Molecular Signature of Pseudomonas aeruginosa with Simultaneous Nanomolar Detection of Quorum Sensing Signaling Molecules at a Boron-Doped Diamond Electrode

Emerging advances in biosensor technologies for quorum sensing signal molecules

Quorum sensing is a physiological phenomenon of microbial cell-to-cell information exchange, which relies on the quorum sensing signal molecules (QSSMs) to communicate and coordinate collective processes. Quorum sensing enables bacteria to alter their behavior as the population density and species composition of the bacterial community change. Effective detection of QSSMs is paramount for regulating microbial community behavior. However, traditional detection methods face the shortcomings of complex operation, high costs, and lack of portability. By combining the advantage of biosensing and nanomaterials, the biosensors play a pivotal significance in QSSM detection. In this review, we first briefly describe the QSSM classification and common detection techniques. Then, we provide a comprehensive summary of research progress in biosensor-based QSSM detection according to the transduction mechanism. Finally, challenges and development trends of biosensors for QSSM detection are discussed. We believe it offers valuable insights into this burgeoning research area.

Inhibition of Acinetobacter nosocomialis twitching motility by quinolones produced by Pseudomonas aeruginosa

Twitching motility in A. nosocomialis is a key virulence factor linked to antibiotic resistance and pathogenicity. This study revealed that the Pseudomonas quinolone signal (PQS) and hydroxy-containing quinolones significantly inhibit motility without affecting bacterial growth, highlighting their potential as targets for controlling bacterial virulence.

Comprehensive insights into UTIs: from pathophysiology to precision diagnosis and management

Urinary tract infections (UTIs) are the second most common infectious disease, predominantly impacting women with 150 million individuals affected globally. It increases the socio-economic burden of society and is mainly caused by Escherichia coli, Proteus mirabilis, Klebsiella pneumoniae, Enterobacter spp., and Staphylococcus spp. The severity of the infection correlates with the host factors varying from acute to chronic infections. Even with a high incidence rate, the diagnosis is mainly based on the symptoms, dipstick analysis, and culture analysis, which are time-consuming, labour-intensive, and lacking sensitivity and specificity. During this period, medical professionals prescribe empirical antibiotics, which may increase the antimicrobial resistance rate. Timely and precise UTI diagnosis is essential for addressing antibiotic resistance and improving overall quality of life. In response to these challenges, new techniques are emerging. The review provides a comprehensive overview of the global burden of UTIs, associated risk factors, implicated organisms, traditional and innovative diagnostic methods, and approaches to UTI treatment and prevention.

Development of Novel Surface-Enhanced Raman Spectroscopy-Based Biosensors by Controlling the Roughness of Gold/Alumina Platforms for Highly Sensitive Detection of Pyocyanin Secreted from Pseudomonas aeruginosa

Pyocyanin is considered a maker of Pseudomonas aeruginosa (P. aeruginosa) infection. Pyocyanin is among the toxins released by the P. aeruginosa bacteria. Therefore, the development of a direct detection of PYO is crucial due to its importance. Among the different optical techniques, the Raman technique showed unique advantages because of its fingerprint data, no sample preparation, and high sensitivity besides its ease of use. Noble metal nanostructures were used to improve the Raman response based on the surface-enhanced Raman scattering (SERS) technique. Anodic metal oxide attracts much interest due to its unique morphology and applications. The porous metal structure provides a large surface area that could be used as a hard template for periodic nanostructure array fabrication. Porous shapes and sizes could be controlled by controlling the anodization parameters, including the anodization voltage, current, temperature, and time, besides the metal purity and the electrolyte type/concentration. The anodization of aluminum foil results in anodic aluminum oxide (AAO) formation with different roughness. Here, we will use the roughness as hotspot centers to enhance the Raman signals. Firstly, a thin film of gold was deposited to develop gold/alumina (Au/AAO) platforms and then applied as SERS-active surfaces. The morphology and roughness of the developed substrates were investigated using scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques. The Au/AAO substrates were used for monitoring pyocyanin secreted from Pseudomonas aeruginosa microorganisms based on the SERS technique. The results showed that the roughness degree affects the enhancement efficiency of this sensor. The high enhancement was obtained in the case of depositing a 30 nm layer of gold onto the second anodized substrates. The developed sensor showed high sensitivity toward pyocyanin with a limit of detection of 96 nM with a linear response over a dynamic range from 1 µM to 9 µM.

