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Background   Clostridium thermocellum  is a model thermophilic organism for the production of biofuels from lignocellulosic substrates. The majority of publications studying the physiology of this organism use substrate concentrations of ≤10 g/L. However, industrially relevant concentrations of substrate start at 100 g/L carbohydrate, which corresponds to approximately 150 g/L solids. To gain insight into the physiology of fermentation of high substrate concentrations, we studied the growth on, and utilization of high concentrations of crystalline cellulose varying from 50 to 100 g/L by  C. thermocellum .    Results  Using a defined medium, batch cultures of  C. thermocellum  achieved 93% conversion of cellulose (Avicel) initially present at 100 g/L. The maximum rate of substrate utilization increased with increasing substrate loading. During fermentation of 100 g/L cellulose, growth ceased when about half of the substrate had been solubilized. However, fermentation continued in an uncoupled mode until substrate utilization was almost complete. In addition to commonly reported fermentation products, amino acids - predominantly L-valine and L-alanine - were secreted at concentrations up to 7.5 g/L. Uncoupled metabolism was also accompanied by products not documented previously for  C. thermocellum , including isobutanol,  meso - and RR/SS-2,3-butanediol and trace amounts of 3-methyl-1-butanol, 2-methyl-1-butanol and 1-propanol. We hypothesize that  C. thermocellum  uses overflow metabolism to balance its metabolism around the pyruvate node in glycolysis.    Conclusions   C. thermocellum  is able to utilize industrially relevant concentrations of cellulose, up to 93 g/L. We report here one of the highest degrees of crystalline cellulose utilization observed thus far for a pure culture of  C. thermocellum , the highest maximum substrate utilization rate and the highest amount of isobutanol produced by a wild-type organism.    Electronic supplementary material  The online version of this article (doi:10.1186/s13068-014-0155-1) contains supplementary material, which is available to authorized users.

The exometabolome of Clostridium thermocellum reveals overflow metabolism at high cellulose loading