Chemostat experiments performed to date have been conducted with fast-growing microorganisms at relatively high growth rates(between 0.02 and 0.05 h−1) where energy flux was sufficient to sustain the expression of many catabolic pathways. In contrast, continuous cultures with biomass retention (retentostats)can be considered as good candidates for mimicking environmental conditions, since they allow the investigation of bacterial physiology at extremely low growth rates. Substrate inflow is kept constant while biomass increases, causing the amount of substrate available per bacterial cell to decrease over time leading to extremely low growth rates with doubling times of up to one year