Influence of ferric iron on gene expression and rhamnolipid synthesis during batch cultivation of Pseudomonas aeruginosa PAO1

Bioprocesses based on sustainable resources and rhamnolipids in particular have become increasingly attractive in recent years. These surface-active glycolipids with various chemical and biological properties have diverse biotechnological applications and are naturally produced by Pseudomonas aeruginosa. Their production, however, is tightly governed by a complex growth-dependent regulatory network, one of the major obstacles in the way to upscale production. P. aeruginosa PAO1 was grown in shake flask cultures using varying concentrations of ferric iron. Gene expression was assessed using quantitative PCR. A strong increase in relative expression of the genes for rhamnolipid synthesis, rhlA and rhlC, as well as the genes of the pqs quorum sensing regulon was observed under iron-limiting conditions. Iron repletion on the other hand caused a down-regulation of those genes. Furthermore, gene expression of different iron regulation-related factors, i.e. pvdS, fur and bqsS, was increased in response to iron limitation. Ensuing from these results, a batch cultivation using production medium without any addition of iron was conducted. Both biomass formation and specific growth rates were not impaired compared to normal cultivation conditions. Expression of rhlA, rhlC and pvdS, as well as the gene for the 3-oxo-C12-HSL synthetase, lasI, increased until late stationary growth phase. After this time point, their expression steadily decreased. Expression of the C4-HSL synthetase gene, rhlI, on the other hand, was found to be highly increased during the entire process.