Fermentative hydrogen production was evaluated using two anaerobic fluidized bed reactors (AFBRs). The reactors were fed cassava processing wastewater and operated with varying organic loading rates (OLR: 4-30 kg COD m(-3) d(-1)) for up to 160 d under mesophilic conditions. The effects and roles of lactic acid bacteria (LAB) and their antimicrobial peptides in hydrogen-producing bioreactors and on hydrogen producers were evaluated. A maximum hydrogen yield of 2.0 mmol g(-1) COD and a maximum hydrogen production rate of 2.1 L H-2 d(-1) L-1 were achieved with OLRs of 10 kg COD m(-3) d(-1) and 14 kg COD m(-3) d(-1), respectively, which coincided with the absence of bacteriocin-producing lactic acid bacteria. The major metabolites were lactic acid, ethanol, methanol, acetic acid and butyric acid. After growth on MRS agar, gram-staining and catalase tests, 47 strains were classified as presumptive lactic acid bacteria, with counts ranging from <10 to 1.5 x 10(9) CFU mL(-1) for both reactors, providing evidence that lactic acid bacteria are able to survive and persist in the reactors. Thirty-nine pure cultures of the LAB community were successfully identified by 16S rRNA sequencing. For AFBR1 (4-12 kg COD m(-3) d(-1)), there was a prevalence of Lactobacillus sp. (45.4%), followed by Lactococcus lactis (31.8%). Lactobacillus sp. (78.6%) was the prevalent genus for AFBR2 (14-30 kg COD m(-3) d(-1)). (C) 2015 Elsevier B.V. All rights reserved.