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Like other hydrocarbon-oxidising bacteria, rhodococci respond to the presence of alkanes by producing biosurfactant molecules to improve their ability to utilise these hydrophobic compounds as growth substrates. In the rhodococci these surfactants are predominantly glycolipids, the majority of which remain cell-bound during unrestricted growth. Most work has been done on the trehalose mycolates formed by Rhodococcus erythropolis, but nitrogen-limited conditions lead to the production of anionic trehalose tetraesters also. As surfactants, these compounds, whether purified or in crude form, are able to reduce the surface tension of water from 72 mN m-1 to a low of 26, thus making them among the most potent biosurfactants known. They are also able 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). Biosurfactants have about a ten- to 40-fold lower critical micelle concentration than synthetic surfactants. Such properties suggest a range of industrial applications, where a variety of surface-active characteristics are appropriate. Interest in biosurfactants as industrial chemicals results from the toxicity of many petrochemical-derived surfactants. Currently world-wide surfactant production is on a very large scale, and the demand for them is increasing. However, the drive towards less environmentally damaging chemicals 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. This problem is being addressed using several strategies. An approach where there is great scope for improvement with the rhodococci is an understanding of the genetic basis of glycolipid production, which is largely unknown. They may find applications in the near future in the environmental remediation industries, where the requirement for purified molecules is of less importance. This review summarises knowledge of the chemistry, biochemistry and production of Rhodococcus surface-active lipids. Where they have been used, or there is potential for use, in industrial applications is discussed.
Biodegradation, Environmental↗