Understanding time-growth rate relationship for slow-growing microbial cells in retentostat culturing systems

Generally, microorganisms in nature are severely growth-limited because of nutrient availability, competition, or physico-chemical parameters. To simulate these growing conditions, the use of continuous culturing systems has been proposed. One modification of these systems is the so-called retentostat. It corresponds to a system closed to cells and open to nutrients, and a nutrient is limiting growth. In this system, cells are forced to reduce progressively their growth rate as a result of increasing biomass sharing the same resources. In these systems, near-zero growth rates (corresponding to long doubling times) can be achieved within relatively reasonable time frames. Nevertheless, in these systems, the relationship between growth rate and time remains to be further analyzed. Herein, we evaluate a retentostat culturing system for different prokaryotic species. We established a general relationship between growth rate and incubation time. This allows us to easily estimate either the growth rate reached after a given incubation time or the time required to obtain cells at a specific slow growth rate. The results suggested progressive adaptation of cells at decreasing growth rates. This contribution simplifies estimates for incubation time-dependent growth rates and shows a retentostat as a species-independent culturing system where growth rates depend on time and dilution rate.