Transformation of indole by methanogenic and sulfate-reducing microorganisms isolated from digested sludge.

In the present study, mineralization of an aromaticN-heterocyclic molecule, indole, by microorganisms present in anaerobically digested sewage sludge was examined. The first step in indole mineralization was the formation of a hydroxylated intermediate, oxindole. The rate of transformation of indole to oxindole and its subsequent disappearance was dependent on the concentration of inoculum and indole and the incubation temperature. Methanogenesis appeared to be the dominant process in the mineralization of indole in 10% digested sludge even in the presence of high concentrations of sulfate. Enrichment of the digested sludge with sulfate as an electron acceptor allowed the isolation of a metabolically stable mixed culture of anaerobic bacteria which transformed indole to oxindole and acetate, and ultimately to methane and carbon dioxide. This mixed culture exhibited a predominance of sulfate-reducers over methanogens with more than 75% of the substrate mineralized to carbon dioxide. The investigation demonstrates that indole can be transformed by both methanogenic and sulfate-reducing microbial populations.