Bioassay-based screening of microorganisms that degrade dioxin using substrate-immobilized microtubes

In the current study, we attempted to develop a method for bioassay-based screening of microorganisms that degrade dioxin. However, a crucial problem encountered was that the standard dioxin 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) added to bacterial medium immediately disappeared from the liquid phase due to its adsorption onto polypropylene (PP) tubes. Among other aromatic hydrocarbons, adsorption onto PP tubes was also observed in β-naphthoflavone but not in benzo[a]pyrene. Adsorption of TCDD was observed not only onto PP tubes but also onto polystyrene, glass, and PP tubes with low affinity for DNA or protein. Silanization was not effective at preventing adsorption of TCDD. TCDD immobilized onto PP tubes was recovered by organic solvents, including ethanol, methanol, and dimethyl sulfoxide (DMSO). The elution efficiency of the immobilized TCDD by DMSO was approximately 85%. Based on these findings, screening of bacteria that degrade dioxin was attempted as follows. First, TCDD was immobilized onto PP tubes. Second, bacterial suspension was added to the tubes and incubated for biodegradation of TCDD. Third, remaining, immobilized TCDD was eluted by DMSO and subjected to a reporter bioassay to evaluate the level of TCDD. Using this method, we demonstrated successful screening of bacteria that have the potential for degradation of dioxin.