Glycerol or crude glycerol as substrates make Pseudomonas aeruginosa achieve anaerobic production of rhamnolipids

Does regulation hold the key to optimizing lipopeptide production in Pseudomonas for biotechnology?

Lipopeptides (LPs) produced by Pseudomonas spp. are specialized metabolites with diverse structures and functions, including powerful biosurfactant and antimicrobial properties. Despite their enormous potential in environmental and industrial biotechnology, low yield and high production cost limit their practical use. While genome mining and functional genomics have identified a multitude of LP biosynthetic gene clusters, the regulatory mechanisms underlying their biosynthesis remain poorly understood. We propose that regulation holds the key to unlocking LP production in Pseudomonas for biotechnology. In this review, we summarize the structure and function of Pseudomonas-derived LPs and describe the molecular basis for their biosynthesis and regulation. We examine the global and specific regulator-driven mechanisms controlling LP synthesis including the influence of environmental signals. Understanding LP regulation is key to modulating production of these valuable compounds, both quantitatively and qualitatively, for industrial and environmental biotechnology.

Highly efficient production of rhamnolipid in P. putida using a novel sacB-based system and mixed carbon source

Pseudomonas putida KT2440, a GRAS strain, has been used for synthesizing bulk and fine chemicals. However, the gene editing tool to metabolically engineer KT2440 showed low efficiency. In this study, a novel sacB-based system pK51mobsacB was established to improve the efficiency for marker-free gene disruption. Then the rhamnolipid synthetic pathway was introduced in KT2440 and genes of the competitive pathways were deleted to lower the metabolic burden based on pK51mobsacB. A series of endogenous and synthetic promoters were used for fine tuning rhlAB expression. The limited supply of dTDP-L-rhamnose was enhanced by heterologous rmlBDAC expression. Cell growth and rhamnolipid production were well balanced by using glucose/glycerol as mixed carbon sources. The final strain produced 3.64 g/L at shake-flask and 19.77 g/L rhamnolipid in a 5 L fermenter, the highest obtained among metabolically engineered KT2440, which implied the potential of KT2440 as a promising microbial cell factory for industrial rhamnolipid production.

The aggregated biofilm dominated by Delftia tsuruhatensis enhances the removal efficiency of 2,4-dichlorophenol in a bioelectrochemical system

Bioelectrochemical system is a prospective strategy in organic-contaminated groundwater treatment, while few studies clearly distinguish the mechanisms of adsorption or biodegradation in this process, especially when dense biofilm is formed. This study employed a single chamber microbial electrolysis cell (MEC) with two three-dimensional electrodes for removing a typical organic contaminant, 2,4-dichlorophenol (DCP) from groundwater, which inoculated with anaerobic bacteria derived from sewage treatment plant. Compared with the single biodegradation system without electrodes, the three-dimensional electrodes with a high surface enabled an increase of alpha diversity of the microbial community (increased by 52.6% in Shannon index), and provided adaptive ecological niche for more bacteria. The application of weak voltage (0.6 V) furtherly optimized the microbial community structure, and promoted the aggregation of microorganisms with the formation of dense biofilm. Desorption experiment proved that the contaminants were removed from the groundwater mainly via adsorption by the biofilm rather than biodegradation, and compared with the reactor without electricity, the bioelectrochemical system increased the adsorption capacity from 50.0% to 74.5%. The aggregated bacteria on the surface of electrodes were mainly dominated by Delftia tsuruhatensis (85.0%), which could secrete extracellular polymers and has a high adsorption capacity (0.30 mg/g electrode material) for the contaminants. We found that a bioelectrochemical system with a three-dimensional electrode could stimulate the formation of dense biofilm and remove the organic contaminants as well as their possible more toxic degradation intermediates via adsorption. This study provides important guidance for applying bioelectrochemical system in groundwater or wastewater treatment.

Optimization of Medium Components for Fed-Batch Fermentation Using Central Composite Design to Enhance Lichenysin Production by Bacillus licheniformis Ali5

