In silico-guided engineering of Pseudomonas putida towards growth under micro-oxic conditions

Strategy for Optimizing Vitamin B12 Production in Pseudomonas putida KT2440 Using Metabolic Modeling

Background/Objectives: Vitamin B12 is very important for human health, as it is a cofactor for enzymatic activities and plays various roles in human physiology. It is highly valued in the pharmaceutical, food, and additive production industries. Some of the bacteria currently used for the vitamin production are difficult to modify with gene-editing tools and may have slow growth. We propose the use of the bacteria Pseudomonas putida KT2440 for the production of vitamin B12 because it has a robust chassis for genetic modifications. The present wok evaluates P. putida KT2440 as a host for vitamin B12 production and explore potential gene-editing optimization strategies. Methods: We curated and modified a genome-scale metabolic model of Pseudomonas putida KT2440 and evaluated different strategies to optimize vitamin B12 production using the knockin and OptGene algorithms from the COBRA Toolbox. Furthermore, we examined the presence of riboswitches as cis-regulatory elements and calculated theoretical biomass growth yields and vitamin B12 production using a flux balance analysis (FBA). Results: According to the flux balance analysis of P. putida KT2440 under culture conditions, the biomass production values could reach 1.802 gDW-1·h1·L-1, and vitamin B12 production could reach 0.359 µmol·gDW-1·h-1·L-1. The theoretical vitamin B12 synthesis rate calculated using P. putida KT2040 with two additional reactions was 14 times higher than that calculated using the control, Pseudomonas denitrificans, which has been used for the industrial production of this vitamin. Conclusions: We propose that, with the addition of aminopropanol linker genes and the modification of riboswitches, P. putida KT2440 may become a suitable host for the industrial production of vitamin B12.

Production of bio-indigo from engineered Pseudomonas putida KT2440 harboring tryptophanase and flavin-containing monooxygenase

Indigo is a unique blue dye that has been used in the textile industry for centuries and is currently mass-produced commercially through chemical synthesis. However, the use of toxic substrates and reducing agents for chemical synthesis is associated with environmental concerns, necessitating the development of eco-friendly alternatives based on microbial production. In this study, a robust industrial strategy for indigo production was developed using Pseudomonas putida KT2440 as the host strain, which is characterized by its excellent ability to degrade aromatic compounds and high resistance to environmental stress. By introducing the genes tryptophanase (tnaA) and Flavin-containing monooxygenase (FMO), a P. putida HI201 strain was constructed to produce indigo from tryptophan. To enhance the indigo yield, culture conditions, including medium composition, temperature, tryptophan concentration, and shaking speed, were optimized. Under optimal conditions such as TB medium, 15 mM tryptophan, 30°C, 200 rpm, P. putida HI201 biosynthesized 1.31 g/L indigo from tryptophan in a fed-batch fermentation system. The introduction of tnaA and FMO genes also enabled the production of indigo in various P. putida species, and the indigo-producing strain had a blue color, which served as a visual indicator. This study presents a strategy for using P. putida as a host for robust and sustainable microbial production of indigo, highlighting the strain's applicability and efficiency in environment friendly dye synthesis.

Systems biology of electrogenic Pseudomonas putida - multi-omics insights and metabolic engineering for enhanced 2-ketogluconate production

BACKGROUND

Pseudomonas putida KT2440 has emerged as a promising host for industrial bioproduction. However, its strictly aerobic nature limits the scope of applications. Remarkably, this microbe exhibits high bioconversion efficiency when cultured in an anoxic bio-electrochemical system (BES), where the anode serves as the terminal electron acceptor instead of oxygen. This environment facilitates the synthesis of commercially attractive chemicals, including 2-ketogluconate (2KG). To better understand this interesting electrogenic phenotype, we studied the BES-cultured strain on a systems level through multi-omics analysis. Inspired by our findings, we constructed novel mutants aimed at improving 2KG production.

