Rhizoremediation of lindane by root-colonizing Sphingomonas

We used a two-step enrichment approach to isolate root-colonizing hexachlorocyclohexane (HCH)-degrading microorganisms. The first step consists of the use of classical liquid enrichment to isolate γ-HCH degraders. The γ-HCH-degrading microbes were attached in mass to corn seeds sown in soil with γ-HCH, and after plant development we rescued bacteria growing on root tips. Bacteria were then subjected to a second enrichment round in which growth on liquid medium with γ-HCH and inoculation of corn seeds were repeated. We then isolated bacteria on M9 minimal medium with γ-HCH from root tips. We were able to isolate four Sphingomonas strains, all of which degraded α-, β-, γ- and δ-HCH. Two of the strains were particularly good colonizers of corn roots, reaching high cell density in vegetated soil and partly removing γ-HCH. In contrast, these bacteria performed poorly in unplanted soils. This study supports the hypothesis that the removal of persistent toxic chemicals can be accelerated by combinations of plants and bacteria, a process generally known as rhizoremediation.