Lichenysin, an amphiphilic biosurfactant with structural and physicochemical properties similar to surfactin, is produced by Bacillus licheniformis. Its low toxicity, good environmental compatibility, solubilization, foaming, emulsification and detergent activities have led to a wide range of applications in agricultural biocontrol, enhanced oil recovery, foaming agents for cosmetics and detergents for household cleaning products. However, despite the extraordinary surface-active properties and potential applications of lichenysin, the number of wild bacteria found so far is relatively low. Low titers and high costs are the main limiting factors for widespread industrial applications. In this study, a factorial design was used to optimize the composition of the medium for the production of lichenysin by Bacillus licheniformis Ali5. Firstly, the solutions of carbon, nitrogen, amino acids, inorganic salts and trace elements in the medium were evaluated in flasks using a single-factor optimization method. Meanwhile, the operating conditions were optimized in the same way. Afterwards, a partial factorial design was used to investigate the effect of six variables (five medium compositions and inoculum size) on lichenysin production. Based on the results obtained, the concentrations of sucrose and ammonium nitrate and the inoculum size were considered to be important for lichenysin production. Subsequently, a full factorial design was used to optimize these three variables. The optimized medium composition were sucrose 19.8 g/L, NH4NO3 3.9 g/L, K2HPO4·3H2O 4.0 g/L, MgSO4·7H2O 0.6 g/L, FeSO4·7H2O 0.1 g/L, CaCl2 0.01 g/L, NaCl 3.0, trace elements 1.2 mL/L. Finally, the titer of lichenysin after fed-batch fermentation reached 1425.85 mg/L, which was approximately 5.5 times higher than the titer of lichenysin from the original medium. Consequently, the method was further demonstrated to be suitable for lichenysin production.

Evolution and regulation of microbial secondary metabolism

Microbes have disproportionate impacts on the macroscopic world. This is in part due to their ability to grow to large populations that collectively secrete massive amounts of secondary metabolites and alter their environment. Yet, the conditions favoring secondary metabolism despite the potential costs for primary metabolism remain unclear. Here we investigated the biosurfactants that the bacterium Pseudomonas aeruginosa makes and secretes to decrease the surface tension of surrounding liquid. Using a combination of genomics, metabolomics, transcriptomics, and mathematical modeling we show that the ability to make surfactants from glycerol varies inconsistently across the phylogenetic tree; instead, lineages that lost this ability are also worse at reducing the oxidative stress of primary metabolism on glycerol. Experiments with different carbon sources support a link with oxidative stress that explains the inconsistent distribution across the P. aeruginosa phylogeny and suggests a general principle: P. aeruginosa lineages produce surfactants if they can reduce the oxidative stress produced by primary metabolism and have excess resources, beyond their primary needs, to afford secondary metabolism. These results add a new layer to the regulation of a secondary metabolite unessential for primary metabolism but important to change physical properties of the environments surrounding bacterial populations.

Bacillus licheniformis: The unexplored alternative for the anaerobic production of lipopeptide biosurfactants?

Microbial biosurfactants have attracted the attention of researchers and companies for the last decades, as they are considered promising candidates to replace chemical surfactants in numerous applications. Although in the last years, considerable advances were performed regarding strain engineering and the use of low-cost substrates in order to reduce their production costs, one of the main bottlenecks is their production at industrial scale. Conventional aerobic biosurfactant production processes result in excessive foaming, due to the use of high agitation and aeration rates necessary to increase dissolved oxygen concentration to allow microbial growth and biosurfactant production. Different approaches have been studied to overcome this problem, although with limited success. A not widely explored alternative is the development of foam-free processes through the anaerobic growth of biosurfactant-producing microorganisms. Surfactin, produced by Bacillus subtilis, is the most widely studied lipopeptide biosurfactant, and the most powerful biosurfactant known so far. Bacillus licheniformis strains produce lichenysin, a lipopeptide biosurfactant which structure is similar to surfactin. However, despite its extraordinary surface-active properties and potential applications, lichenysin has been scarcely studied. According to previous studies, B. licheniformis is better adapted to anaerobic growth than B. subtilis, and could be a good alternative for the anaerobic production of lipopeptide biosurfactants. In this review, the potential and limitations of surfactin and lichenysin production under anaerobic conditions will be analyzed, and the possibility of implementing foam-free processes for lichenysin production, in order to expand the market and applications of biosurfactants in different fields, will be discussed.

Recovery and utilization of crude glycerol, a biodiesel byproduct

Biodiesel production has increased significantly in the past decade because it has been demonstrated to be a viable alternative and renewable fuel. Consequently, the production of crude glycerol, the main byproduct of the transesterification of lipids to biodiesel, has risen as well. Therefore, the effective recovery and utilization of crude glycerol can provide biodiesel with additional value. In this review, we first summarized the state-of-the-art progress on crude glycerol recovery and purification. Subsequently, numerous approaches have been reviewed for the utilization of crude glycerol, including use as animal feeds, for combustion, anaerobic fermentation, and chemical conversion. Finally, an extensive discussion and outlook is presented in relation to the techniques and processes in the chemical conversion of crude glycerol.

In this review, we summarize the latest technologies for the recovery and purification of crude glycerol and applications of crude glycerol, with focus on its chemical conversion.