The extent of inhibition of two strains of Clostridium thermocellum by a Populus hydrolysate was investigated. A Monod-based model of wild type (WT) and Populus hydrolysate tolerant mutant (PM) strains of the cellulolytic bacterium C thermocellum was developed to quantify growth kinetics in standard media and the extent of inhibition to a Populus hydrolysate. The PM was characterized by a higher growth rate (mu(max) = 1.223 vs. 0.571 h(-1)) and less inhibition (k(I,gen) = 0.991 vs. 0.757) in 10% v/v Populus hydrolysate compared to the WT. In 17.5% v/v Populus hydrolysate inhibition of PM increased slightly (K-I,K-gen = 0.888), whereas the WT was strongly inhibited and did not grow in a reproducible manner. Of the individual inhibitors tested, 4-hydroxybenzoic acid was the most inhibitory, followed by galacturonic acid. The PM did not have a greater ability to detoxify the hydrolysate than the WT. (C) 2013 Elsevier Ltd. All rights reserved.