Controlling the Production of Pseudomonas Phenazines by Modulating the Genetic Repertoire

Exploring phenazine electron transfer interaction with elements of the respiratory pathways of Pseudomonas putida and Pseudomonas aeruginosa

Pseudomonas aeruginosa phenazines contribute to survival under microaerobic and anaerobic conditions by extracellular electron discharge to regulate cellular redox balances. This electron discharge is also attractive to be used for bioelectrochemical applications. However, elements of the respiratory pathways that interact with phenazines are not well understood. Five terminal oxidases are involved in the aerobic electron transport chain (ETC) of Pseudomonas putida and P. aeruginosa. The latter bacterium also includes four reductases that allow for denitrification. Here, we explored if phenazine-1-carboxylic acid interacts with those elements to enhance anodic electron discharge and drive bacterial growth in oxygen-limited conditions. Bioelectrochemical evaluations of terminal oxidase-deficient mutants of both Pseudomonas strains and P. aeruginosa with stimulated denitrification pathways indicated no direct beneficial interaction of phenazines with ETC elements for extracellular electron discharge. However, the single usage of the Cbb3-2 oxidase increased phenazine production, electron discharge, and cell growth. Assays with purified periplasmic cytochromes NirM and NirS indicated that pyocyanin acts as their electron donor. We conclude that phenazines play an important role in electron transfer to, between, and from terminal oxidases under oxygen-limiting conditions and their modulation might enhance EET. However, the phenazine-anode interaction cannot replace oxygen respiration to deliver energy for biomass formation.

Loss of LasR function leads to decreased repression of Pseudomonas aeruginosa PhoB activity at physiological phosphate concentrations

While the Pseudomonas aeruginosa LasR transcription factor plays a role in quorum sensing (QS) across phylogenetically-distinct lineages, isolates with loss-of-function mutations in lasR (LasR- strains) are commonly found in diverse settings including infections where they are associated with worse clinical outcomes. In LasR- strains, the transcription factor RhlR, which is controlled by LasR, can be alternately activated in low inorganic phosphate (Pi) concentrations via the two-component system PhoR-PhoB. Here, we demonstrate a new link between LasR and PhoB in which the absence of LasR increases PhoB activity at physiological Pi concentrations and raises the Pi concentration necessary for PhoB inhibition. PhoB activity was also less repressed by Pi in mutants lacking different QS regulators (RhlR and PqsR) and in mutants lacking genes required for the production of QS-regulated phenazines suggesting that decreased phenazine production was one reason for decreased PhoB repression by Pi in LasR- strains. In addition, the CbrA-CbrB two-component system, which is elevated in LasR- strains, was necessary for reduced PhoB repression by Pi and a Δcrc mutant, which lacks the CbrA-CbrB-controlled translational repressor, activated PhoB at higher Pi concentrations than the wild type. The ΔlasR mutant had a PhoB-dependent growth advantage in a medium with no added Pi and increased virulence-determinant gene expression in a medium with physiological Pi, in part through reactivation of QS. This work suggests PhoB activity may contribute to the virulence of LasR- P. aeruginosa and subsequent clinical outcomes.

Electrobiochemical skills of Pseudomonas aeruginosa species that produce pyocyanin or pyoverdine for glycerol oxidation in a microbial fuel cell

Pseudomonas aeruginosa can produce pigments, which mediate external electron transfer (EET). Depending on the mediator, this species can be explored in bioelectrosystems to harvest energy or to obtain chemicals from residual organic compounds. This study has compared the performance of microbial fuel cells (MFCs) inoculated with a Pseudomonas aeruginosa isolate, namely EW603 or EW819, which produce pyocyanin and pyoverdine, respectively. The efficiency of these MFCs in glycerol, a typical residue of biodiesel production, were also compared. The MFCs exhibited different performances. The maximum voltage was 411 and 281 mV m², the power density was 40.1 and 21.3 mW m^-2, and the coulombic efficiency was 5.16 and 1.49% for MFC-EW603 and MFC-EW819, respectively. MFC-EW603 and MFC-EW819 achieved maximum current at 560 and 2200 Ω, at 141.2 and 91.3 mA m^-2, respectively. When the system was operated at the respective maximum current output, MFC-EW603 consumed the total glycerol content (11 mmol L^-1), and no products could be detected after 50 h. In turn, acetic and butyric acids were detected at the end of MFC-EW819 operation (75 h). The results suggested that P. aeruginosa metabolism can be steered in the MFC to generate current or microbial products depending on the pigment-producing strain and the conditions applied to the system, such as the external resistance. In addition, gene cluster pathways related to phenazine production (phzA and phzB) and other electrogenic-related genes (mexGHI-opmB) were identified in the strain genomes, supporting the findings. These results open new possibilities for using glycerol in bioelectrochemical systems.