RESULTS

When incubated on glucose, P. putida KT2440 did not grow but produced significant amounts of 2KG, along with minor amounts of gluconate, acetate, pyruvate, succinate, and lactate. 13C tracer studies demonstrated that these products are partially derived from biomass carbon, involving proteins and lipids. Over time, the cells exhibited global changes on both the transcriptomic and proteomic levels, including the shutdown of translation and cell motility, likely to conserve energy. These adaptations enabled the cells to maintain significant metabolic activity for several weeks. Acetate formation was shown to contribute to energy supply. Mutants deficient in acetate production demonstrated superior 2KG production in terms of titer, yield, and productivity. The ∆aldBI ∆aldBII double deletion mutant performed best, accumulating 2KG at twice the rate of the wild type and with an increased yield (0.96 mol/mol).

CONCLUSIONS

By integrating transcriptomic, proteomic, and metabolomic analyses, this work provides the first systems biology insight into the electrogenic phenotype of P. putida KT2440. Adaptation to anoxic-electrogenic conditions involved coordinated changes in energy metabolism, enabling cells to sustain metabolic activity for extended periods. The metabolically engineered mutants are promising for enhanced 2KG production under these conditions. The attenuation of acetate synthesis represents the first systems biology-informed metabolic engineering strategy for enhanced 2KG production in P. putida. This non-growth anoxic-electrogenic mode expands our understanding of the interplay between growth, glucose phosphorylation, and glucose oxidation into gluconate and 2KG in P. putida.

Anaerobic glucose uptake in Pseudomonas putida KT2440 in a bioelectrochemical system

Providing an anodic potential in a bio-electrochemical system to the obligate aerobe Pseudomonas putida enables anaerobic survival and allows the cells to overcome redox imbalances. In this setup, the bacteria could be exploited to produce chemicals via oxidative pathways at high yield. However, the absence of anaerobic growth and low carbon turnover rates remain as obstacles for the application of such an electro-fermentation technology. Growth and carbon turnover start with carbon uptake into the periplasm and cytosol. P. putida KT2440 has three native transporting systems for glucose, each differing in energy and redox demand. This architecture previously led to the hypothesis that internal redox and energy constraints ultimately limit cytoplasmic carbon utilization in a bio-electrochemical system. However, it remains largely unclear which uptake route is predominantly used by P. putida under electro-fermentative conditions. To elucidate this, we created three gene deletion mutants of P. putida KT2440, forcing the cells to exclusively utilize one of the routes. When grown in a bio-electrochemical system, the pathway mutants were heavily affected in terms of sugar consumption, current output and product formation. Surprisingly, however, we found that about half of the acetate formed in the cytoplasm originated from carbon that was put into the system via the inoculation biomass, while the other half came from the consumption of substrate. The deletion of individual sugar uptake routes did not alter significantly the secreted acetate concentrations among different strains even with different carbon sources. This means that the stoichiometry of the sugar uptake routes is not a limiting factor during electro-fermentation and that the low rates might be caused by other reasons, for example energy limitations or a yet-to-be-identified oxygen-dependent regulatory mechanism.

Improving microbial bioproduction under low-oxygen conditions

Microbial bioconversion provides access to a wide range of sustainably produced chemicals and commodities. However, industrial-scale bioproduction process operations are preferred to be anaerobic due to the cost associated with oxygen transfer. Anaerobic bioconversion generally offers limited substrate utilization profiles, lower product yields, and reduced final product diversity compared with aerobic processes. Bioproduction under conditions of reduced oxygen can overcome the limitations of fully aerobic and anaerobic bioprocesses, but many microbial hosts are not developed for low-oxygen bioproduction. Here, we describe advances in microbial strain engineering involving the use of redox cofactor engineering, genome-scale metabolic modeling, and functional genomics to enable improved bioproduction processes under low oxygen and provide a viable path for scaling these bioproduction systems to industrial scales.