Quorum quenching effect of cyclodextrins on the pyocyanin and pyoverdine production of Pseudomonas aeruginosa

Various virulence determinants in Pseudomonas aeruginosa are regulated by the quorum sensing (QS) network producing and releasing signalling molecules. Two of these virulence determinants are the pyocyanin and pyoverdine, which interfere with multiple cellular functions during infection. The application of QS-inhibiting agents, such as cyclodextrins (CDs), appears to be a promising approach. Further to method development, this research tested in large-volume test systems the effect of α- and β-CD (ACD, BCD) at 1, 5, and 10 mM concentrations on the production of pyocyanin in the P. aeruginosa model system. The concentration and time-dependent quorum quenching effect of native CDs and their derivatives on pyoverdine production was tested in a small-volume high-throughput system. In the large-volume system, both ACD and BCD significantly inhibited pyocyanin production, but ACD to a greater extent. 10 mM ACD resulted in 58% inhibition, while BCD only ~40%. Similarly, ACD was more effective in the inhibition of pyoverdine production; nevertheless, the results of RMANOVA demonstrated the significant efficiency of both ACD and BCD, as well as their derivatives. Both the contact time and the cyclodextrin treatments significantly influenced pyoverdine production. In this case, the inhibitory effect of ACD after 48 h at 12.5 mM was 57%, while the inhibitory effect of BCD and its derivatives was lower than 40%. The high-level significant inhibition of both pyocyanin and pyoverdine production by ACD was detectable. Consequently, the potential value of CDs as QS inhibitors and the antivirulence strategy should be considered. KEYPOINTS: • Applicability of a simplified method for quantification of pyocyanin production was demonstrated. • The cyclodextrins significantly affected the pyocyanin and pyoverdine production. • The native ACD exhibited the highest attenuation in pyoverdine production.

A paper biosensor for overcoming matrix effects interfering with the detection of sputum pyocyanin with competitive immunoassays

Detecting sputum pyocyanin (PYO) with a competitive immunoassay is a promising approach for diagnosing Pseudomonas aeruginosa respiratory infections. However, it is not possible to perform a negative control to evaluate matrix-effects in competitive immunoassays, and the highly complex sputum matrix often interferes with target detection. Here, we show that these issues are alleviated by performing competitive immunoassays with a paper biosensor. The biosensing platform consists of a paper reservoir, which contains antibody-coated gold nanoparticles, and a substrate containing a competing recognition element, which is a piece of paper modified with an albumin-antigen conjugate. Detection of PYO with a limit of detection of 4.7·10-3 µM and a dynamic range between 4.7·10-1 µM and 47.6 µM is accomplished by adding the sample to the substrate with the competing element and pressing the reservoir against it for 5 min. When tested with patient samples, the biosensor was able to qualitatively differentiate spiked from non-spiked samples, whereas ELISA did not show a clear cut-off between them. Furthermore, the relative standard deviation was lower when determining sputum with the paper-based biosensor. These features, along with a mild liquefaction step that circumvents the use of harsh chemicals or instruments, make our biosensor a good candidate for diagnosing Pseudomonas infections at the bedside through the detection of sputum PYO.

Rapid fingerprinting of bacterial species using nanocavities created on screen-printed electrodes modified by β-cyclodextrin

Rapid and precise identification of infectious microorganisms is important across a range of applications where microbial contamination can cause serious issues ranging from microbial resistance to corrosion. In this paper a screen-printed, polymeric β-cyclodextrin (β-CD) modified electrode, affording nanocavities for inclusion of the analytes, is shown as a disposable sensor capable of identifying bacteria by their metabolites. Three bacterial species were tested: two from the Pseudomonas genus, Pseudomonas fluorescens (P. fluorescens) and Pseudomonas aeruginosa (P. aeruginosa), and Serratia marcescens (S. marcescens), a member of the family, Enterobacteriaceae. On biofilm formation each species gave distinct, reproducible, redox fingerprints with a detection limit of 4 × 10-8 M. Square wave adsorptive stripping voltammetry (SWAdSV) was used for detection. Scanning electron microscopy (SEM) and cyclic voltammetry (CV) techniques were used to characterize the morphology and electrical conductivity of the modified electrode. In comparison to the bare screen-printed electrode, the modified electrode showed a considerably higher performance and offered an excellent sensitivity along with a relatively fast analysis time.

