady state, the specific growth rate μ of the cells equals D, and substrate concentration S adjusts to satisfy μ = μ(S), commonly described by Monod kinetics μ = μ_max S/(K_s + S).

Because growth is limited by the limiting nutrient, the cells maintain growth at a controlled rate rather than responding to changing overall nutrient levels. If the dilution rate exceeds the maximum μ_max of the organism, washout occurs and the culture declines.

Chemostats are used to study microbial physiology, metabolic fluxes, and evolutionary adaptation, and to produce biomass or metabolites under constant environmental conditions. Advantages include stable environmental conditions and precise control of growth rate; limitations include risk of contamination, accumulation of inhibitory byproducts, and the need for careful calibration of flow and mixing.