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Keywords:

  • biodegradation;
  • γ-hexachlorocylohexane;
  • metabolites;
  • pathway;
  • Xanthomonas

Abstract

Aim:  To isolate γ-hexachlorocyclohexane (HCH)-degrading bacteria from contaminated soil and characterize the metabolites formed and the genes involved in the degradation pathway.

Methods and Results:  A bacterial strain Xanthomonas sp. ICH12, capable of biodegrading γ- HCH was isolated from HCH-contaminated soil. DNA-colony hybridization method was employed to detect bacterial populations containing specific gene sequences of the γ-HCH degradation pathway. linA (dehydrodehalogenase), linB (hydrolytic dehalogenase) and linC (dehydrogenase) from a Sphingomonas paucimobilis UT26, reportedly possessing γ-HCH degradation activity, were used as gene probes against isolated colonies. The isolate was found to grow and utilize γ-HCH as the sole carbon and energy source. The 16S ribosomal RNA gene sequence of the isolate resulted in its identification as a Xanthomonas species, and we designated it as strain ICH12. During the degradation of γ-HCH by ICH12, formation of two intermediates, γ-2,3,4,5,6-pentachlorocyclohexene (γ-PCCH), and 2,5-dichlorobenzoquinone (2,5-DCBQ), were identified by gas chromatography-mass spectrometric (GC-MS) analysis. While γ-PCCH was reported previously, 2,5-dichlorohydroquinone was a novel metabolite from HCH degradation.

Conclusions:  A Xanthomonas sp. for γ-HCH degradation from a contaminated soil was isolated. γ-HCH was utilized as sole source of carbon and energy, and the degradation proceeds by successive dechlorination. Two degradation products γ-PCCH and 2,5-DCBQ were characterized, and the latter metabolite was not known in contrasts with the previous studies. The present work, for the first time, demonstrates the potential of a Xanthomonas species to degrade a recalcitrant and widespread pollutant like γ-HCH.

Significance and Impact of the Study:  This study demonstrates the isolation and characterization of a novel HCH-degrading bacterium. Further results provide an insight into the novel degradation pathway which may exist in diverse HCH-degrading bacteria in contaminated soils leading to bioremediation of γ-HCH.