The present study focused on developing a wild-type actinomycete isolate as a model for a non-pathogenic filamentous producer of biosurfactants. A total of 33 actinomycetes isolates were screened and their extracellular biosurfactants production was evaluated using olive oil as the main substrate. Out of 33 isolates, 32 showed positive results in the oil spreading technique (OST). All isolates showed good emulsification activity (E) ranging from 84.1 to 95.8%. Based on OST and E values, isolate R1 was selected for further investigation in biosurfactant production in an agitated submerged fermentation. Phenotypic and genotypic analyses tentatively identified isolate R1 as a member of the Streptomyces genus. A submerged cultivation of Streptomyces sp. R1 was carried out in a 3-L stirred-tank bioreactor. The influence of impeller tip speed on volumetric oxygen transfer coefficient (k a), growth, cell morphology and biosurfactant production was observed. It was found that the maximum biosurfactant production, indicated by the lowest surface tension measurement (40.5 ± 0.05 dynes/cm) was obtained at highest k a value (50.94 h) regardless of agitation speed. The partially purified biosurfactant was obtained at a concentration of 7.19 g L, characterized as a lipopeptide biosurfactant and was found to be stable over a wide range of temperature (20-121 °C), pH (2-12) and salinity [5-20% (w/v) of NaCl].

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Significance and Impact of the Study: This study highlights the significance of an effective fed-batch strategy for rhamnolipid production in a submerged fermentation using a water-immiscible substrate, based on maximum substrate uptake rate. The impact of this strategy ensured that the substrate was supplied at the rate matching the maximum substrate utilization by the cells without excess feeding, leading to increased rhamnolipid production, yield and productivity. AbstractA fed-batch strategy was established based on the maximum substrate uptake rate (MSUR) of Pseudomonas aeruginosa USM-AR2 grown in diesel to produce rhamnolipid. This strategy matches the substrate feed rates with the substrate demand based on the real-time measurements of dissolved oxygen (DO). The MSUR was estimated by determining the time required for consumption of a known amount of diesel. The MSUR trend paralleled the biomass profile of Ps. aeruginosa USM-AR2, where the MSUR increased throughout the exponential phase indicating active substrate utilization and then decreased when cells entered stationary phase. Rhamnolipid yield on diesel was enhanced from 0Á047 (g/g) in batch to 0Á110 (g/g) in pulse-pause fed-batch and 0Á123 (g/g) in MSUR fed-batch. Rhamnolipid yield on biomass was also improved from 0Á421 (g/g) in batch, 3Á098 (g/g) in pulse-pause fed-batch to 3Á471 (g/g) using MSUR-based strategy. Volumetric productivity increased from 0Á029 g l À1 h À1 in batch, 0Á054 g l À1 h À1 in pulse-pause fed-batch to 0Á076 g l À1 h À1 in MSUR fed-batch.

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Rigidoporus microporus is the main causal of white root disease (WRD) in rubber trees (Hevea brasiliensis). The present study investigates the use of rhamnolipid, a biosurfactant produced by Pseudomonas aeruginosa USM-AR2 against R. microporus. In vitro dose-responses towards rhamnolipid were determined on different isolates of R. microporus using the poisoned food technique (PFT). Inhibition of mycelial growth was found to be dose-dependent, with the highest inhibition of 76.74% at 200 ppm (pH 6.29) on SEG isolate. On the contrary, the lowest concentration of rhamnolipid applied at 10 ppm (pH 5.97) had effectively inhibited the growth of RL 19 to 34.36%. AM isolate was assumed to be the most aggressive pathogen due to the lowest inhibition recorded on all rhamnolipid concentrations tested. At the same time, RL 19 was the least aggressive pathogen compared to the other R. microporus isolates. The rhamnolipid concentrations (ppm), which reduced mycelial growth at 50% (EC50), were recorded at 17.82 ppm for AM isolate, 12.52 ppm for RL 26, and 11.80 ppm for RL 19 isolate. This result indicated that rhamnolipid concentrations to inhibit 50% of mycelial growth might vary based on the aggressiveness and the virulence levels of different R. microporus isolates. It was found that pH changes after incorporating rhamnolipid into the PDA were not the main factor affecting the inhibition of R. microporus isolates. It is obvious that rhamnolipid had an inhibitory effect on fungal growth in vitro. It is the first report on rhamnolipid that has been shown to control R. microporus potentially.