Rhamnolipid production by a novel thermophilic hydrocarbon-degrading Pseudomonas aeruginosa AP02-1
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Thermophilic bacterial cultures were isolated from a hot spring environment on hydrocarbon containing mineral salts media. One strain identified as Pseudomonas aeruginosa AP02-1 was tested for the ability to utilize a range of hydrocarbons both n-alkanes and polycyclic aromatic hydrocarbons as sole carbon source. Strain AP02-1 had an optimum growth temperature of 45°C and degraded 99% of crude oil 1% (v/v) and diesel oil 2% (v/v) when added to a basal mineral medium within 7 days of incubation. Surface activity measurements indicated that biosurfactants, mainly glycolipid in nature, were produced during the microbial growth on hydrocarbons as well as on both water-soluble and insoluble substrates. Mass spectrometry analysis showed different types of rhamnolipid production depending on the carbon substrate and culture conditions. Grown on glycerol, P. aeruginosa AP02-1 produced a mixture of ten rhamnolipid homologues, of which Rha-Rha-C10-C10 and Rha-C10-C10 were predominant. Rhamnolipid-containing culture broths reduced the surface tension to ≈28 mN and gave stable emulsions with a number of hydrocarbons and remained effective after sterilization. Microscopic observations of the emulsions suggested that hydrophobic cells acted as emulsion-stabilizing agents.
This work was partially supported by the CEC EU Structural Funds, Building Sustainable Prosperity, Measure 5.1 ‘Sustainable Management of the Environment and Promotion of the Natural and Built Heritage (BSP7473), Environment and Heritage Service, N. Ireland. We also like to thank Dr. A.R. Taddei of the Centre for Electron Microscopy at University of Tuscia, Italy, for EM negative stain analyses.
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