Electrochemical Sensors Based on MoSx -Functionalized Laser-Induced Graphene for Real-Time Monitoring of Phenazines Produced by Pseudomonas aeruginosa

Pseudomonas aeruginosa (P. aeruginosa) is an opportunistic pathogen causing infections in blood and implanted devices. Traditional identification methods take more than 24 h to produce results. Molecular biology methods expedite detection, but require an advanced skill set. To address these challenges, this work demonstrates functionalization of laser-induced graphene (LIG) for developing flexible electrochemical sensors for P. aeruginosa based on phenazines. Electrodeposition as a facile approach is used to functionalize LIG with molybdenum polysulfide (MoSx ). The sensor's limit of detection (LOD), sensitivity, and specificity are determined in broth, agar, and wound simulating medium (WSM). Control experiments with Escherichia coli, which does not produce phenazines, demonstrate specificity of sensors for P. aeruginosa. The LOD for pyocyanin (PYO) and phenazine-1-carboxylic acid (PCA) is 0.19 × 10-6  and 1.2 × 10-6  m, respectively. Furthermore, the highly stable sensors enable real-time monitoring of P. aeruginosa biofilms over several days. Comparing square wave voltammetry data over time shows time-dependent generation of phenazines. In particular, two configurations-"Normal" and "Flipped"-are studied, showing that the phenazines time dynamics vary depending on how cells interact with sensors. The reported results demonstrate the potential of the developed sensors for integration with wound dressings for early diagnosis of P. aeruginosa infection.

Highly Sensitive Detection of PQS Quorum Sensing in Pseudomonas Aeruginosa Using Screen-Printed Electrodes Modified with Nanomaterials

The rapid diagnosis of Pseudomonas aeruginosa infection is very important because this bacterium is one of the main sources of healthcare-associated infections. Pseudomonas quinolone signal (PQS) is a specific molecule for quorum sensing (QS) in P. aeruginosa, a form of cell-to-cell bacterial communication and its levels can allow the determination of the bacterial population. In this study, the development of the first electrochemical detection of PQS using screen-printed electrodes modified with carbon nanotubes (CNT-SPE) is reported. The electrochemical fingerprint of PQS was determined using different electrode materials and screen-printed electrodes modified with different nanomaterials. The optimization of the method in terms of electrolyte, pH, and electrochemical technique was achieved. The quantification of PQS was performed using one of the anodic peaks in the electrochemical fingerprint of the PQS on the CNT-SPE. The sensor exhibited a linear range from 0.1 to 15 µM, with a limit of detection of 50 nM. The sensor allowed the selective detection of PQS, with low interference from other QS molecules. The sensor was successfully applied to analysis of real samples (spiked urine and human serum samples, spiked microbiological growth media, and microbiological cultures).

Label-Free Electrochemical Aptasensor for the Detection of the 3-O-C12-HSL Quorum-Sensing Molecule in Pseudomonas aeruginosa

Pseudomonas aeruginosa, an opportunistic Gram-negative bacterium, is one of the main sources of infections in healthcare environments, making its detection very important. N-3-oxo-dodecanoyl L-homoserine lactone (3-O-C₁₂-HSL) is a characteristic molecule of quorum sensing—a form of cell-to-cell communication between bacteria—in P. aeruginosa. Its detection can allow the determination of the bacterial population. In this study, the development of the first electrochemical aptasensor for the detection of 3-O-C₁₂-HSL is reported. A carbon-based screen-printed electrode modified with gold nanoparticles proved to be the best platform for the aptasensor. Each step in the fabrication of the aptasensor (i.e., gold nanoparticles’ deposition, aptamer immobilization, incubation with the analyte) was optimized and characterized using cyclic voltammetry, differential pulse voltammetry, and electrochemical impedance spectroscopy. Different redox probes in solution were evaluated, the best results being obtained in the presence of [Fe(CN)₆]⁴⁻/[Fe(CN)₆]³⁻. The binding affinity of 106.7 nM for the immobilized thiol-terminated aptamer was determined using surface plasmon resonance. The quantification of 3-O-C₁₂-HSL was performed by using the electrochemical signal of the redox probe before and after incubation with the analyte. The aptasensor exhibited a logarithmic range from 0.5 to 30 µM, with a limit of detection of 145 ng mL⁻¹ (0.5 µM). The aptasensor was successfully applied for the analysis of real samples (e.g., spiked urine samples, spiked microbiological growth media, and microbiological cultures).

