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A novel mechanistic model for the growth of baker's yeast on glucoseis presented. It is based on the fact that glucose degradation proceeds via two pathways under conditions of aerobic ethanol formation. Part is metabolized oxidatively and part reductively, with ethanol being the end product of reductive energy metabolism. The corresponding metabolic state is designated oxidoreductive. Ethanol can be used oxidatively only. Maximum rates of oxidative glucose and ethanol degradation are governed by the respiratory capacity of the cells. The model is formulated by using the stoichiometric growth equations for pure oxidative and reductive (fermentative) glucose and ethanol metabolism. Together with the experimentally determinable yield coefficients (Y(X/S)) for the respective metabolic pathways, the resulting equation system is sufficiently determined. The superiority of the presented model over hitherto published ones is based on two essential novelities. (1) The model was developed on experimentally easily accessible parameters only. (2) For the modeling of aerobic ethanol formation, the substrate flow was split into two simultaneously operating (i.e., in parallel) metabolic pathways that exhibit different but constant energy-generating efficiencies (respiration and fermentation) and consequently different and constant biomass yields (Y(X/S)). The model allows the prediction of experimental data without parameter adaption in a biologically dubious manner.

We isolated transposon TnS-GM-induced mutants of Pseudomonas aeruginosa PG201 that were unable to grow in minimal media containing hexadecane as a carbon source. Some of these mutants lacked extracellular rhamnolipids, as shown by measuring the surface and interfacial tensions of the cell culture supernatants. Furthermore, the concentrated culture media of the mutant strains were tested for the presence of rhamnolipids by thin-layer chromatography and for rhamnolipid activities, including hemolysis and growth inhibition ofBacilus subtilis. Many procaryotic and eucaryotic microorganisms satisfy their carbon and energy requirements by using compounds, such as hydrocarbons, that are poorly soluble in aqueous media. The growth on hydrocarbons is often associated with the production of surface-active compounds. Surface-active molecules contain hydrophilic and hydrophobic components, a property that enables such molecules to concentrate at interfaces and to reduce the surface tensions of aqueous media. Several different microbial products that exhibit surface-active properties have been identified in the past. These so-called biosurfactants are produced by certain bacteria and by a number of yeasts and filamentous fungi. They include low-molecular-weight glycolipids, lipopeptides, and high-molecular-weight lipid-containing polymers such as lipoproteins, lipopolysaccharide-protein complexes, and polysaccharide-protein-fatty acid complexes. The common hydrophobic (lipophilic) moiety in biosurfactants is the hydrocarbon chain of a fatty acid, whereas the polar or hydrophilic group is derived from the ester or alcohol functional groups of neutral lipids, from the carboxylate group of fatty acids or amino acids, or, in the case of glycolipids, from carbohydrates. When the surfactants are extracellular, they cause the emulsification of the hydrocarbon. When they are cell wall associated, they facilitate the penetration of hydrocarbons to the periplasmic space. Many of the biosurfactants known today have been investigated with a view toward possible technical applications. Because biosurfactants are readily biodegradable and can be produced in large amounts by microorganisms and thus are not dependent on petroleum-derived products, they might well be able to replace, in some instances, the traditional synthetic surfactants. The structures, properties, and production of biosurfactants have been reviewed extensively; the overview of Reiser et al. (34) is the most recent. * Corresponding author.The rhamnose-containing glycolipids produced by Pseudomonas spp. (20,22,38) are among the biosurfactants to be studied most intensively. The rhamnolipids from Pseudomonas aeruginosa were first described in 1949 (24), and studies on the biosynthesis of these compounds were carried out in vivo by , who showed that these glycolipids were secreted into the medium during the stationary phase of growth. They also defined the optimal conditions for rhamnolipid production by this organism from various radioactive precursors, such as acetate, gly...

\The respiratory capacity of Saccharomyces cerevisiae growing in continuous culture on glucose and on mixtures of glucose and ethanol was investigated. An oxygen uptake rate of 8 mmol g-l h-l was found to limit the ability of the organism to degrade a substrate purely oxidatively. On glucose as sole energy and carbon source, this respiration rate was invariably achieved at an identical growth rate and thus at an identical substrate uptake rate when the inlet glucose concentration was varied. The rate of ethanol co-consumption together with glucose was strictly governed by this limiting maximum respiratory capacity and no repression of respiration was observed at dilution rates where ethanol was excreted by the cells. Hence, a limitation in some step in the oxidative branch of catabolism is likely to be responsible for incomplete oxidation of glucose at high growth rates rather than an undefined action of glucose repression.

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