The gradostat: a bidirectional compound chemostat and its application in microbiological research.

A multistage continuous culture system is described in which solutes are transferred between vessels in opposite directions simultaneously. The system, called a gradostat, produces opposing solute gradients and is a good laboratory model of many natural microbial ecosystems in which solute gradients are important. Theoretical predictions concerning solute transfer were confirmed under steady-state and non steady-state conditions, using a coloured dye. Paracoccus denitrificans grew anaerobically in the gradostat at the intersection between opposing gradients of succinate and nitrate. Opposing gradients of glucose and oxygen separated the growth of a Bacillus sp. (a facultative anaerobe) and Clostridium butyricum (an obligate anaerobe). Viable counts for both species fell exponentially away from their growth positions at the ends of the gradostat. The potential value of the gradostat and possible alternative conformations are discussed.