Background: Steviol glycosides such as stevioside have attracted the attention of the food and beverage industry. Recently, efforts were made to produce these natural sweeteners in microorganisms using metabolic engineering. Nonetheless, the steviol titer is relatively low in metabolically engineered microorganisms, and therefore a steviol-biosynthetic pathway in heterologous microorganisms needs to be metabolically optimized. The purpose of this study was to redesign and reconstruct a steviol-biosynthetic pathway via synthetic-biology approaches in order to overproduce steviol in Escherichia coli. Results: A genome-engineered E. coli strain, which coexpressed 5′ untranslated region (UTR)-engineered geranylgeranyl diphosphate synthase, copalyl diphosphate synthase, and kaurene synthase, produced 623.6 ± 3.0 mg/L ent-kaurene in batch fermentation. Overexpression of 5′-UTR-engineered, N-terminally modified kaurene oxidase of Arabidopsis thaliana yielded 41.4 ± 5 mg/L ent-kaurenoic acid. Enhanced ent-kaurenoic acid production (50.7 ± 9.8 mg/L) was achieved by increasing the cellular NADPH/NADP + ratio. The expression of a fusion protein, UtrCYP714A2-AtCPR2 derived from A. thaliana, where trCYP714A2 was 5′-UTR-engineered and N-terminally modified, gave 38.4 ± 1.7 mg/L steviol in batch fermentation. Conclusions: 5′-UTR engineering, the fusion protein approach, and redox balancing improved the steviol titer in flask fermentation and bioreactor fermentation. The expression engineering of steviol-biosynthetic enzymes and the genome engineering described here can serve as the basis for producing terpenoids-including steviol glycosides and carotenoids-in microorganisms.

The use of ultrasound to enhance the efficiency of surfactant-aided soil washing was investigated with a series of laboratory tests on diesel-contaminated soil. Test conditions included the power and duration of sonic energy, the surfactant type, and mixing time. The results of the study show that the use of ultrasound in the soil washing process significantly enhanced the remediation of the contaminated soil. The degree of enhancement varied with sonication energy, treatment time, and mixing duration.

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The goal of this study is to enhance secession ability of adsorbed water in the fine particles of sludge from waste treatment plant using ultrasonic wave energy. Results of the treatment involving reduction in the final sludge cake affect the usage of energy required for sludge combustion. Excess activated sludge before dewatering process was sampled in the local area and the test conditions included application time and power of the ultrasonic energy and types of flocculants to investigate the dewatering behaviors. Capillary suction time(CST) and viscosity of the tested sample were the main indicator for the dewaterbility of the treated sludge. The results of the study show that the ultrasonic treatment decreases the dewaterbility of sludge sample significantly. The degree of the effectiveness, however, varies with the test conditions. Energy and cost reduction caused by the ultrasonic treatment are also discussed in detail.