Fatty acid and hydroxy acid adaptation in three gram-negative hydrocarbon-degrading bacteria in relation to carbon source

The lipids of three gram-negative bacteria, Acineto-bacter calcoaceticus, Marinobacter aquaeolei, and Pseudomonas oleovorans grown on mineral media supplemented with ammonium acetate or hydrocarbons, were isolated, purified, and their structures determined. Three pools of lipids were isolated according to a sequential procedure: unbound lipids extracted with organic solvents, comprising metabolic lipids and the main part of membrane lipids, OH⁻-labile lipids (mainly ester-bound in the lipopolysaccharides, LPS) and H⁺-labile lipids (mainly amidebound in the LPS). Unsaturated FA composition gave evidence for an aerobic desaturation pathway for the synthesis of these acids in A. calcoaceticus and M. aquaeolei, a nonclassic route in gram-negative bacteria. Surprisingly, both aerobic and anaerobic pathways are operating in the studied strain of P. oleovorans. The increase of the proportion of saturated FA observed for the strain of P. oleovorans grown on light hydrocarbons would increase the temperature transition of the lipids for maintaining the inner membrane fluidity. An opposite phenomenon occurs in A. calcoaceticus and M. aquaeolei grown on solid or highly viscous C₁₉ hydrocarbons. The increases of FA<C₁₈ when the bacteria were grown on n-nonadecane, or of iso-FA in cultures on iso-nonadecane would decrease the transition temperature of the lipids, to maintain the fluidity of the inner membranes. Moreover, P. oleovorans grown on hydrocarbons greatly decreases the proportion of β-hydroxy acids of LPS, thus likely maintaining the physical properties of the outer membrane. By contrast, no dramatic change in hydroxy acid composition occurred in the other two bacteria.