Production of soluble lignin-rich fragments (APPL) from wheat lignocellulose by Streptomyces viridosporus and their partial metabolism by natural bacterial isolates
The actinomycete Streptomyces viridosporus attacks wheat lignocellulose releasing soluble lignin-rich fragments (APPL). Chemical analyses indicated that these APPL contain 20% less guaiacyl and 70% less glucose residues than the original substrate. In order to determine the effect of non-filamentous bacteria on APPL, natural isolates were selected in synthetic media containing APPL as sole carbon source. From a total of eighty cultures, two strains (Pseudomonas spp. B23 and E21) and a consortium of two strains (Enterobacter sp. V1 and Pseudomonas sp. V2) were selected for further studies. Strains Pseudomonas fluorescens biovar I and Pseudomonas acidovorans D3, previously isolated on β-1 and β-O-4 lignin model compounds, respectively, were included for comparative purposes. Analysis of APPL recovered after bacterial growth indicated that Pseudomonas B23 attacked preferentially both guaiacyl and syringyl lignin units while barely affected their carbohydrate content. On the other hand, Pseudomonas E21 and the consortium metabolized sugar moieties without modifying the aromatic residues of APPL. P. fluorescens biovar I and P. acidovorans D3, in spite of being able to cleave lignin linkages on dimeric model compounds, exhibited very limited growth on APPL. The former only partially degraded guaiacyl residues, while the latter did not produce detectable changes on APPL structure.