Batch, fed-batch and continuous

A key decision is how to run the bioreactor. There are three modes.

Batch. A closed system: all the nutrients are put in at the start, and nothing is added or removed later (except air). The culture passes through the phases of growth: lag, log, stationary, death. As the cells multiply, the substrate falls and the biomass rises.

Fed-batch. Sometimes a substrate limits growth, or is toxic at high levels. Then it is fed in gradually, a little at a time or continuously. Since nothing is removed, the volume increases. Keeping the substrate low avoids its toxic effects. The substrate concentration, and the rate at which it is used, stay constant, while the biomass and product keep rising.

Continuous. Fresh medium is added and used medium removed at the same rate, so the volume stays the same and product flows out continuously. The new cells formed balance the cells washed out, giving a steady state. At steady state, the growth rate, the rate of product formation, the substrate concentration and the rate of substrate use all stay constant.

In short: the reactor is closed, fed, or flowing.