Abstract
This study was conducted to identify and characterize bacteria capable of degrading spent oil contaminated soil. The physicochemical parameters of used engine oil were analyzed according to existing standards, while the total heterotrophic plate counts (HBC) and hydrocarbon utilizing bacteria counts were ascertained with the pour plate methods using nutrient agar and minimal salt agar (MSA) media respectively. The results indicated a mean total HBC ranging from 2.86 ± 0.08 to 5.76 log10 CFU/g and mean hydrocarbon utilizing bacterial counts from 1.32 ± 0.09 to 3.82 ± 0.25 log10 CFU/g, with samples enriched with Glycine max (Soybean) recorded to have the highest bacterial counts. The phenotypic identification of the hydrocarbon utilizing bacteria as sources of carbon and energy showed the presence of two primary bacterial genera: Bacillus and Micrococcus.However, from the overall 50 counted colonies, the frequency of occurrence for Bacillus was 41 (82 %) whereas, the Micrococcus was (9) 18%. The obtained data, confirmed the breakdown capacity of autochthonous (indigenous) organisms notably; Bacillus in the reduction of pollutants linked with oil spillage. This provides for reliable and promising approach of ameliorating crude oil pollutants and its inherent threats. 
 Keywords: Soil, spent oil, Glycine max, degrading bacteria, isolation and bioremediation
Highlights
This study was conducted to identify and characterize bacteria capable of degrading spent oil contaminated soil
Spent engine oil or used motor oil abounds in oil mechanic workshops and industrial sources that may occur as hydraulic oil, turbine, process and brake oils and consists of large number of long-chain saturated hydrocarbons
The crude oil is a mixture of hydrocarbon and hydrocarbon-like, polycyclic aromatic hydrocarbons (PAHs) and heavy metals (Cappello et al, 2007)
Summary
Sample Collections: One kilogram of soil sample contaminated with spent engine oil was obtained from a motor mechanic workshop along Mission Road, in Okada, Ovia North East Local Government Area, Edo State. Ten grams (10 g) of the contaminated soil sample from the sampling site was blended with soybean, measured into 2 polyethene bags, label A and B Another 10 g of the contaminated spent engine oil was taken into 2 other polyethene bags as samples C and D, while an uncontaminated soil sample was collected from the Igbinedion University, Okada botanical garden. Isolation and Enumeration of bacteria: Isolation of bacteria from spent engine oil contaminated soil samples was performed according to methods established (Atlas, 2004; Udeani et al, 2009). Identification and characterization: Individual bacterial colonies were picked based on different colony morphology and color and further sub-cultured by pouring onto fresh nutrient agar slants and incubated at room temperature for 48 hr. The bacterial colonies on agar plates were counted and subsequently expressed as hydrocarbon utilizing bacteria per ml of spent oil. Identification and characterization of hydrocarbon utilizing bacteria: The isolates were identified and characterized using morphological and biochemical tests respectively (Mandigan et al, 2017)
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