Isolation and characterization of efficient isoxaben‐transforming Microbacterium sp strains from four European soils

Abstract Nutrient‐agar plates containing isoxaben (500 mg litre −1 ) were used to isolate isoxaben‐metabolising bacteria from four European soils incubated with the herbicide under laboratory conditions. In flask experiments, inoculation of a basal salts medium containing nitrogen and [ phenyl ‐U‐ 14 C]isoxaben with an isolate (B2b) resulted in 33% recovery of the initial radioactivity as [ 14 C]carbon dioxide after 2 weeks. A major metabolite identified by GC–MS and NMR analysis as 3‐(1‐ethyl‐1‐methylpropyl)isoxazol‐5‐ylamine accumulated both in basal salts and nutrient broth media. 2,6‐Dimethoxybenzoic acid, a suspected metabolite of isoxaben, was not detected in either liquid media. However, the capability of the B2b isolate to use 2,6‐dimethoxybenzoic acid as a source of carbon was demonstrated. Soil inoculation with the B2b strain resulted in an increase in the recovery of [ 14 C]carbon dioxide from both [ phenyl ‐U‐ 14 C] and [ isoxazole ‐5‐ 14 C]isoxaben. The metabolite identified as 3‐(1‐ethyl‐1‐methylpropyl)isoxazole‐5‐ylamine only accumulated if the soil was autoclaved before inoculation. This metabolite was rapidly mineralized by the microflora of a natural soil without history of isoxaben treatment. Homology patterns of sequenced 16S rDNA between isoxaben‐transforming isolates and reference strains showed that the four isolates identified belonged to the genus Microbacterium . © 2002 Society of Chemical Industry