PeakDecoder enables machine learning-based metabolite annotation and accurate profiling in multidimensional mass spectrometry measurements

Multidimensional measurements using state-of-the-art separations and mass spectrometry provide advantages in untargeted metabolomics analyses for studying biological and environmental bio-chemical processes. However, the lack of rapid analytical methods and robust algorithms for these heterogeneous data has limited its application. Here, we develop and evaluate a sensitive and high-throughput analytical and computational workflow to enable accurate metabolite profiling. Our workflow combines liquid chromatography, ion mobility spectrometry and data-independent acquisition mass spectrometry with PeakDecoder, a machine learning-based algorithm that learns to distinguish true co-elution and co-mobility from raw data and calculates metabolite identification error rates. We apply PeakDecoder for metabolite profiling of various engineered strains of Aspergillus pseudoterreus, Aspergillus niger, Pseudomonas putida and Rhodosporidium toruloides. Results, validated manually and against selected reaction monitoring and gas-chromatography platforms, show that 2683 features could be confidently annotated and quantified across 116 microbial sample runs using a library built from 64 standards.

Screening of natural phenazine producers for electroactivity in bioelectrochemical systems

Mediated extracellular electron transfer (EET) might be a great vehicle to connect microbial bioprocesses with electrochemical control in stirred-tank bioreactors. However, mediated electron transfer to date is not only much less efficient but also much less studied than microbial direct electron transfer to an anode. For example, despite the widespread capacity of pseudomonads to produce phenazine natural products, only Pseudomonas aeruginosa has been studied for its use of phenazines in bioelectrochemical applications. To provide a deeper understanding of the ecological potential for the bioelectrochemical exploitation of phenazines, we here investigated the potential electroactivity of over 100 putative diverse native phenazine producers and the performance within bioelectrochemical systems. Five species from the genera Pseudomonas, Streptomyces, Nocardiopsis, Brevibacterium and Burkholderia were identified as new electroactive bacteria. Electron discharge to the anode and electric current production correlated with the phenazine synthesis of Pseudomonas chlororaphis subsp. aurantiaca. Phenazine-1-carboxylic acid was the dominant molecule with a concentration of 86.1 μg/ml mediating an anodic current of 15.1 μA/cm² . On the other hand, Nocardiopsis chromatogenes used a wider range of phenazines at low concentrations and likely yet-unknown redox compounds to mediate EET, achieving an anodic current of 9.5 μA/cm² . Elucidating the energetic and metabolic usage of phenazines in these and other species might contribute to improving electron discharge and respiration. In the long run, this may enhance oxygen-limited bioproduction of value-added compounds based on mediated EET mechanisms.

Phosphite synthetic auxotrophy as an effective biocontainment strategy for the industrial chassis Pseudomonas putida

The inclusion of biosafety strategies into strain engineering pipelines is crucial for safe-by-design biobased processes. This in turn might enable a more rapid regulatory acceptance of bioengineered organisms in both industrial and environmental applications. For this reason, we equipped the industrially relevant microbial chassis Pseudomonas putida KT2440 with an effective biocontainment strategy based on a synthetic dependency on phosphite, which is generally not readily available in the environment. The produced PSAG-9 strain was first engineered to assimilate phosphite through the genome-integration of a phosphite dehydrogenase and a phosphite-specific transport complex. Subsequently, to deter the strain from growing on naturally assimilated phosphate, all native genes related to its transport were identified and deleted generating a strain unable to grow on media containing any phosphorous source other than phosphite. PSAG-9 exhibited fitness levels with phosphite similar to those of the wild type with phosphate, and low levels of escape frequency. Beyond biosafety, this strategy endowed P. putida with the capacity to be cultured under non-sterile conditions using phosphite as the sole phosphorous source with a reduced risk of contamination by other microbes, while displaying enhanced NADH regenerative capacity. These industrially beneficial features complement the metabolic advantages for which this species is known for, thereby strengthening it as a synthetic biology chassis with potential uses in industry, with suitability towards environmental release.

