Physiological adaptations involved in alkane assimilation at a low temperature by Rhodococcus sp strain Q15

We examined physiological adaptations which allow the psychrotroph Rhodococ cus sp. strain Q15 to assimilate alkanes at a low temperature (alkanes are contaminants which are generally insoluble and/or solid at low temperatures ). During growth at 5 degrees C on hexadecane or diesel fuel, strain Q15 pr oduced a cell surface-associated biosurfactant(s) and, compared to glucose- acetate-grown cells, exhibited increased cell surface hydrophobicity. A tra nsmission electron microscopy examination of strain Q15 grown at 5 degrees C revealed the presence of intracellular electron-transparent inclusions an d flocs of cells connected by an extracellular polymeric substance (EPS) wh en cells were grown on a hydrocarbon and morphological differences between the EPS of glucose-acetate-grown and diesel fuel-grown cells. A lectin bind ing analysis performed by using confocal scanning laser microscopy (CSLM) s howed that the EPS contained a complex mixture of glycoconjugates, dependin g on both the growth temperature and the carbon source. Two glycoconjugates [beta-D-Gal-(1-3)-D-GlcNAc and alpha-L-fucose] were detected only on the s urfaces of cells groan on diesel fuel at 5 degrees C. Using scanning electr on microscopy, we observed strain Q15 cells on the surfaces of octacosane c rystals, and using CSLM, we observed strain Q15 cells covering the surfaces of diesel fuel microdroplets; these findings indicate that this organism a ssimilates both solid and liquid alkane substrates at a low temperature by adhering to the alkane phase. Membrane fatty acid analysis demonstrated tha t strain Q15 adapted to growth at a low temperature by decreasing the degre e of saturation of membrane lipid fatty acids, but it did so to a lesser ex tent when it was groan on hydrocarbons at 5 degrees C; these findings sugge st that strain Q15 modulates membrane fluidity in response to the counterac ting influences of low temperature and hydrocarbon toxicity.