uThe effect of two different inoculum pretreatments, thermal and cell wash-out (A1 and A2, respectively) on the performance of anaerobic fluidized bed reactors for hydrogen production was determined. The reactors were operated for 112 days under the same operational conditions using glucose as substrate at increasing organic loading rates and decreasing hydraulic retention times. Both treatments were effective avoiding methanogenesis. Reactor A2 showed better performance and stability than reactor Al in each one of the different operational conditions. Cell wash-out treatment produced higher hydrogen volumetric production rates and yields than thermal treatment (7 L H-2/L-d, 3.5 mol H-2/mol hexose, respectively). DGGE analysis revealed that the microbial communities developed were affected by the inoculum treatment. Organisms from the genera Clostridium and Lactobacillus predominated in both reactors, with their relative abundances linked to hydrogen production. Resilience was observed in both reactors after a period of starvation. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.