Identification, Isolation and Physiological Properties of Lactate-Degrading Bacteria Predominant in a Thermophilic Anaerobic Digestion Reactor Treating Poly(L-lactic acid)

摘要

One ideal treatment of poly(L-lactic acid) (PLLA) wastes contaminated with food and beverages is to convert to methane gas by anaerobic digestion reactors. Although a thermophilic anaerobic digestion method using PLLA whose chemical hydrolizability is enhanced by pretreatment have been developed, little information on lactate-degrading bacteria in the reactor have been available. Therefore, the aims of this study are to isolate lactate-degrading bacteria predominant in the reactor, and to characterize phenotypic and genotypic traits of isolates. 16S rRNA gene amplicon sequencing showed that > 20% of predominant bacteria in the reactor were microbes belonging to genera Thermodesulfovibrio, Coprothermobacter, and Moorella. Enrichment cultures were established with a basal medium (pH 7.0) supplemented with L-lactic acid (10 mM) and thermophilic digester sludge as an inoculum. Our attempt has succeeded in predominant cultivation of bacteria belonging to genus Thermodesulfovibrio. Subsequent several serial-dilution cultivation and roll-tube cultivation methods achieved the isolation of Thermodesulfovibrio bacteria, designated as strain K.uro-1. Strain K.uro-1 was able to convert L-lactic acid to acetate and hydrogen, and to produce acetate and methane from L-lactic acid in co-cultivation with hydrogenotrophic methanogens. Further investigation of the phenotypic and genotypic characteristics would lead to stably operate the thermophilic anaerobic digestion reactor using PLLA wastes.