Turbidostat

A turbidostat is a type of continuous culture device that maintains a microbial culture at a predetermined cell density by adjusting the inflow of fresh medium and the outflow of culture to keep the culture turbidity at a set value. It relies on an optical measurement of turbidity, typically an optical density reading, to infer cell concentration and to drive a feedback control system.

The operating principle involves monitoring the culture’s turbidity with a sensor and using a controller to regulate a pump or valve that delivers fresh medium and removes culture as needed. When the measured density rises above the setpoint, fresh medium is added and culture is removed to dilute the culture back to the target density; when the density is below the setpoint, dilution is reduced or ceased. In contrast to a chemostat, which maintains a constant dilution rate through a fixed nutrient inflow, a turbidostat continuously adjusts dilution to maintain constant cell density.

Typical components include a culture vessel, a reservoir of fresh medium, a waste outlet, a sensor for turbidity, a pump or valve system, and a control unit (often implementing a PID control loop). The setpoint is chosen based on the desired cell density, with common operating ranges depending on organism and sensor linearity (often a low to moderate optical density).

Applications include maintaining log-phase growth at high density, conducting evolution or selection experiments, and stabilizing populations with high plasmid burden. Related concepts are the chemostat and other continuous-culture systems; microturbidostats are used in microfluidic setups.