A Plasmid Induces Biodegradation of Pyrene by Bacillus cereus C7 and Identification of Its Degradation Metabolites

Pyrene is a ubiquitous, high molecular weight organic pollutant known for its high recalcitrance and toxicity. The Pyrene degrading bacterium, C7 was isolated from coal deposited soil samples by using selective enrichment culture technique, and its ability of Bacillus cereus C7 to degrade pyrene was determined by a 2,6-DCPIP assay. The bacterial isolate was characterized by 16S-rDNA sequencing as Bacillus cereus. B. cereus efficiently degraded 82.62% of the pyrene and concomitant increases in growth and protein concentration with an associated decrease in pyrene concentration were observed during the course of incubation. GC-MS analysis revealed that B. cereus degraded pyrene into seven metabolites viz. 4,5-dihydroxy pyrene, phenanthrene-4-carboxylic acid, 1-hydroxy-2-naphthaldehyde, 1-hydroxy-2-naphthoic acid, 1,2-dihydroxy naphthalene, salicylic acid and catechol. This study is the first to report salicylic acid and catechol as terminal end products of pyrene catabolism. Plasmid curing studies performed to determine the role of plasmids in pyrene degradation found drastic reduction in cell growth and pyrene degradation in cured culture. Thus, pyrene degradation in B. cereus is certainly plasmid-associated. The isolated plasmid has a molecular size of 11.27 kbp and contains restriction sites for the SmaI, StuI, NotI, BglII, PuvII and EcoRI enzymes. The plasmid encoding pyrene degradation in B. cereus C7 may have practical application in bioremediation and construction of genetically engineered strains with improved PAH degradation efficiency.