Isolation and classification of a soil actinomycete capable of sulphur-specific biotransformation of dibenzothiophene, benzothiophene and thianthrene.

AIM: To isolate actinomycete spp with the ability to desulphurize sulphur-containing heterocyclic compounds present in petroleum. METHODS AND RESULTS: Enrichment cultures were set up to select and isolate sulphur heterocycle metabolizing soil micro-organisms. Screening of the microbial isolates for the desulphurization property led to isolation of R3. The isolate was characterized by PCR screening of 16S rRNA genes and classical taxonomic investigations. HPLC analysis of the desulphurization assays with R3 showed ~85% transformation of dibenzothiophene (270 µmol l(-1)), present as the sole sulphur source in basal salt medium, in 4 days. Production of the desulphurized dibenzothiophene metabolite, 2-hydroxybiphenyl, was confirmed by GC/MS analyses. GC/MS analyses also established the ability of R3 to transform benzothiophene to benzothiophene-1-oxide and benzothiophene-1, 1-dioxide, and thianthrene to thianthrene-5-oxide. PCR primers computed based on the desulphurization operon (dszABC) of Rhodococcus erythropolis IGTS8 yielded the predicted amplification products with R3 genomic DNA as template. Southern hybridization and restriction endonuclease digestion profiles indicated that R3 amplicons were homologous to dsz AB. CONCLUSIONS: The enrichment method used in this study yielded an environmental isolate with the ability to transform multiple sulphur heterocycles. The isolate R3 has taxonomic proximity to the Oerskovia sp, order Actinomycetales. The isolate R3 selectively removes sulphur from dibenzothiophene yielding 2-hydroxybiphenyl and sulphate. R3 also transforms benzothiophene and thianthrene in a sulphur-targeted manner. The desulphurization genes in R3 bear similarity to those in R. erythropolis IGTS8. SIGNIFICANCE AND IMPACT OF THE STUDY: The actinomycetes present in soil can remove sulphur from different sulphur heterocycle substrates and have potential as biodesulphurization catalysts.

Degradation of dibenzothiophene and its metabolite 3-hydroxy-2-formylbenzothiophene by an environmental isolate.

Microbial degradation of dibenzothiophene (DBT) beyond 3-hydroxy-2-formylbenzothiophene (HFBT), a commonly detected metabolite of the Kodama pathway for DBT metabolism, and the catabolic intermediates leading to its mineralization are not fully understood. The enrichment cultures cultivated from crude oil contaminated soil led to isolation of ERI-11; a natural mixed culture, selected for its ability to deplete DBT in basal salt medium (BSM). A bacterial strain isolated from ERI-11, and tentatively named A11, degraded more than 90 % of the initial DBT (270 µM), present as the sole carbon and sulfur source, in 72 h. Gas chromatography-mass spectrophotometry (GC-MS) analyses of the DBT degrading A11 culture medium extracts led to detection of HFBT. The metabolite HFBT, produced using A11, was used in degradation assays to evaluate its metabolism by the bacteria isolated in this study. Ultra violet-visible spectrophotometry and high-performance liquid chromatography analyses established the ability of the strain A11 to deplete HFBT, present as the sole sulfur and carbon source in BSM. GC-MS analyses showed the presence of 2-mercaptobenzoic acid in the HFBT degrading A11 culture extracts. The findings in this study establish that the environmental isolate A11 possesses the metabolic capacity to degrade DBT beyond the metabolite HFBT. The compound 2-mercaptobenzoic acid is an intermediate formed on HFBT degradation by A11.