Submitted to: Applied and Environmental Microbiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/18/2000
Publication Date: 7/1/2000
Citation: Interpretive Summary: Chemicals with the properties of detergents are sometimes used to aid in the cleanup of soil and water using biological processes. These compounds act by helping to bring chemicals that are not water soluble into solution so that microbes can attack them. Most detergents are synthetic chemicals. Rhamnolipids are class of natural compounds with detergent-like properties. They are produced by numerous strains of bacteria and help those bacteria degrade difficult substrates. The rhamnolipids produced by bacteria can have several forms, differing in the number of rhamnose moieties attached to the molecule (they can have 1 or 2) or in the form of the fatty acid moieties attached. This study investigated the production of rhamnolipid by a strain of the bacterium Pseudomonas aeruginosa. The effect of different culture techniques on the amount and type of rhamnolipid produced were studied. The greatest amount of rhamnolipid was produced when the bacteria were fed corn oil as carbon source. Spacing out the addition of nitrogen to the culture also increased the amount of rhamnolipid produced. In no case, did different culture conditions change the ratio of rhamnolipid types in the mixture that was produced. This information will be useful to scientists and engineers involved in the cleanup of soil and water and to the cosmetic and food industries where rhamnolipids are used.
Technical Abstract: The production of rhamnolipid biosurfactants by P. aeruginosa UG2 was examined under different culture conditions. Rhamnolipid yield was affected by the nature of the carbon sources, the nutrient concentrations, pH, and age of the culture. Hydrophobic substrates like corn oil, lard (rich in unsaturated and saturated fat), and long chain alcohols maximized biosurfactant production (100-165 mg/g substrate). Hydrophilic substrates like glucose, and succinic acid delivered poor yields (12-36 mg/g substrate). Rhamnolipid production was greater when N as (NH4)2SO4 and trace metals were added in several periodic doses rather than at the beginning of the process. Increased biosurfactant production was seen in cultures maintained at neutral pH relative to cultures allowed to develop acidic conditions (pH = 6.25). Although the level of rhamnolipid production was affected by culture conditions, the distribution of rhamnolipid subspecies did not vary between cultures. A dirhamnolipid species containing two 10 carbon alpha-hydroxy fatty acids [Rh2C10C10] was the most abundant in the mixtures (60.6% mol), while the levels of the monorhamnolipid [RhC10C10] (20.7%) and two dirhamno variants containing both 10 carbon and 12 carbon alpha-hydroxy fatty acids [Rh2C10C12 and it's dehydro variant Rh2C10C12-H2 ] (18.7%) were similar. Biosurfactant mixtures produced with corn oil as sole carbon source solubilized the herbicide trifluralin [2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl)benzamine] to a greater extent. This suggests that the presence of incompletely metabolized hydrophobic by-products acting as cosolvents can increase the solubilization capacity of biosurfactant mixtures.