The thermophilic bacterium Caldicellulosiruptor bescii grows at 78 degrees C on high concentrations (200 g L-1) of both crystalline cellulose and unpretreated switchgrass, while low concentrations (<20 g L-1) of acid-pretreated switchgrass inhibit growth. Degradation of crystalline cellulose, but not that of unpretreated switchgrass, was limited by nitrogen and vitamin (folate) availability. Under optimal conditions, C. bescii solubilized approximately 60% of the crystalline cellulose and 30% of the unpretreated switchgrass using initial substrate concentrations of 50 g L-1. Further fermentation of crystalline cellulose and of switchgrass was inhibited by organic acid end-products and by a specific inhibitor of C. bescii growth that did not affect other thermophilic bacteria, respectively. Soluble mono-and oligosaccharides, organic acids, carbon dioxide, and microbial biomass, quantitatively accounted for the crystalline cellulose and plant biomass carbon utilized. C. bescii therefore degrades industrially-relevant concentrations of lignocellulosic biomass that have not undergone pretreatment thereby demonstrating its potential utility in biomass conversion. (C) 2013 Elsevier Ltd. All rights reserved.