Mercury bioremediation by engineered Pseudomonas putida KT2440 with adaptationally optimized biosecurity circuit

Hazardous materials, such as heavy metals, are the major sources of health risk. Using genetically modified organisms (GMOs) to dispose heavy metals has the advantages of strong environmental compatibility and high efficiency. However, the biosecurity of GMOs used in the environment is a major concern. In this study, a self-controlled genetic circuit was designed and carefully fine-tuned for programmable expression in Pseudomonas putida KT2440, which is a widely used strain for environmental bioremediation. The cell behaviours were controlled by automatically sensing the variation of Hg²⁺ concentration without any inducer requirement or manual interventions. More than 98% Hg²⁺ was adsorbed by the engineered strain with a high cell recovery rate of 96% from waterbody. The remaining cells were killed by the suicide module after the mission was accomplished. The escape frequency of the engineered P. putida strain was lower than 10^-9 , which meets the recommendation of US NIH guideline for GMOs release (<10^-8 ). The same performance was achieved in a model experiment by using natural lake water with addition of Hg²⁺ . The microbial diversity analysis further confirmed that the remediation process made little impact on the indigenous ecosystem. Thus, this study provides a practical method for environmental remediation by using GMOs.

An automated DIY framework for experimental evolution of Pseudomonas putida

Adaptive laboratory evolution (ALE) is a general and effective strategy for optimizing the design of engineered genetic circuits and upgrading metabolic phenotypes. However, the specific characteristics of each microorganism typically ask for exclusive conditions that need to be adjusted to the biological chassis at stake. In this work, we have adopted a do-it-yourself (DIY) approach to implement a flexible and automated framework for performing ALE experiments with the environmental bacterium and metabolic engineering platform Pseudomonas putida. The setup includes a dual-chamber semi-continuous log-phase bioreactor design combined with an anti-biofilm layout to manage specific traits of this bacterium in long-term cultivation experiments. As a way of validation, the prototype was instrumental for selecting fast-growing variants of a P. putida strain engineered to metabolize D-xylose as sole carbon and energy source after running an automated 42 days protocol of iterative regrowth. Several genomic changes were identified in the evolved population that pinpointed the role of RNA polymerase in controlling overall physiological conditions during metabolism of the new carbon source.

Pseudomonas putida KT2440 endures temporary oxygen limitations

The obligate aerobic nature of Pseudomonas putida, one of the most prominent whole-cell biocatalysts emerging for industrial bioprocesses, questions its ability to be cultivated in large-scale bioreactors, which exhibit zones of low dissolved oxygen tension. P. putida KT2440 was repeatedly subjected to temporary oxygen limitations in scale-down approaches to assess the effect on growth and an exemplary production of rhamnolipids. At those conditions, the growth and production of P. putida KT2440 were decelerated compared to well-aerated reference cultivations, but remarkably, final biomass and rhamnolipid titers were similar. The robust growth behavior was confirmed across different cultivation systems, media compositions, and laboratories, even when P. putida KT2440 was repeatedly exposed to dual carbon and oxygen starvation. Quantification of the nucleotides ATP, ADP, and AMP revealed a decrease of intracellular ATP concentrations with increasing duration of oxygen starvation, which can, however, be restored when re-supplied with oxygen. Only small changes in the proteome were detected when cells encountered oscillations in dissolved oxygen tensions. Concluding, P. putida KT2440 appears to be able to cope with repeated oxygen limitations as they occur in large-scale bioreactors, affirming its outstanding suitability as a whole-cell biocatalyst for industrial-scale bioprocesses.

