Thermally treated grass fibers as colonizable substrate for beneficial bacterial inoculum

This study investigates how thermally treated (i.e., torrefied) grass, a new prospective ingredient of potting soils, is colonized by microorganisms. Torrefied grass fibers (TGF) represent a specific colonizable niche, which is potentially useful to establish a beneficial microbial community that improves plant growth. TGF and torrefied grass extracts (TGE) were inoculated with a suspension of microorganisms obtained from soil. Sequential microbial enrichment steps were then performed in both substrates. The microbial communities developing in the substrates were assessed using cultivation-based and cultivation-independent approaches. Thus, bacterial isolates were obtained, and polymerase chain reaction-denaturing gradient gel electrophoresis (DGGE) analyses for bacterial communities were performed. Partial sequencing of the 16S ribosomal RNA gene from isolates and bands from DGGE gels showed diverse communities after enrichment in TGE and TGF. Bacterial isolates affiliated with representatives of the alpha-proteobacteria (Methylobacterium radiotolerans, Rhizobium radiobacter), gamma-proteobacteria (Serratia plymuthica, Pseudomonas putida), Cytophaga-Flavobacterium-Bacteroides (CFB) group (Flavobacterium denitrificans), beta-proteobacteria (Ralstonia campinensis), actinobacteria (Cellulomonas parahominis, Leifsonia poae, L. xyli subsp. xyli, and Mycobacterium anthracenicum), and the firmicutes (Bacillus megaterium) were found. In TGE, gamma-proteobacteria were dominant (61.5% of the culturable community), and 20% belonged to the CFB group, whereas actinobacteria (67.4%) and alpha-proteobacteria (21.7%) were prevalent in TGF. A germination assay with lettuce seeds showed that the phytotoxicity of TGF and TGE decreased due to the microbial enrichment.