Amrita University
Amritapuri, Vallikavu, Kerala 690525
India
Ganesh Kumar S, Kalimuthu K, Solomon Robinson David Jebakumar, Vimalan J
Molecular and physicochemical characterization of rhamnolipid biosurfactant produced by Pseudomonas sp JSK6
Interest in microbial surfactants like rhamnolipids has been progressively increasing in recent years due to their diversity, biodegradability and possibility of large scale production (Pinzon et al. 2013). However, traditional engineering by random and targeted genetic alteration, process design and recombinant strategies in rhamnolipid production were not studied in detail. Based on the environmental conditions, wide diversity of rhamnolipid congeners and homologues was produced in various bacterial strains (Abdel-Mawgoud et al. 2010). In this study, a biosurfactant producing bacterial strain, Pseudomonas sp. JSK6 was isolated from hydrocarbons contaminated sites at Madurai district of South India. The isolate produced mixture of both mono and di-rhamnolipids with excellent surfactant properties. The critical micelle concentration (CMC) of the produced rhamnolipids was 30 mg/L. The emulsification index of 56.4% with diesel and 55.3% with kerosene was quite stable after 24 h. The culture condition optimisation showed that highest rhamnolipid (1.6 g/L) production was achieved at pH 7 and 35◦C. The rhamnolipid was stable over a wide range of temperature (upto 100◦C) and pH (upto 10). The enzyme rhamnosyltransferase-1 which is responsible for biosynthesis of mono-rhamnolipid is encoded by the rhIAB genes, which are organized in rhlABRI operon. From the isolate JSK6, mono-rhamnolipid encoding gene cluster rhlABRI (4.3 kb) was successfully amplified and sequenced by primer walking. The sequence analysis showed that the rhlABRI gene cluster was perfectly organized in the strain JSK6. The rhamnolipid produced by strain JSK6 was analysed in NMR and LC-ESI-MS for structural characterisation. The chemical shifts in 1HNMR (0.86 ppm, 1.267 ppm, 2.546 ppm, 4.134ppm and 5.427 ppm) and 13CNMR (14.466ppm, 23.004–34.951ppm, 171.833ppm and 174.056ppm) and ESI-MS analysis suggested that extracted biosurfactant was present as mono-rhamnolipid and di-rhamnolipid as ten different homologues. This study concluded that the isolate JSK6 is a potential strain with capabilities to produce rhamnolipid biosurfactants with unique physicochemical properties. Cloning of rhlABRI gene cluster of strain JSK6 in a suitable expression system for large scale production is under progress.