Evaluation of inhibitory compounds produced by bacteria isolated from a hydrogen-producing bioreactor during the self-fermentation of wheat straw.

Abstract

The objective of this study was to evaluate the inhibitory activity of compounds secreted by bacteria isolated from a hydrogen-producing bioreactor to understand how these microorganisms interact in this community. In vitro inhibitory assays were performed using samples secreted by bacteria subject to different treatments to determine if their inhibitory effect was due to organic acids, non-proteinaceous compounds or bacteriocin-like inhibitory substances (BLIS). Bacterial isolated were suppressed 43%, 30% and 27% by neutralized, precipitated and non-neutralized cell-free supernatants, respectively. Non-hydrogen producers (non-H2 P) lactic acid bacteria (LAB) (Lactobacillus plantarum LB1, Lactobacillus pentosus LB7, Pediococcus acidilactici LB4) and hydrogen producers (H2 P) LAB (Enterococcus faecium F) were inhibited by the production of organic acids, non-proteinaceous compounds and BLIS. Meanwhile, the obligate anaerobe H2 P (Clostridium beijerinckii B) inhibited by the production of non-proteinaceous compounds and BLIS. The presence of BLIS was confirmed when proteolytic enzymes affected the inhibitory activity of secreted proteins in values ranging from 20% to 42%. The BLIS produced by L. plantarum LB1, P. acidilactici LB4, L. pentosus LB7 and E. faecium F showed molecular masses of ~11, 25, 20 and 11 kDa, respectively. It was demonstrated antagonistic interactions between Lactobacillus-Enterococcus and Pediococcus-Enterococcus species, generated by the secretion of organic acids, non-proteinaceous compounds and BLIS. We report the interactions between LAB isolated from hydrogen-producing bioreactors. These interactions might impact the dynamics of the microbial population during hydrogen generation. Our work lays a foundation for strategies that allow controlling bacteria that can affect hydrogen production.