The anoxic electrode-driven fructose catabolism of Pseudomonas putida KT2440

Pseudomonas putida (P. putida) is a microorganism of interest for various industrial processes, yet its strictly aerobic nature limits application. Despite previous attempts to adapt P. putida to anoxic conditions via genetic engineering or the use of a bioelectrochemical system (BES), the problem of energy shortage and internal redox imbalance persists. In this work, we aimed to provide the cytoplasmic metabolism with different monosaccharides, other than glucose, and explored the physiological response in P. putida KT2440 during bioelectrochemical cultivation. The periplasmic oxidation cascade was found to be able to oxidize a wide range of aldoses to their corresponding (keto-)aldonates. Unexpectedly, isomerization of the ketose fructose to mannose also enabled oxidation by glucose dehydrogenase, a new pathway uncovered for fructose metabolism in P. putida KT2440 in BES. Besides the isomerization, the remainder of fructose was imported into the cytoplasm and metabolized. This resulted in a higher NADPH/NADP+ ratio, compared to glucose. Comparative proteomics further revealed the upregulation of proteins in the lower central carbon metabolism during the experiment. These findings highlight that the choice of a substrate in BES can target cytosolic and periplasmic oxidation pathways, and that electrode-driven redox balancing can drive these pathways in P. putida under anaerobic conditions.

Micro-aerobic production of isobutanol with engineered Pseudomonas putida

Pseudomonas putida KT2440 is emerging as a promising microbial host for biotechnological industry due to its broad range of substrate affinity and resilience to physicochemical stresses. Its natural tolerance towards aromatics and solvents qualifies this versatile microbe as promising candidate to produce next generation biofuels such as isobutanol. In this study, we scaled-up the production of isobutanol with P. putida from shake flask to fed-batch cultivation in a 30 L bioreactor. The design of a two-stage bioprocess with separated growth and production resulted in 3.35 gisobutanol L-1. Flux analysis revealed that the NADPH expensive formation of isobutanol exceeded the cellular catabolic supply of NADPH finally causing growth retardation. Concomitantly, the cell counteracted to the redox imbalance by increased formation of 2-ketogluconic thereby providing electrons for the respiratory ATP generation. Thus, P. putida partially uncoupled ATP formation from the availability of NADH. The quantitative analysis of intracellular pyridine nucleotides NAD(P)+ and NAD(P)H revealed elevated catabolic and anabolic reducing power during aerobic production of isobutanol. Additionally, the installation of micro-aerobic conditions during production doubled the integral glucose-to-isobutanol conversion yield to 60 mgisobutanol gglucose -1 while preventing undesired carbon loss as 2-ketogluconic acid.

Identification of Oxygen-Independent Pathways for Pyridine Nucleotide and Coenzyme A Synthesis in Anaerobic Fungi by Expression of Candidate Genes in Yeast

Neocallimastigomycetes are unique examples of strictly anaerobic eukaryotes. This study investigates how these anaerobic fungi bypass reactions involved in synthesis of pyridine nucleotide cofactors and coenzyme A that, in canonical fungal pathways, require molecular oxygen. Analysis of Neocallimastigomycetes proteomes identified a candidate l-aspartate-decarboxylase (AdcA) and l-aspartate oxidase (NadB) and quinolinate synthase (NadA), constituting putative oxygen-independent bypasses for coenzyme A synthesis and pyridine nucleotide cofactor synthesis. The corresponding gene sequences indicated acquisition by ancient horizontal gene transfer (HGT) events involving bacterial donors. To test whether these enzymes suffice to bypass corresponding oxygen-requiring reactions, they were introduced into fms1Δ and bna2Δ Saccharomyces cerevisiae strains. Expression of nadA and nadB from Piromyces finnis and adcA from Neocallimastix californiae conferred cofactor prototrophy under aerobic and anaerobic conditions. This study simulates how HGT can drive eukaryotic adaptation to anaerobiosis and provides a basis for elimination of auxotrophic requirements in anaerobic industrial applications of yeasts and fungi. IMPORTANCE NAD (NAD⁺) and coenzyme A (CoA) are central metabolic cofactors whose canonical biosynthesis pathways in fungi require oxygen. Anaerobic gut fungi of the Neocallimastigomycota phylum are unique eukaryotic organisms that adapted to anoxic environments. Analysis of Neocallimastigomycota genomes revealed that these fungi might have developed oxygen-independent biosynthetic pathways for NAD⁺ and CoA biosynthesis, likely acquired through horizontal gene transfer (HGT) from prokaryotic donors. We confirmed functionality of these putative pathways under anaerobic conditions by heterologous expression in the yeast Saccharomyces cerevisiae. This approach, combined with sequence comparison, offers experimental insight on whether HGT events were required and/or sufficient for acquiring new traits. Moreover, our results demonstrate an engineering strategy for enabling S. cerevisiae to grow anaerobically in the absence of the precursor molecules pantothenate and nicotinate, thereby contributing to alleviate oxygen requirements and to move closer to prototrophic anaerobic growth of this industrially relevant yeast.

