Biodegradation of imidacloprid by an isolated soil microorganism

Imidacloprid (1-[(6-chloro-3-pyridinyl)-methyl]-N-nitro-2-imidazolidinimine), a chloronicotinyl insecticide used to control biting and sucking insects, is very persistent in the soil with a half-life often greater than 100 days. Although a few soil metabolites have been reported in the literature, there are no reports of imidacloprid-degrading soil microorganisms. Our objectives were to discover, isolate, and characterize microorganisms capable of degrading imidacloprid in soil. Two soil-free stable enrichment cultures in N-limited media were obtained that degraded 19 mg L-1 (43%) and 11 mg L-1 (16%) of the applied imidacloprid, and produced about 19 mg L-1 6-chloronicotinic acid in three weeks. Enrichment media without microorganisms had no loss of imidacloprid. Strain PC-21, obtained from the enrichment cultures, degraded 37% to 58% of 25 mg L-1 imidacloprid in tryptic soy broth containing 1 g L-1 succinate and D-glucose at 27 degrees C incubation over a period of three weeks. Trace amounts of NO3-/NO2- were produced and six metabolites were characterized by high performance liquid chromatography (HPLC) using C-14-methylene-imidacloprid and liquid chromatograph-electrospray-mass spectrometer (LC-MS). Two of the metabolites were identified as imidacloprid-guanidine and imidacloprid-urea by HPLC standards and LC-MS. During the experiment, 6-chloronicotinic acid was not produced. Less than 1% of the applied C-14 was incorporated into the microbial biomass and no (CO2)-C-14 was detected. Strain PC-21, identified as a species of Leifsonia by PCR amplification of a 500 bp sequence of 16s rRNA, cometabolized imidacloprid.