Microbial biosurfactants
Surfactants and emulsifiers are widely used for food, cosmetic and pharmaceutical applications. Most compounds are still synthesized chemically from hydrocarbons, however surface-active molecules of biological origin -so called biosurfactants -have gained considerable interest in recent years. Biosurfactants can be obtained either by chemical syntheses from renewable resources, by microbial fermentation processes, or by enzymatic syntheses. Surfactants from renewable resources are reviewed in .Biosurfactants display a wide variety of molecular structures. Their hydrophilic moiety often contains a mono-, oligo-or polysaccharide, an amino acid, peptide or protein, whereas the hydrophobic part is composed of saturated or unsaturated fatty acids, hydroxy fatty acids or fatty alcohols . According to Schoerken and Kempers [3], biosurfactants can be classified in (i) glycolipids (e.g. rhamnolipids, sophorolipids, trehalose lipids), (ii) lipopeptides (e.g. surfactin, liposan), (iii) phospholipids, (iv) neutral lipids (e.g. corynomycolic acid), (v) polymeric surfactants (e.g. emulsan, liposan) and (vi) particulate biosurfactants (vesicles, whole cells). The physicochemical properties of many biosurfactants are comparable to chemically synthesized compounds. Advantageously, they can be produced from renewable resource substrates or industrial waste products and are biodegradable. In addition, several biosurfactants have been reported to have manifold biological activities and include antibiotics, fungicides, insecticides, antiviral and antitumor agents, immunomodulators, or specific toxins or enzyme inhibitors.Of special interest for industrial use are microbial glycolipids: they occur either as an important part of the microbial cell surface, enabling the contact with hydrophobic substrates, or they are secreted as emulsifiers in the culture broth when grown on water immiscible or oily substrates. The main group of low molecular weight biosurfactants contain carbohydrate moieties bound to fatty acid or hydroxy-fatty acid chains. These surfactants possess many commercially attractive properties and offer clear advantages over their synthetic counterparts. The most important members of this group are sophorolipids, rhamnolipids and trehalose lipids, which are all covered by the present special issue of EJLST.Although the production of microbial glycolipids has been studied extensively in the last decades, commercial scale production has been realized only in few cases [3], and to date, they are only found in niche applications due to their distinctly higher prices compared to traditional surfactants . An interesting option for the future could be their use for microbial enhanced oil recovery (MEOR), provided that the production costs could be reduced .The reasons for the still limited use of microbial glycolipids in industry include the use of expensive substrates, limited product concentrations, and low yields -often caused by substrate and/or product inhibitions, and the formation of product mixtures rather than pure compounds. These are the causes of high costs of the downstream processing. In some cases, the microbial production strains are even pathogenic or difficult to handle on a larger scale because of oxygen transfer problems and extensive foam formation requiring the use of antifoaming agents.According to Mukherjee et al.[2] three main strategies could be used to make the production of biosurfactants more cost-competitive: (i) screening for overproducing nonpathogenic wild type, mutant or recombinant strains, (ii) the use of cheaper and/or waste substrates to lower the initial raw material costs involved in the process and (iii) development of more efficient bioprocesses including optimization of culture conditions, as well as costeffective separation processes for maximum biosurfactant recovery.This special issue of EJLST focuses on the production and application of microbial glycolipids with emphasis on sophorolipids from Candida bombicola, rhamnolipids from nonpathogenic bacterial strains and trehaloselipids. It covers different aspects of microbial