Adaptive Laboratory Evolution Restores Solvent Tolerance in Plasmid-Cured Pseudomonas putida S12: a Molecular Analysis

Pseudomonas putida S12 is inherently solvent tolerant and constitutes a promising platform for biobased production of aromatic compounds and biopolymers. The megaplasmid pTTS12 of P. putida S12 carries several gene clusters involved in solvent tolerance, and the removal of this megaplasmid caused a significant reduction in solvent tolerance. In this study, we succeeded in restoring solvent tolerance in plasmid-cured P. putida S12 using adaptive laboratory evolution (ALE), underscoring the innate solvent tolerance of this strain. Whole-genome sequencing identified several single nucleotide polymorphisms (SNPs) and a mobile element insertion enabling ALE-derived strains to survive and sustain growth in the presence of a high toluene concentration (10% [vol/vol]). We identified mutations in an RND efflux pump regulator, arpR, that resulted in constitutive upregulation of the multifunctional efflux pump ArpABC. SNPs were also found in the intergenic region and subunits of ATP synthase, RNA polymerase subunit β', a global two-component regulatory system (GacA/GacS), and a putative AraC family transcriptional regulator, Afr. Transcriptomic analysis further revealed a constitutive downregulation of energy-consuming activities in ALE-derived strains, such as flagellar assembly, FoF1 ATP synthase, and membrane transport proteins. In summary, constitutive expression of a solvent extrusion pump in combination with high metabolic flexibility enabled the restoration of the solvent tolerance trait in P. putida S12 lacking its megaplasmid.IMPORTANCE Sustainable production of high-value chemicals can be achieved by bacterial biocatalysis. However, bioproduction of biopolymers and aromatic compounds may exert stress on the microbial production host and limit the resulting yield. Having a solvent tolerance trait is highly advantageous for microbial hosts used in the biobased production of aromatics. The presence of a megaplasmid has been linked to the solvent tolerance trait of Pseudomonas putida; however, the extent of innate, intrinsic solvent tolerance in this bacterium remained unclear. Using adaptive laboratory evolution, we successfully adapted the plasmid-cured P. putida S12 strain to regain its solvent tolerance. Through these adapted strains, we began to clarify the causes, origins, limitations, and trade-offs of the intrinsic solvent tolerance in P. putida This work sheds light on the possible genetic engineering targets to enhance solvent tolerance in Pseudomonas putida as well as other bacteria.

A navigation guide of synthetic biology tools for Pseudomonas putida

Pseudomonas putida is a microbial chassis of huge potential for industrial and environmental biotechnology, owing to its remarkable metabolic versatility and ability to sustain difficult redox reactions and operational stresses, among other attractive characteristics. A wealth of genetic and in silico tools have been developed to enable the unravelling of its physiology and improvement of its performance. However, the rise of this microbe as a promising platform for biotechnological applications has resulted in diversification of tools and methods rather than standardization and convergence. As a consequence, multiple tools for the same purpose have been generated, whilst most of them have not been embraced by the scientific community, which has led to compartmentalization and inefficient use of resources. Inspired by this and by the substantial increase in popularity of P. putida, we aim herein to bring together and assess all currently available (wet and dry) synthetic biology tools specific for this microbe, focusing on the last 5 years. We provide information on the principles, functionality, advantages and limitations, with special focus on their use in metabolic engineering. Additionally, we compare the tool portfolio for P. putida with those for other bacterial chassis and discuss potential future directions for tool development. Therefore, this review is intended as a reference guide for experts and new 'users' of this promising chassis.

