Pseudomonas fragi and Achromobacter aerogenes isolated from used engine oil polluted soils were grown in minimal salts medium (MSM) supplemented with used engine oil as sole carbon and energy source to evaluate their ability to biodegrade used engine oil. The two organisms utilized 73.3 and 80.0% of the oil with a degradation rate of 0.073 and 0.08 ml/day respectively. The utilization rate of the mixed culture did not differ significantly with an 80.0% utilization and 0.08 ml/day degradation rate. However the rate of utilization was reduced significantly after repeated sub culturing of the organisms on nutrient agar for six months with percentage utilization dropping to 33.3, 26.7 and 30.0% respectively for A. aerogenes, P. fragi and the mixed culture. This suggests that the presence of hydrocarbons in the growth medium is necessary for the stability of hydrocarbon utilization potentials of the isolates.
Key words: used engine oil, biodegradation, hydrocarbons, bacteria.
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