ABSTRACT
RhamnolipidsRhamnolipids are mainly synthesized by the genus Pseudomonas, especially P. aeruginosa strains; under growth limiting conditions with water immiscible carbon sources (cf. Section 17.3.3). P. aeruginosa is an opportunistic pathogen. It is a Gram-negative, monotrichous and ubiquitous soil bacterium. P. aeruginosa synthesizes different cell associated and extracellular virulence factors, one of them being rhamnolipids. 17.2.1 Rhamnolipid Structure
The di-rhamnolipid (α-l-rhamnopyranosyl-(1-2)-α-l-rhamnopyranosyl-3-hydroxydecanoyl-3-hydroxydecanoate also called rhamno-lipid 3 or Rha-Rha-C10-C10) characterized by Jarvis and Johnson (1949) is only one of over 28 rhamnolipid congeners described for P. aeruginosa, varying mostly in fatty acid chain length and degree of saturation (Déziel et al., 1999). Chain lengths vary from octanoic (-C8) to dodecanoic (-C12) acids. Single double bonds are observed for all chain lengths (e.g., -C10:1, -C12:1). The major part of the rhamnolipids contains two fatty acid residues, generally of the same length. Most rhamnolipid congeners are produced in traces. Apart from di-rhamnolipid, a mono-rhamnolipid (α-l-rhamnopyranosyl-
3-hydroxydecanoyl-3-hydroxydecanoate also called rhamnolipid 1 or Rha-C10-C10) is predominant in P. aeruginosa cultivations. The so-called rhamnolipids 2 and 4, are a di-and a mono-rhamnolipid (Rha-Rha-C10 and Rha-C10, respectively), with only one β-hydroxydecanoic acid residue, respectively. They were reported for resting cells of Pseudomonas sp. DSM 2874 cultivated at 37°C (Syldatk et al., 1985a, 1985b). The generalized chemical structure and nomenclature for mono-and di-rhamnolipids is presented in Fig. 17.1.The rhamnolipids 1-4 are termed after their order of appearance on a thin-layer chromatography of an extract from Pseudomonas sp. DSM 2874 (Syldatk et al., 1985a, 1985b). Although the terms rhamnolipid 1-4 are obsolete, they are still often used. However, in the following sections the generalized nomenclature will be used (cf. Fig. 17.1). Two further rhamnolipids are αC10:1-Rha-C10-C10 (rhamnolipid A) and αC10:1-Rha-Rha-C10-C10 (rhamnolipid B), which were reported by Yamaguchi and Sato (1976) and represent α-decenoic acid (αC10:1) acylated derivatives of Rha-C10-C10(rhamnolipid 1) and Rha-Rha-C10-C10 (rhamnolipid 3), respectively.