The Bacillus cereus toxin alveolysin disrupts the intestinal epithelial barrier by inducing microtubule disorganization through CFAP100

Bacillus cereus is a Gram-positive bacterium that mainly causes self-limiting emetic or diarrheal illness but can also cause skin infections and bacteremia. Symptoms of B. cereus ingestion depend on the production of various toxins that target the gastric and intestinal epithelia. From a screen of bacterial isolates from human stool samples that compromised intestinal barrier function in mice, we identified a strain of B. cereus that disrupted tight and adherens junctions in the intestinal epithelium. This activity was mediated by the pore-forming exotoxin alveolysin, which increased the production of the membrane-anchored protein CD59 and of cilia- and flagella-associated protein 100 (CFAP100) in intestinal epithelial cells. In vitro, CFAP100 interacted with microtubules and promoted microtubule polymerization. CFAP100 overexpression stabilized microtubules in intestinal epithelial cells, leading to disorganization of the microtubule network and perturbation of tight and adherens junctions. The disruption of cell junctions by alveolysin depended on the increase in CFAP100, which in turn depended on CD59 and the activation of PI3K-AKT signaling. These findings demonstrate that, in addition to forming membrane pores, B. cereus alveolysin can permeabilize the intestinal epithelium by disrupting epithelial cell junctions in a manner that is consistent with intestinal symptoms and may allow the bacteria to escape the intestine and cause systemic infections. Our results suggest the potential value of targeting alveolysin or CFAP100 to prevent B. cereus-associated intestinal diseases and systemic infections.

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.

The two faces of pyocyanin - why and how to steer its production?

The ambiguous nature of pyocyanin was noted quite early after its discovery. This substance is a recognized Pseudomonas aeruginosa virulence factor that causes problems in cystic fibrosis, wound healing, and microbiologically induced corrosion. However, it can also be a potent chemical with potential use in a wide variety of technologies and applications, e.g. green energy production in microbial fuel cells, biocontrol in agriculture, therapy in medicine, or environmental protection. In this mini-review, we shortly describe the properties of pyocyanin, its role in the physiology of Pseudomonas and show the ever-growing interest in it. We also summarize the possible ways of modulating pyocyanin production. We underline different approaches of the researchers that aim either at lowering or increasing pyocyanin production by using different culturing methods, chemical additives, physical factors (e.g. electromagnetic field), or genetic engineering techniques. The review aims to present the ambiguous character of pyocyanin, underline its potential, and signalize the possible further research directions.

Recent Developments in the Biological Activities, Bioproduction, and Applications of Pseudomonas spp. Phenazines

Phenazines are a large group of heterocyclic nitrogen-containing compounds with demonstrated insecticidal, antimicrobial, antiparasitic, and anticancer activities. These natural compounds are synthesized by several microorganisms originating from diverse habitats, including marine and terrestrial sources. The most well-studied producers belong to the Pseudomonas genus, which has been extensively investigated over the years for its ability to synthesize phenazines. This review is focused on the research performed on pseudomonads' phenazines in recent years. Their biosynthetic pathways, mechanism of regulation, production processes, bioactivities, and applications are revised in this manuscript.

