ABSTRACT
I. INTRODUCTION Certain microorganisms are able to convert preferably carbohydrates, nalkanes (C 10-C20), and triglycerides (fatty acids C10-C22) into biosurfactants. The carbon sources may be used separately or in combination with each other. Representative examples are low molecular mass glycolipids and lipopeptides as well as high molecular mass lipopolysaccharides and lipoproteins. Summaries on the microbial "workhorses," the chemical structures of biosurfactants, the microbial production data and, additionally, their potential commercial applications were presented recently [1-5]. Precondition for the overproduction of biosurfactants is, in principle, that first sufficient biomass, which provides the corresponding biosynthetic enzymes, has to be produced (Fig. 1). After initially studying a lot of parameters for the best cultivation conditions of the microorganism, e.g., nutrients, pH, temperature, aeration rate, etc., subsequently the conversion of special carbon sources to individual biosurfactants has to be investigated in detail (Fig. 2). Because of different substrates capable to be converted to glycolipids, e.g., their general biosynthesis pathways concerning degradation, gluconeogenesis, and lipid synthesis are presented in Fig. 3. If biosynthesis details of certain biosurfactants are amenable, they have been reported in later chapters. As for the cultivation methods to produce biosurfactants, generally the following routes are possible:
1. Growth-associated production 2. Production under growth-limiting conditions 3. Semicontinuous production with resting cells (free or immobilized) 4. Fed-batch and continuous production
In most cases, the yields of method 1 are substantially lower than those of methods 2-4. Depending on the carbon sources used for cultivation, the low
FIG. 3 Possible routes for the microbial dissimilation and assimilation of different carbon sources leading to glycolipids.
