Benzo(a)pyrene degradation pathway in Bacillus subtilis BMT4i (MTCC 9447)

Background: Benzo(a)pyrene (BaP), a high molecular weight pentacyclic aromatic hydrocarbon, is a priority pollutant of extreme concern. Bacillus subtilis BMT4i (MTCC 9447) degrades BaP through chromosomally encoded pathway. Nevertheless, inadequate information is available on BaP degradation pathway in genus Bacillus despite of its species being shown as potent BaP degrader. The objective of this study was to elucidate BaP degradation pathway in B. subtilis strain BMT4i by identifying metabolites through UHPLC-MS. Materials and methods: Batch experiments were conducted to characterize metabolic pathway of BaP in the bacterium B. subtilis BMT4i. The metabolites were separated and characterized by UHPLC-MS. Results: The major intermediates of BaP metabolism that had accumulated in the culture media after 15 days of incubation were benzo(a)pyrene-11,12-epoxide, 7,8,9,10- tetrahydrobenzo[pqr] tetraphene-7,8,9,10-tetraol, benzo(a)pyrene-cis-7,8-dihydrodiol, 8-carboxy-7-hydroxy pyrene,chrysene-4 or 5-carboxylicacid,cis-4-(8-hydroxypyrene-7yl)-2-oxobut-3-enoic acid, hydroxymethoxybenzo(a) pyrene and dimethoxybenzo(a)pyrene. Among above, 8-carboxy-7-hydroxy pyrene, chrysene-4 or 5-carboxylic acid, and cis-4-(8-hydroxypyrene-7yl)-2-oxobut-3-enoic acid are ring cleavage products of BaP. Conclusion: The identified metabolites indicated that BMT4i initially oxidized BaP with monooxygenases and dioxygenases at C-11,12 or and C-7,8 and C-9,10 positions, suggesting operation of multiple pathways for BaP degradation in B. subtilis. Further studies are essential to find out whether the entire biodegradation process in B. subtilis results into metabolic detoxification of BaP or not.