Characterization of Rhodococcus opacus R7, a strain able to degrade naphthalene and o-xylene isolated from a polycyclic aromatic hydrocarbon-contaminated soil

Rhodococcus opacus R7 was isolated from a soil contaminated with polycyclic aromatic hydrocarbons for its ability to grow on naphthalene. The strain w as also able to degrade o-xylene, the isomer of xylenes most recalcitrant t o microbial degradation. The catabolic pathways for naphthalene and o-xylen e were investigated by identification of metabolites in R. opacus R7 cultur es performed with the two hydrocarbons and by evaluation of some enzymes in volved in the metabolism of these compounds. 1,2-Dihydro-1,2-dihydroxynapht halene, salicylic and gentisic acids were identified as metabolites in cult ures exposed to naphthalene. This suggests that the degradation occurs thro ugh the dioxygenation of the aromatic ring with the formation of 1,2-dihydr o-1,2-dihydroxynaphthalene, dehydrogenated to the corresponding 1,2-dihydro xy derivative which is further oxidized to salicylic acid, a key intermedia te of naphthalene metabolism; this compound is converted to gentisic acid c leaved by a gentisate 1,2-dioxygenase. From R. opacus R7 cultures supplied with o-xylene, 2,3-dimethylphenol and 3,4-dimethylcatechol were observed. T he pathway of o-xylene involves the monooxygenation of the benzene nucleus leading to dimethylphenol which is further metabolised to 3,4-dimethylcatec hol, followed by a meta cleavage reaction, catalyzed by the catechol 2,3-di oxygenase. R. opacus R7 is the first strain thus far described both in Gram -negative and Gram-positive bacteria which has the ability to degrade both a polycyclic aromatic hydrocarbon such as naphthalene and a monocyclic arom atic hydrocarbon such as o-xylene. (C) 2001 Editions scientifiques et medic ales Elsevier SAS.