Abstract
BackgroundMicrobial population dynamics in bioreactors depend on both nutrients availability and changes in the growth environment. Research is still ongoing on the optimization of bioreactor yields focusing on the increase of the maximum achievable cell density.ResultsA new process-based model is proposed to describe the aerobic growth of Saccharomyces cerevisiae cultured on glucose as carbon and energy source. The model considers the main metabolic routes of glucose assimilation (fermentation to ethanol and respiration) and the occurrence of inhibition due to the accumulation of both ethanol and other self-produced toxic compounds in the medium. Model simulations reproduced data from classic and new experiments of yeast growth in batch and fed-batch cultures. Model and experimental results showed that the growth decline observed in prolonged fed-batch cultures had to be ascribed to self-produced inhibitory compounds other than ethanol.ConclusionsThe presented results clarify the dynamics of microbial growth under different feeding conditions and highlight the relevance of the negative feedback by self-produced inhibitory compounds on the maximum cell densities achieved in a bioreactor.
Highlights
Microbial population dynamics in bioreactors depend on both nutrients availability and changes in the growth environment
Model simulations were compared to classic experiments of yeast growth in both batch [53] and fed-batch cultures [12], and new experiments carried out in an aerated fedbatch bioreactor with two strains belonging to the CEN
The model presented was able to reproduce the dynamic behaviour of several yeast strains growing both in batch and fed-batch cultures
Summary
Microbial population dynamics in bioreactors depend on both nutrients availability and changes in the growth environment. Microbial cell populations growing in a closed vessel (batch culture), under suitable environmental and substrate conditions typically show an initial exponential proliferation followed by a decline in growth rate and transition to stationary phase [1] Such dynamics, different from Malthus’ law prediction, have been essentially ascribed to either exhaustion of nutrients according to the Monod model [2], or accumulation of toxic compounds in the culture medium [3], both affecting the Metabolic shift between respiration and fermentation Glucose catabolism of the yeast S. cerevisiae may follow two different pathways: aerobic respiration to CO2. The overflow metabolism hypothesis attributes aerobic fermentation to the saturation of a limited respiratory capacity leading to an overflow reaction at pyruvate level [11] Such effect is observed within seconds after exposure to high glucose concentrations and has been distinguished from more long-term effects concerning repression of respiration [8]. The latter has been reported to involve different signal transduction pathways activated by levels of either extracellular glucose or intracellular yet un-identified metabolites [16]
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