An alternative consolidated bioprocessing approach is the use of a co-culture containing cellulolytic and solventogenic clostridia. It has been demonstrated that the rate of cellulose utilization in the co-culture of Clostridium acetobutylicum and Clostridium cellulolyticum is improved compared to the mono-culture of C. cellulolyticum, suggesting the presence of syntrophy between these two species. However, the metabolic interactions in the co-culture are not well understood. To understand the metabolic interactions in the co-culture, we developed a genome-scale metabolic model of C. cellulolyticum comprising of 431 genes, 621 reactions, and 603 metabolites. The C. cellulolyticum model can successfully predict the chemostat growth and byproduct secretion with cellulose as the substrate. However, a growth arrest phenomenon, which occurs in batch cultures of C. cellulolyticum at cellulose concentrations higher than 6.7 g/L, cannot be predicted by dynamic flux balance analysis due to the lack of understanding of the underlying mechanism. These genome-scale metabolic models of the pure cultures have also been integrated using a community modeling framework to develop a dynamic model of metabolic interactions in the co-culture. Co-culture simulations suggest that cellobiose inhibition cannot be the main factor that is responsible for improved cellulose utilization relative to mono-culture of C. cellulolyticum.

BackgroundClostridial co-culture containing cellulolytic and solventogenic species is a potential consolidated bioprocessing (CBP) approach for producing biochemicals and biofuels from cellulosic biomass. It has been demonstrated that the rate of cellulose utilization in the co-culture of Clostridium acetobutylicum and Clostridium cellulolyticum is improved compared to the mono-culture of C. cellulolyticum (BL 5:119-124, 1983). However, the metabolic interactions in this co-culture are not well understood. To investigate the metabolic interactions in this co-culture we dynamically characterized the physiology and microbial composition using qPCR.ResultsThe qPCR data suggested a higher growth rate of C. cellulolyticum in the co-culture compared to its mono-culture. Our results also showed that in contrast to the mono-culture of C. cellulolyticum, which did not show any cellulolytic activity under conditions similar to those of co-culture, the co-culture did show cellulolytic activity even superior to the C. cellulolyticum mono-culture at its optimal pH of 7.2. Moreover, experiments indicated that the co-culture cellulolytic activity depends on the concentration of C. acetobutylicum in the co-culture, as no cellulolytic activity was observed at low concentration of C. acetobutylicum, and thus confirming the essential role of C. acetobutylicum in improving C. cellulolyticum growth in the co-culture. Furthermore, butanol concentration of 350 mg/L was detected in the co-culture batch experiments.ConclusionThese results suggest the presence of synergism between these two species, while C. acetobutylicum metabolic activity significantly improves the cellulolytic activity in the co-culture, and allows C. cellulolyticum to survive under harsh co-culture conditions, which do not allow C. cellulolyticum to grow and metabolize cellulose independently. It is likely that C. acetobutylicum improves the cellulolytic activity of C. cellulolyticum in the co-culture through exchange of metabolites such as pyruvate, enabling it to grow and metabolize cellulose under harsh co-culture conditions.

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