A metabolic and physiological design study of Pseudomonas putida KT2440 capable of anaerobic respiration

BACKGROUND

Pseudomonas putida KT2440 is a metabolically versatile, HV1-certified, genetically accessible, and thus interesting microbial chassis for biotechnological applications. However, its obligate aerobic nature hampers production of oxygen sensitive products and drives up costs in large scale fermentation. The inability to perform anaerobic fermentation has been attributed to insufficient ATP production and an inability to produce pyrimidines under these conditions. Addressing these bottlenecks enabled growth under micro-oxic conditions but does not lead to growth or survival under anoxic conditions.

RESULTS

Here, a data-driven approach was used to develop a rational design for a P. putida KT2440 derivative strain capable of anaerobic respiration. To come to the design, data derived from a genome comparison of 1628 Pseudomonas strains was combined with genome-scale metabolic modelling simulations and a transcriptome dataset of 47 samples representing 14 environmental conditions from the facultative anaerobe Pseudomonas aeruginosa.

CONCLUSIONS

The results indicate that the implementation of anaerobic respiration in P. putida KT2440 would require at least 49 additional genes of known function, at least 8 genes encoding proteins of unknown function, and 3 externally added vitamins.

Coupling an Electroactive Pseudomonas putida KT2440 with Bioelectrochemical Rhamnolipid Production

Sufficient supply of oxygen is a major bottleneck in industrial biotechnological synthesis. One example is the heterologous production of rhamnolipids using Pseudomonas putida KT2440. Typically, the synthesis is accompanied by strong foam formation in the reactor vessel hampering the process. It is caused by the extensive bubbling needed to sustain the high respirative oxygen demand in the presence of the produced surfactants. One way to reduce the oxygen requirement is to enable the cells to use the anode of a bioelectrochemical system (BES) as an alternative sink for their metabolically derived electrons. We here used a P. putida KT2440 strain that interacts with the anode using mediated extracellular electron transfer via intrinsically produced phenazines, to perform heterologous rhamnolipid production under oxygen limitation. The strain P. putida RL-PCA successfully produced 30.4 ± 4.7 mg/L mono-rhamnolipids together with 11.2 ± 0.8 mg/L of phenazine-1-carboxylic acid (PCA) in 500-mL benchtop BES reactors and 30.5 ± 0.5 mg/L rhamnolipids accompanied by 25.7 ± 8.0 mg/L PCA in electrode containing standard 1-L bioreactors. Hence, this study marks a first proof of concept to produce glycolipid surfactants in oxygen-limited BES with an industrially relevant strain.

Enabling anoxic acetate assimilation by electrode-driven respiration in the obligate aerobe, Pseudomonas putida

This study examined the obligate aerobe, Pseudomonas putida, using acetate as the sole carbon and energy source, and respiration via an anode as the terminal electron acceptor under anoxic conditions. P. putida showed significantly different acetate assimilation in a closed-circuit microbial fuel cell (CC-MFC) compared to an open circuit MFC (OC-MFC). More than 72% (2.6 mmol) of acetate was consumed during 84 hrs in the CC-MFC in contrast to the no acetate consumption observed in the OC-MFC. The CC-MFC produced 150 μA (87 C) from acetate metabolization. Electrode-based respiration reduced the NADH/NAD⁺ ratio anaerobically, which is similar to the aerobic condition. The CC-MFC showed significantly higher acetyl-CoA synthetase activity than the OC-MFC (0.028 vs. 0.001 μmol/min/mg), which was comparable to the aerobic condition (circa 60%). Overall, electrode-based respiration enables P. putida to metabolize acetate under anoxic conditions and provide a platform to regulate the bacterial redox balance without oxygen.