Study on the degradation pathway of benzo[a]pyrene by Bacillus sp. strain M1

Bacillus sp. strain M1, originally isolated from soil contaminated with polyaromatic hydrocarbons during earlier work of our group, has a strong ability to degrade benzo[a]pyrene (BaP). Here we show that the bacteria can utilize phthalic acid, protocatechuic acid, salicylic acid, catechol or gentisic acid as the sole carbon source, with utilization of 65.50-92.28% within 2 days. The metabolites formed during culture of Bacillus sp. strain M1 in mineral salt medium with BaP as the sole carbon source were detected by GC-MS. Intermediate metabolites were identified that included 4,5-dimethylphenanthrene, 9-ethyl-10-methylanthracene, 2-ethylacridine, phthalate derivatives, silylated catechol, and salicylic acid. The involvement of six relevant intracellular enzymes, namely phthalate dioxygenase, salicylate hydroxylase, catechol-1,2-dioxygenase, catechol-2,3-dioxygenase, gentisic acid dioxygenase and dehydrogenases, was demonstrated, and their activities correlated with the level of BaP degradation. Incorporating all findings, we propose a degradation pathway of BaP by strain M1 that starts with an enzymic attack of the C-9/C-10 or the C-7/C-8 bond. The pathway is summarized as BaP → cis-4,5-pyrene dihydrodiol → 4,5-dimethyl phenanthrene → 1-hydroxy-2-naphthalic acid. In addition, two new products, 9-ethyl-10-methylanthracene and 2-ethylacridine, were discovered.