Investigation of the metabolic inhibition observed in solid-substrate cultivation of Clostridium thermocellum on cellulose

Metabolic inhibition of Clostridium thermocellum, when grown in a high solids environment, was investigated by comparing submerged fermentation (SmF), solid-substrate cultivation (SSC) and solid-substrate cultivation with media replacement by periodic flushing (FSSC). Cellulose conversion extent and end-product concentrations were measured over time. SmF converted ∼65% of the cellulose in 240 h (10 days), whereas SSC converted <8% in the same period. FSSC converted approximately 25% and 47% of initial substrate after 240 h; 45% and 71% of initial substrate after 25 days, with media replacement every 24 and 12 h, respectively. The SSC experienced higher initial production rates for all fermentation products, but could not sustain production rates. When acetate concentrations reached a critical point, the acetate decreased the intracellular volume of C. thermocellum cell suspensions at pH values similar to those observed in SSC. Acids produced by fermentation exacerbated the already unfavorable osmotic condition of SSC, resulting in metabolic inhibition. Consistent with this finding, approximately constant amounts of ethanol, acetate and lactate were produced during each flush of the FSSC. Flushed solid-substrate cultivation maintained favorable growth conditions for C. thermocellum even up to 25 days, allowing more total product to be formed than in the other cultivation methods.