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Rhamnolipids are biosurfactants with interesting physico-chemical properties. However, the main obstacles towards an economic production are low productivity, high raw-material costs, relatively expensive downstream processing, and a lack of understanding the rhamnolipid production regulation in bioreactor systems. This study shows that the sequenced Pseudomonas aeruginosa strain PAO1 is able to produce high quantities of rhamnolipid during 30 L batch bioreactor cultivations with sunflower oil as sole carbon source and nitrogen limiting conditions. Thus PAO1 could be an appropriate model for rhamnolipid production in pilot plant bioreactor systems. In contrast to well-established production strains, PAO1 allows knowledge-based systems biotechnological process development combined with the frequently used heuristic bioengineering approach. The maximum rhamnolipid concentration obtained was 39 g/L after 90 h of cultivation. The volumetric productivity of 0.43 g/Lh was comparable with previous described production strains. The specific rhamnolipid productivity showed a maximum between 40 and 70 h of process time of 0.088 g(RL)/g(BDM)h. At the same time interval, a shift of the molar di- to mono-rhamnolipid ratio from 1:1 to about 2:1 was observed. PAO1 not only seems to be an appropriate model, but surprisingly has the potential as a strain of choice for actual biotechnological rhamnolipid production.

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The Burkholderia plantarii type strain DSM 9509T was shown to produce extracellular rhamnolipids that clearly differ in structure and properties from the Pseudomonas aeruginosa rhamnolipids. The type strain has not been previously described as rhamnolipid producer. B. plantarii DSM 9509T was cultivated in a parallel bioreactor system and produced up to 45.74 mg/L of rhamnolipids when grown in glucose supplemented rich medium. The structure of the main rhamnolipid Rha 2 -C 14 -C 14 (L-rhamnopyranosyl-L-rhamnopyranosyl-b-hydroxytetradecanoyl-b-hydroxytetradecanoate) was verified by MS/MS. The purified rhamnolipid Rha 2 -C 14 -C 14 lowered the equilibrium surface tension of distilled water to 29.4 mN/m. The CMC was between 15 and 20 mg/L. In this study, growth and product formation were investigated under biotechnological aspects. Practical applications: B. plantarii DSM 9509T produces a rhamnolipid that differs in structure from P. aeruginosa rhamnolipids and exhibits excellent surfactant properties. An essential advantage in comparison to P. aeruginosa is that B. plantarii DSM 9509T is a Biosafety Level 1 microorganism and is not pathogenic in humans. A biotechnological production of the Burkholderia rhamnolipids could lead to their application in detergents, pharmaceuticals, and other industries providing new products on the biosurfactant market.

A lack of understanding of the quantitative rhamnolipid production regulation in bioreactor cultivations of Pseudomonas aeruginosa and the absence of respective comparative studies are important reasons for achieving insufficient productivities for an economic production of these biosurfactants. The Pseudomonas strains DSM 7108 and DSM 2874 are described to be good rhamnolipid over-producers. The strain PAO1 on the other hand is the best analyzed type strain for genetic regulation mechanisms in the species P. aeruginosa. These three strains were cultivated in a 30-L bioreactor with a medium containing nitrate and sunflower oil as sole C-source at 30 and 37 °C. The achieved maximum rhamnolipid concentrations varied from 7 to 38 g/L, the volumetric productivities from 0.16 to 0.43 g/(L·h), and the cellular yield from 0.67 to 3.15 g/g, with PAO1 showing the highest results for all of these variables. The molar di- to mono-rhamnolipid ratio changed during the cultivations; it was strain dependent but not significantly influenced by the temperature. This study explicitly shows that the specific rhamnolipid synthesis rate per cell follows secondary metabolite-like courses coinciding with the transition to the stationary phase of typical logistic growth behavior. However, the rhamnolipid synthesis was already induced before N-limitation occurred.

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