Growth inhibition of Clostridium cellulolyticum by an inefficiently regulated carbon flow

Carbon flow in Clostridium cellulolyticum was investigated either in batch or continuous culture using a synthetic medium with cellobiose as the sole source of carbon and energy. Previous experiments carried out using a complex growth medium led to the conclusion that the carbon flow was stopped by intracellular NADH. In this study, results showed that cells cultured in a synthetic medium were better able to control electron flow since the NADH/NAD(+) ratios were in the range 0.3-0.7, whereas a ratio as high as 57 was previously found in cells cultured on a complex medium. Furthermore, a specific rate of cellobiose consumption of 2.13 mmol (g cells)(-1) h(-1) was observed on synthetic medium whereas the highest value obtained on complex medium was 0.68 mmol (g cells)(-1) h(-1). When C. cellulolyticum was grown in continuous culture and cellobiose in the feed medium was increased from 5.84 to 17.57 mM in stepwise fashion, there was an increase in cellobiose utilization without growth inhibition. In contrast, when the reactor was fed directly with 14.62 mM cellobiose, residual cellobiose was observed (4.24 mM) and growth was limited. These data indicate that C, cellulolyticum is not able to optimize its growth and carbon flaw in response to a sudden increase in the concentration of growth substrate cellobiose. This interpretation was confirmed (i) by the study of cellobiose batch fermentation where it was demonstrated that growth inhibition was not due to nutritional limitation or inhibition by fermentation products but was associated with carbon excess and (ii) by the growth of C. cellulolyticum in dialysis culture where no growth inhibition was observed due to the limitation of carbon flow by the low rate of cellobiose diffusion through the dialysis tubing.