Sulfur source-mediated transcriptional regulation of the rhlABC genes involved in biosurfactants production by Pseudomonas sp. strain AK6U

Abstract

Despite the nutritional significance of sulfur, its influence on biosurfactants production has not been sufficiently studied. We investigated the expression of key biosurfactants production genes, rhlABC, in cultures of Pseudomonas sp. AK6U grown with inorganic or organic sulfur sources. AK6U grew with either inorganic sulfate (MgSO4), dibenzothiophene (DBT), or DBT-sulfone as a sole sulfur source in the presence of glucose as a carbon source. The AK6U cultures produced variable amounts of biosurfactants depending on the utilized sulfur source. Biosurfactants production profile of the DBT cultures was significantly different from that of the DBT-sulfone and inorganic sulfate cultures. The last two cultures were very similar in terms of biosurfactants productivity. Biosurfactants yield in the DBT cultures (1.3 g/L) was higher than that produced by the DBT-sulfone (0.5 g/L) and the inorganic sulfate (0.44 g/L) cultures. Moreover, the surface tension reduction in the DBT cultures (33 mN/m) was much stronger than that measured in the DBT-sulfone (58 mN/m) or inorganic sulfate (54 mN/m) cultures. RT-qPCR revealed variations in the expression levels of the rhlABC genes depending on the sulfur source. The DBT cultures had higher expression levels for the three genes as compared to the DBT-sulfone and inorganic sulfate cultures. There was no significant difference in the expression profiles between the DBT-sulfone and the MgSO4 cultures. The increased expression of rhlC in the DBT cultures is indicative for production of higher amounts of dirhamnolipids compared to the DBT-sulfone and inorganic sulfate cultures. The gene expression results were in good agreement with the biosurfactants production yields and surface tension measurements. The sulfur source mediates a fine-tuned mechanism of transcriptional regulation of biosurfactants production genes. Our findings can have an impact on industrial production of biosurfactants and other biotechnological processes like biodesulfurization.

Stimulation of rhamnolipid biosurfactants production in Pseudomonas aeruginosa AK6U by organosulfur compounds provided as sulfur sources

“…Moreover, in line with our results, it has been reported that rhamnolipid composition depends on many factors such as the producing strain, culture conditions, and medium composition [27], [29]. The higher proportion of di-rhamnolipids as compared to mono-rhamnolipids (in the DBT and DBT-sulfone cultures) is in a good agreement with the previously reported higher expression level of the rhlC gene compared to the rhlB gene [21]. Furthermore, the predominance of the di-rhamnolipid L-Rha–L-Rha–C 10 –C 10 congener in all the analyzed cultures agrees with data reported in the literature [40].…”

“…Such qualitative and quantitative differences were exhibited in the rhamnolipid congeners produced in cultures containing organosulfur (DBT, DBT-sulfone) and inorganic sulfur substrates. The variations in rhamnolipid congener profiles reflect the recently reported differences in the rhlABC gene expression among the DBT, DBT-sulfone, and inorganic sulfate cultures [21]. Moreover, in line with our results, it has been reported that rhamnolipid composition depends on many factors such as the producing strain, culture conditions, and medium composition [27], [29].…”

The Role of Microorganisms and Productions in Biodesulfurization of Fossiel Fuels

“…This mosaic nature of AK6 is expected to fit better than axenic monocultures for the development of biotechnological processes targeting desulfurization of a broad range of sulfur compounds present in crude oil and diesel. Furthermore, deployment of mixed cultures may prevent the decay of the biodesulfurization activity resulting from the accumulation of the inhibitory intermediates of the 4S pathway [37,42].In fact, all detected 16S rRNA gene sequences are related to those of bacterial genera that are known as biodesulfurization-competent, hydrocarbon degraders, or inhabitants of hydrocarbons-polluted environments [43,44].…”

Editorial: Microbiotechnology Based Surfactants and Their Applications

“…Not only the less-researched strains and classes but a significant leap is required investigating the carbon sources that would work best for high biosurfactant production. Addressing this area are the original research articles by Antoniou et al ( 2015 ), Gudiña et al ( 2015 ) and Ismail et al ( 2014 ), and a review by Banat et al ( 2014 ).…”

“…strain AK6U greatly varies depending on the sulfur source. They showed that a biosurfactant yield of 1.3 g/l was obtained in presence of dibenzothiophene (DBT) as a carbon source which was higher than obtained in presence of DBT-sulfone (0.5 g/l) and the inorganic sulfate (0.44 g/l) (Ismail et al, 2014 ). To bring together these types of “carbon-source” based studies for “low-cost” biosurfactant production technologies Ibrahim M. Banat and co-authors wrote an intensive review where they discussed how and why despite so many developments on biosurfactants their commercialization remain difficult, costly and to a large extent irregular and what role does the low-cost renewable raw substrates and fermentation technology play in reducing the overall production cost.…”