Production of rhamnolipids by Pseudomonas chlororaphis, a nonpathogenic bacterium

Rhamnolipids, naturally occurring biosurfactants constructed of rhamnose sugar molecules and β-hydroxyalkanoic acids, have a wide range of potential commercial applications. In the course of a survey of 33 different bacterial isolates, we have identified, using a phenotypic assay for rhamnolipid production, a strain of the nonpathogenic bacterial species Pseudomonas chlororaphis that is capable of producing rhamnolipids. Rhamnolipid production by P. chlororaphis was achieved by growth at room temperature in static cultures of a mineral salts medium containing 2% glucose. We obtained yields of roughly 1 g/liter of rhamnolipids, an amount comparable to the production levels reported in Pseudomonas aeruginosa grown with glucose as the carbon source. The rhamnolipids produced by P. chlororaphis appear to be exclusively the mono-rhamnolipid form. The most prevalent molecular species had one monounsaturated hydroxy fatty acid of 12 carbons and one saturated hydroxy fatty acid of 10 carbons. P. chlororaphis, a nonpathogenic saprophyte of the soil, is currently employed as a biocontrol agent against certain types of plant fungal diseases. The pathogenic nature of all bacteria previously known to produce rhamnolipids has been a major obstacle to commercial production of rhamnolipids. The use of P. chlororaphis therefore greatly simplifies this matter by removing the need for containment systems and stringent separation processes in the production of rhamnolipids.