Surface-active lipids in rhodococci

Like other hydrocarbon-oxidising bacteria, rhodococci respond to the presen ce of alkanes by producing biosurfactant molecules to improve their ability to utilise these hydrophobic compounds as growth substrates. In the rhodoc occi these surfactants are predominantly glycolipids, the majority of which remain cell-bound during unrestricted growth. Most work has been done on t he trehalose mycolates formed by Rhodococcus erythropolis, but nitrogen-lim ited conditions lead to the production of anionic trehalose tetraesters als o. As surfactants, these compounds, whether purified or in crude form, are abl e to reduce the surface tension of water from 72 mN m-l to a low of 26, thu s making them among the most potent biosurfactants known. They are also abl e to reduce the interfacial tension between water and a hydrophobic phase ( e.g. n-hexadecane) from 43 mN m(-1) to values less than one (Table 1). Bios urfactants have about a ten- to 40-fold lower critical micelle concentratio n than synthetic surfactants. Such properties suggest a range of industrial applications, where a variety of surface-active characteristics are approp riate. Interest in biosurfactants as industrial chemicals results from the toxicity of many petrochemical-derived surfactants. Currently world-wide su rfactant production is on a very large scale, and the demand for them is in creasing. However, the drive towards less environmentally damaging chemical s makes biosurfactants attractive as they have lower toxicity. The reason they have not achieved a significant market share is the cost of production, which is considerably higher than for synthetic surfactants. T his problem is being addressed using several strategies. An approach where there is great scope for improvement with the rhodococci is an understandin g of the genetic basis of glycolipid production, which is largely unknown. They may find applications in the near future in the environmental remediat ion industries, where the requirement for purified molecules is of less imp ortance. This review summarises knowledge of the chemistry, biochemistry and product ion of Rhodococcus surface-active lipids. Where they have been used, or the re is potential for use, in industrial applications is discussed.