Abstract
The quantitative and qualitative effect of water immiscible and miscible carbon-rich substrates on the production of biosurfactants, surfactin and rhamnolipids, by Bacillus amyloliquefaciens ST34 and Pseudomonas aeruginosa ST5, respectively, was analysed. A small-scale high throughput 96 deep-well micro-culture method was utilised to cultivate the two strains in mineral salt medium (MSM) supplemented with the water miscible (glucose, glycerol, fructose and sucrose) and water immiscible carbon sources (diesel, kerosene and sunflower oil) under the same growth conditions. The biosurfactants produced by the two strains were isolated by acid precipitation followed by an organic solvent extraction. Ultra-performance liquid chromatography coupled to electrospray ionisation mass spectrometry was utilised to analyse yields and characterise the biosurfactant variants. For B. amyloliquefaciens ST34, maximum surfactin production was observed in the MSM supplemented with fructose (28 mg L−1). In addition, four surfactin analogues were produced by ST34 using the different substrates, however, the C13–C15 surfactins were dominant in all extracts. For P. aeruginosa ST5, maximum rhamnolipid production was observed in the MSM supplemented with glucose (307 mg L−1). In addition, six rhamnolipid congeners were produced by ST5 using different substrates, however, Rha–Rha–C10–C10 and Rha–C10–C10 were the most abundant in all extracts. This study highlights that the carbon sources utilised influences the yield and analogues/congeners of surfactin and rhamnolipids produced by B. amyloliquefaciens and P. aeruginosa, respectively. Additionally, glucose and fructose were suitable substrates for rhamnolipid and surfactin, produced by P. aeruginosa ST5 and B. amyloliquefaciens ST34, which can be exploited for bioremediation or as antimicrobial agents.
Highlights
Biosurfactants are an important class of microbially synthesised compounds that have been extensively researched due to their diverse biological properties and functions (Van Hamme et al 2006; Gudiña et al 2013; Kiran et al 2016)
The B. amyloliquefaciens ST34 and P. aeruginosa ST5 strains utilised in the current study, were previously shown to produce surfactin and rhamnolipids, respectively when cultivated in mineral salt medium (MSM) supplemented with glycerol as a sole carbon source (Ndlovu 2017)
The ultraperformance liquid chromatography coupled with mass spectrometry (UPLC–MS) profiles of the surfactin standard and the extracts produced by ST34 cultivated in MSM supplemented with the water miscible substrates and water immiscible substrates revealed four major peaks/peak clusters with retention times (Rt) between 10 and 13 min (Fig. 1)
Summary
Biosurfactants are an important class of microbially synthesised compounds that have been extensively researched due to their diverse biological properties and functions (Van Hamme et al 2006; Gudiña et al 2013; Kiran et al 2016) Owing to their low toxicity and biodegradable nature, they exhibit potential for various commercial. Rhamnolipid biosynthesis by P. aeruginosa occurs in consecutive steps of glycosyl transfer reactions catalysed by different rhamnosyl-transferases, yielding separate activated precursor hydrophilic (mono- or dirhamnose) and hydrophobic (hydoxyfatty acids) moieties. These are dimerised by the rhamnosyl-transferases and other enzymes (Soberón–Chávez et al 2005). The microbially produced rhamnolipid mixtures display varying properties that depend on the type and proportion of the homologs, which differ, based on the bacterial strain used, culture conditions, medium composition and the type of carbon source used for growth (Déziel et al 1999; Abalos et al 2001; Das et al 2009; Singh et al 2014)
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