The operation of a bioreactor that involves the continuous addition of fresh sterile media and its withdrawal at the constant rate. The media contains all the necessary nutrients required to promote biological growth of the microorganisms within the bioreactor. The volume of the bioreactor can be kept constant by an overflow weir arrangement. The dilution rate is used to characterize the operation which is defined as the ratio of the flow of feed to the volume of the bioreactor. Chemostat cultivation involves the continuous feed of substrate in which at least one nutrient is limiting. Turbidostat cultivation involves maintaining a constant biomass concentration by varying the dilution rate.