Biodegradation of heavy petroleum polycyclic aromatic hydrocarbons (PAHs) in polluted soil by biofilm-forming Bacillus tropicus UCB and Pseudomonas aeruginosa SYLI isolated from crude oil-contaminated sludge

Crude oil pollution poses a threat to soil ecosystems, particularly in oil-producing regions. This study assessed the biodegradation potential of biofilm-forming Bacillus tropicus UCB and Pseudomonas aeruginosa SYLI isolated from crude oil sludge. Sludge samples were seasonally collected and bacterial counts determined using standard methods while microbial enrichment was conducted in mineral salt medium containing 1% crude oil. Biofilm formation was assessed using Congo red agar and microplate assays. Isolates were identified through cultural, biochemical, and 16S rDNA analysis. Dose–response toxicity test examined degradation across 1%, 3%, 7%, and 10% crude oil concentrations, while PAHs degradation in soil microcosm was analysed using GC–MS. Seasonal variations significantly influenced bacterial populations, with highest count (1.53 × 10⁸ CFU/mL) in the dry season and the least 3.17 × 10⁶ CFU/mL) during wet season. Optical density peaked at 2.86 nm in enrichment III. Results revealed molecular identities of the isolates as B. tropicus UCB and P. aeruginosa SYLI. Both isolates metabolized crude oil from 1 to 10%, with B. tropicus producing 601 mg/L CO₂ with 10% at day 12 and P. aeruginosa yielding 616 mg/L with 1% at day 4. In addition, results showed over 99% removal of low molecular weight PAHs and 75% degradation of high molecular weight PAHs, upon biostimulation. These findings highlight complementary strengths of B. tropicus on high-oil loads and P. aeruginosa rapid initial degradation at lower concentrations. This study suggests that biofilm formation coupled with biostimulation may improve bacterial efficiency in bioremediation. It also represents the first in vitro report on PAHs degradation by Bacillus tropicus.