Bioremediation of fipronil by a Bacillus firmus isolate from soil

• Bacillus firmus proved its potential in efficient metabolization of fipronil.
• Fipronil were not detected after 42 d in soil when fortified @ 1.50 mg kg−1.
• Fipronil sulfide was found to be the main metabolite followed by sulfone and amide.
• Total fipronil residues were not found to follow the first order kinetics.

Persistence of fipronil, a new molecule in extensive use against various insect pests is causing serious problems to the environment. Bacillus firmus was isolated by selective enrichment from soil samples collected from sugar fields with known history of pesticide usage and evaluated for metabolization of fipronil in clay loam soil. Soil samples in 50 g aliquotes were fortified with fipronil @ 0.50–1.50 mg kg−1 and inoculated with B. firmus cells (45 × 107 CFU mL−1) and incubated at 25 °C. Each sample in triplicates was drawn periodically up to 56 d and residual fipronil contents analyzed by gas liquid chromatograph. Fipronil residues were not detected after 35 d at lower doses of fipronil (@ 0.50, 0.75 and 1.00 mg kg−1). However, at higher doses (@ 1.25 and 1.50 mg kg−1) than this total metabolization of fipronil could be observed after 35 and 42 d, respectively. Thus whereas, B. firmus proved its potential in efficient metabolization of fipronil, the period required for the same was dose dependent. Amongst various metabolites of fipronil degradation, fipronil sulfide was found to be the main metabolite followed by fipronil sulfone and fipronil amide. Though, desulfinyl metabolite earlier reported as one of the main metabolite of fipronil degradation, the existence of the same was not detected in any of the treatment.