Biomedical subjects

G S Shreve

Publications and source records attributed to G S Shreve.

3 recordsLinked to original sources

PubMed1999

Kinetic model of biosurfactant-enhanced hexadecane biodegradation by Pseudomonas aeruginosa.

Many sites of environmental concern contain groundwater contaminated with nonaqueous phase liquids (NAPL). In such sites interfacial processes may affect both the equilibrium and kinetic behavior of the system. In particular, insoluble hydrocarbon partitioning and microbial biodegradation of insoluble hydrocarbon are influenced by the physicochemical and interfacial characteristics of the system. A mechanistic model describing the influence of biological surfactants on microbial biodegradation of liquid-phase insoluble hydrocarbon and subsequent reduction of nonaqueous-phase liquid hydrocarbon is presented. The model consists of six coupled differential equations which use lumped kinetic parameters to describe surfactant micelle formation and diffusion to the microbial cell, nonlinear kinetic expressions for microbial growth and degradation of insoluble hydrocarbon, kinetic spatial descriptions of the change in NAPL-phase droplet size and the organic phase volume fraction with time, as well as equilibrium partitioning expressions for hydrophobic organic contaminant partioning into the surfactant micelle. The model is validated by comparison to data obtained for hexadecane degradation in a well-mixed batch system by the biosurfactant producing microorganism Pseudomonas aeruginosa strain PG201 as well as for nonproducing mutants' growth and hexadecane biodegradation in the presence of exogenously added biosurfactant. Experimentally determined biological growth parameters, as well as physical parameters such as hydrocarbon droplet size, were applied in the kinetic model. Parameter sensitivity analysis was performed on the physical and biological parameters in the model. The parameter sensitivity analysis indicates that for the biological system examined the rate of hydrocarbon solubilization and micellar transport to the cell controls the rate at which cellular uptake and biodegradation of insoluble hydrocarbon occurs. Practical aspects relating to use of the model for support of surfactant-based bioremediation efforts are discussed.

Alkanes↗

PubMed1995

Rhamnolipid biosurfactant enhancement of hexadecane biodegradation by Pseudomonas aeruginosa.

Mutants of Pseudomonas aeruginosa that produce and do not produce rhamnolipid biosurfactant are used to investigate the influence of cell-associated biosurfactant on cellular association with the hydrocarbon-water interface and on hydrocarbon uptake. Rhamnolipid-nonproducing mutant 65E12 of P. aeruginosa is unable to grow in minimal media containing hexadecane as a carbon source in the absence of exogenously added surfactant. Mutant PG201::rhlR grows very slowly in the absence of exogenously added surfactants. Both mutants are deficient in the positive regulatory gene controlling the activation of rhamnolipid synthesis. 65E12 is a double mutant that is also deficient in lipopolysaccharide synthesis. However, growth on hexadecane may be restored to varying degrees when small amounts of purified rhamnolipids or the synthetic anionic surfactant alkyl benzene sulfonate (ABS) is added to the cultures. Rhamnolipid biosurfactant is shown to be approximately 9 times more effective than the structurally similar synthetic anionic surfactant ABS in solubilizing hydrocarbon into the aqueous phase. Physical characteristics of the rhamnolipid and ABS micelles as determined by laser light scattering are described to explain the greater effectiveness of the rhamnolipid in solubilizing hexadecane. The cellular attachment to hydrocarbon-water interfaces and cellular aggregation of the wild-type and mutant strains are examined in the presence and absence of rhamnolipid or synthetic ABS surfactants. Differences in observed hexadecane degradation rates are explained on the basis of emulsified hexadecane concentration, cell surface hydrophobicity, and cellular localization in the culture.

Alkanes↗

PubMedDate not supplied

Solvent selection and productivity in multiphase biotransformation systems.

The 11-beta hydroxylation of the steroid precursor Reichstein's substance S (cortexolone) to hydrocortisone by Curvularia lunata cells is used as a model system for investigating the basis of the effect of solvent choice on multiphase biotransformation reaction kinetics. The effect of solvent choice and phase ratio on cellular toxicity and productivity is examined in batch suspension cultures. In general, the greater the miscibility of the solvent with the aqueous phase, the more cellular toxicity it exhibits. However, the presence of solvent at concentrations above the solubility limit showed no increased toxic effect over that achieved with saturation concentrations. Toxicity experiments indicate that of the solvents examined octane is the least toxic to the Curvularia lunata substance S biotransformation system under consideration. The production of hydrocortisone is determined to be approximately 20 times greater during the growth phase than during the stationary phase. The phase ratio of the organic solvent added has a significant effect on the productivity of the system. It was found that there is an increase in productivity with an increase in the phase ratio of the organic solvent added. Reaction kinetics are examined, and the increase in productivity with increasing amounts of organic solvent is attributed to the increase in concentration of the steroid reactant in the system.

Biotransformation↗