The dynamic influence of cells on the formation of stable emulsions in organic–aqueous biotransformations

SIGHT-A System for Solvent-Tight Incubation and Growth Monitoring in High Throughput

Physiological characterization of microorganisms in the context of solvent tolerance is a tedious process with a high investment of manual labor while often being limited in throughput capability simultaneously. Therefore, we developed a small-scale solvent-impervious cultivation system consisting of screw cap-sealed glass vials in combination with a 3D-printed vial holder for the Growth Profiler (EnzyScreen) platform. Components and cultivation conditions were empirically tested, and a suitable setup was found for the intended application. To demonstrate the capability of this cultivation system, an adaptive laboratory evolution was performed to further increase the tolerance of Pseudomonas taiwanensis GRC3 toward styrene. This approach yielded heterogenic cultures with improved growth performances in the presence of styrene from which individual clones were isolated and characterized in high throughput. Several clones with improved growth in the presence of 1% (v/v) styrene were analyzed through whole-genome sequencing, revealing mutations in the co-chaperone-encoding gene dnaJ, RNA polymerase α subunit-encoding gene rpoA, and loss-of-function mutations in the ttgGHI solvent efflux pump repressor encoded by ttgV. The developed cultivation system has proven to be a very useful extension of the Growth Profiler, as it reduces manual workload and allows high-throughput characterization.

Demulsification of Bacteria-Stabilized Pickering Emulsions Using Modified Silica Nanoparticles

Pickering emulsions stabilized by bacteria acting as particle emulsifiers are new platforms for microbial transformations of hydrophobic chemicals. However, their high stability often hampers demulsification during downstream processing. Since the existing methods (like addition of surfactants) to demulsify bacteria-stabilized Pickering emulsions have negative effects, new practical methods need to be developed. Here, using chemically modified fumed silica particles with different hydrophobicity, the demulsification of W/O Pickering emulsions stabilized by Mycobacterium neoaurum whole cells was first studied. The binary particle-stabilized emulsions exhibited phase inversion and dewatering induced by the coalescence of W/O emulsions or creaming of O/W emulsions. The silica particle hydrophobicity and concentration were the important parameters influencing the emulsion type, droplet morphology, and dewatering rate. The highest dewatering rate and largest droplet size were obtained at the inversion point from W/O to O/W. Confocal microscopy showed that no interaction between the bacteria and silica particles existed and the silica particle adsorption at the interface induced the detachment of bacteria from the interface, revealing that there was competitive adsorption between the binary particles at the interface. Based on these results, we suggested that the average hydrophobicity of the binary particles at the interface would determine the emulsion type and stability. Finally, this strategy was successfully applied to the demulsification of the Pickering emulsion formed during microbial transformation of sterols. Overall, this study provides a new strategy to demulsify Pickering emulsions by addition of another particle emulsifier. This is also the first example of separation of products as well as organic phases after microbial transformation in Pickering emulsions.

Crystal substrate inhibition during microbial transformation of phytosterols in Pickering emulsions

Water-oil interface of bacterial cell-stabilized Pickering emulsions is an exceptional habitat for microbial assimilation of both hydrophobic nutrients solubilized in oil phase and hydrophilic ones solubilized in water phase. Crystal substrate inhibition, i.e., decreasing phytosterol degradation with the increase loading of crystal phytosterols, is always observed during microbial transformation of phytosterols into steroid synthons in Mycolicibacterium sp (China Center of Industrial Culture Collection, CICC 21,097) cell-stabilized Pickering emulsions. In the present work, we confirmed that crystal substrate inhibition was attributed to the interaction between M. neoaurum and phytosterol crystals that led to the detachment of bacterial cells from the oil-water interfaces in bacterial cell-stabilized Pickering emulsions. Under the selected operation condition (25 ml BEHP per 40 ml water, 60 g/L glucose, 25 g/L phytosterols), the product androst-4-ene-3, 17-dione (AD) and androsta-1, 4-dien-3, 17-dione (ADD) concentration increased linearly with the progress of microbial transformation and reached almost 6 g/L at the 11^th day. This is a paradigm for microbial transformation of crystal substrates as well as in the presence of other surface active additives (such as chitosan and nonionic surfactants) in bacterial cell-stabilized Pickering emulsions. KEY POINTS: • Microbial transformation of crystal phytosterols in Pickering emulsions • Crystal substrate inhibition occurring during microbial transformation • Interaction between phytosterol crystals and bacterial cells leading to demulsification.

Improving Product Specificity of Whole-Cell Alkane Oxidation in Nonconventional Media: A Multivariate Analysis Approach

Two-liquid-phase reaction media have long been used in bioconversions to supply or remove hydrophobic organic reaction substrates and products to reduce inhibitory and toxic effects on biocatalysts. In case of the terminal oxyfunctionalization of linear alkanes by the AlkBGT monooxygenase the excess alkane substrate is often used as a second phase to extract the alcohol, aldehyde, and acid products. However, the selection of other carrier phases or surfactants is complex due to a large number of parameters that are involved, such as biocompatibility, substrate bioavailability, and product extraction selectivity. This study combines systematic high-throughput screening with chemometrics to correlate physicochemical parameters of a range of cosolvents to product specificity and yield using a multivariate regression model. Partial least-squares regression shows that the defining factor for product specificity is the solubility properties of the reaction substrate and product in the cosolvent, as measured by Hansen solubility parameters. Thus the polarity of cosolvents determines the accumulation of either alcohol or acid products. Whereas usually the acid product accumulates during the reaction, by choosing a more polar cosolvent the 1-alcohol product can be accumulated. Especially with Tergitol as a cosolvent, a 3.2-fold improvement in the 1-octanol yield to 18.3  mmol  L^-1 is achieved relative to the control reaction without cosolvents.

Applied catastrophic phase inversion: a continuous non-centrifugal phase separation step in biphasic whole-cell biocatalysis

Biphasic whole-cell biotransformations are known to be efficient alternatives to common chemical synthesis routes, especially for the production of, e.g. apolar enantiopure organic compounds. They provide high stereoselectivity combined with high product concentrations owing to the presence of an organic phase serving as substrate reservoir and product sink. Industrial implementation suffers from the formation of stable Pickering emulsions caused by the presence of cells. State-of-the-art downstream processing includes inefficient strategies such as excessive centrifugation, use of de-emulsifiers or thermal stress. In contrast, using the catastrophic phase inversion (CPI) phenomenon (sudden switch of emulsion type caused by addition of dispersed phase), Pickering-type emulsions can be destabilized efficiently. Within this work a model system using bis(2-ethylhexyl) phthalate (BEHP) as organic phase in combination with E. coli, JM101 was successfully separated using a continuous mixer settler setup. Compared to the state-of-the-art centrifugal separations, this process allows complete phase separation with no detectable water content or cells in the organic phase with no utilities/additives required. Furthermore, the concentration of the product is not affected by the separation. It is therefore a simple applicable method that can be used for separation of stable Pickering-type emulsions based on the knowledge of the point of inversion.