Secondary Metabolites Produced during Aspergillus fumigatus and Pseudomonas aeruginosa Biofilm Formation

In cystic fibrosis (CF), mucus plaques are formed in the patient's lungs, creating a hypoxic condition and a propitious environment for colonization and persistence of many microorganisms. There is clinical evidence showing that Aspergillus fumigatus can cocolonize CF patients with Pseudomonas aeruginosa, which has been associated with lung function decline. P. aeruginosa produces several compounds with inhibitory and antibiofilm effects against A. fumigatus in vitro; however, little is known about the fungal compounds produced in counterattack. Here, we annotated fungal and bacterial secondary metabolites (SM) produced in mixed biofilms under normoxia and hypoxia conditions. We detected nine SM produced by P. aeruginosa. Phenazines and different analogs of pyoverdin were the main compounds produced by P. aeruginosa, and their secretion levels were increased by the fungal presence. The roles of the two operons responsible for phenazine production (phzA1 and phzA2) were also investigated, and mutants lacking one of those operons were able to produce partial sets of phenazines. We detected a total of 20 SM secreted by A. fumigatus either in monoculture or in coculture with P. aeruginosa. All these compounds were secreted during biofilm formation in either normoxia or hypoxia. However, only eight compounds (demethoxyfumitremorgin C, fumitremorgin, ferrichrome, ferricrocin, triacetylfusigen, gliotoxin, gliotoxin E, and pyripyropene A) were detected during biofilm formation by the coculture of A. fumigatus and P. aeruginosa under normoxia and hypoxia conditions. Overall, we showed how diverse SM secretion is during A. fumigatus and P. aeruginosa mixed culture and how this can affect biofilm formation in normoxia and hypoxia. IMPORTANCE The interaction between Pseudomonas aeruginosa and Aspergillus fumigatus has been well characterized in vitro. In this scenario, the bacterium exerts a strong inhibitory effect against the fungus. However, little is known about the metabolites produced by the fungus to counterattack the bacteria. Our work aimed to annotate secondary metabolites (SM) secreted during coculture between P. aeruginosa and A. fumigatus during biofilm formation in both normoxia and hypoxia. The bacterium produces several different types of phenazines and pyoverdins in response to presence of the fungus. In contrast, we were able to annotate 29 metabolites produced during A. fumigatus biofilm formation, but only 8 compounds were detected during biofilm formation by the coculture of A. fumigatus and P. aeruginosa upon either normoxia or hypoxia. In conclusion, we detected many SM secreted during A. fumigatus and P. aeruginosa biofilm formation. This analysis provides several opportunities to understand the interactions between these two species.

Beyond Soil-Dwelling Actinobacteria: Fantastic Antibiotics and Where to Find Them

Bacterial secondary metabolites represent an invaluable source of bioactive molecules for the pharmaceutical and agrochemical industries. Although screening campaigns for the discovery of new compounds have traditionally been strongly biased towards the study of soil-dwelling Actinobacteria, the current antibiotic resistance and discovery crisis has brought a considerable amount of attention to the study of previously neglected bacterial sources of secondary metabolites. The development and application of new screening, sequencing, genetic manipulation, cultivation and bioinformatic techniques have revealed several other groups of bacteria as producers of striking chemical novelty. Biosynthetic machineries evolved from independent taxonomic origins and under completely different ecological requirements and selective pressures are responsible for these structural innovations. In this review, we summarize the most important discoveries related to secondary metabolites from alternative bacterial sources, trying to provide the reader with a broad perspective on how technical novelties have facilitated the access to the bacterial metabolic dark matter.

Role of phenazine-enzyme physiology for current generation in a bioelectrochemical system

Pseudomonas aeruginosa produces phenazine-1-carboxylic acid (PCA) and pyocyanin (PYO), which aid its anaerobic survival by mediating electron transfer to distant oxygen. These natural secondary metabolites are being explored in biotechnology to mediate electron transfer to the anode of bioelectrochemical systems. A major challenge is that only a small fraction of electrons from microbial substrate conversion is recovered. It remained unclear whether phenazines can re-enter the cell and thus, if the electrons accessed by the phenazines arise mainly from cytoplasmic or periplasmic pathways. Here, we prove that the periplasmic glucose dehydrogenase (Gcd) of P. aeruginosa and P. putida is involved in the reduction of natural phenazines. PYO displayed a 60-fold faster enzymatic reduction than PCA; PCA was, however, more stable for long-term electron shuttling to the anode. Evaluation of a Gcd knockout and overexpression strain showed that up to 9% of the anodic current can be designated to this enzymatic reaction. We further assessed phenazine uptake with the aid of two molecular biosensors, which experimentally confirm the phenazines' ability to re-enter the cytoplasm. These findings significantly advance the understanding of the (electro) physiology of phenazines for future tailoring of phenazine electron discharge in biotechnological applications.