Main Metabolites of Pseudomonas aeruginosa: A Study of Electrochemical Properties

Pseudomonas aeruginosa is a ubiquitously distributed soil and water bacterium and is considered an opportunistic pathogen in hospitals. In cystic fibrosis patients, for example, infections with P. aeruginosa can be severe and often lead to chronic or even fatal pneumonia. Therefore, rapid detection and further identification are of major importance in hospital hygiene and infection control. This work shows the electrochemical properties of five P. aeruginosa key metabolites considering their potential use as specific signaling agents in an electrochemical sensor system. The pure solutes of pyocyanin (PYO), Pseudomonas quinolone signal (PQS), pyochelin (PCH), 2-heptyl-4-hydroxyquinoline (HHQ), and 2-heptyl-4-hydroxyquinoline N-oxide (HQNO) were analyzed by different electrochemical techniques (cyclic and square wave voltammetry) and measured using a Gamry Reference 600+ potentiostat. Screen-printed electrodes (DropSens DRP110; carbon working and counter, silver reference electrode) were used to determine signal specificities, detection limits, as well as pH dependencies of the substances. All of the compounds were electrochemically inducible with well-separated oxidation and/or reduction peaks at specific peak potentials relative to the reference electrode. Additionally, all analytes exhibited linear concentration dependency in ranges classically reported in the literature. The demonstration of these properties is a promising step toward direct multiplexed detection of P. aeruginosa in environmental and clinical samples and thus, can make a significant contribution to public health and safety.

Advanced detection and sensing strategies of Pseudomonas aeruginosa and quorum sensing biomarkers: A review

Pseudomonas aeruginosa (P. aeruginosa), a ubiquitous opportunistic pathogen, can frequently cause chronic obstructive pulmonary disease, cystic fibrosis and chronic wounds, and potentially lead to severe morbidity and mortality. Timely and adequate treatment of nosocomial infection in clinic depends on rapid detection and accurate identification of P. aeruginosa and its early-stage antibiotic susceptibility test. Traditional methods like plating culture, polymerase chain reaction, and enzyme-linked immune sorbent assays are time-consuming and require expensive equipment, limiting the rapid diagnostic application. Advanced sensing strategy capable of fast, sensitive and simple detection with low cost has therefore become highly desired in point of care testing (POCT) of nosocomial pathogens. Within this review, advanced detection and sensing strategies for P. aeruginosa cells along with associated quorum sensing (QS) molecules over the last ten years are discussed and summarized. Firstly, the principles of four commonly used sensing strategies including localized surface plasmon resonance (LSPR), surface-enhanced Raman spectroscopy (SERS), electrochemistry, and fluorescence are briefly overviewed. Then, the advancement of the above sensing techniques for P. aeruginosa cells and its QS biomarkers detection are introduced, respectively. In addition, the integration with novel compatible platforms towards clinical application is highlighted in each section. Finally, the current achievements are summarized along with proposed challenges and prospects.

Recent advances in electrochemical strategies for bacteria detection

Introduction: Bacterial infections have always been a major threat to public health and humans' life, and fast detection of bacteria in various samples is significant to provide early and effective treatments. Cell-culture protocols, as well-established methods, involve labor-intensive and complicated preparation steps. For overcoming this drawback, electrochemical methods may provide promising alternative tools for fast and reliable detection of bacterial infections. Methods: Therefore, this review study was done to present an overview of different electrochemical strategy based on recognition elements for detection of bacteria in the studies published during 2015-2020. For this purpose, many references in the field were reviewed, and the review covered several issues, including (a) enzymes, (b) receptors, (c) antimicrobial peptides, (d) lectins, (e) redox-active metabolites, (f) aptamer, (g) bacteriophage, (h) antibody, and (i) molecularly imprinted polymers. Results: Different analytical methods have developed are used to bacteria detection. However, most of these methods are highly time, and cost consuming, requiring trained personnel to perform the analysis. Among of these methods, electrochemical based methods are well accepted powerful tools for the detection of various analytes due to the inherent properties. Electrochemical sensors with different recognition elements can be used to design diagnostic system for bacterial infections. Recent studies have shown that electrochemical assay can provide promising reliable method for detection of bacteria. Conclusion: In general, the field of bacterial detection by electrochemical sensors is continuously growing. It is believed that this field will focus on portable devices for detection of bacteria based on electrochemical methods. Development of these devices requires close collaboration of various disciplines, such as biology, electrochemistry, and biomaterial engineering.

Electrochemical and spectroelectrochemical characterization of bacteria and bacterial systems

Microbes, such as bacteria, can be described, at one level, as small, self-sustaining chemical factories. Based on the species, strain, and even the environment, bacteria can be useful, neutral or pathogenic to human life, so it is increasingly important that we be able to characterize them at the molecular level with chemical specificity and spatial and temporal resolution in order to understand their behavior. Bacterial metabolism involves a large number of internal and external electron transfer processes, so it is logical that electrochemical techniques have been employed to investigate these bacterial metabolites. In this mini-review, we focus on electrochemical and spectroelectrochemical methods that have been developed and used specifically to chemically characterize bacteria and their behavior. First, we discuss the latest mechanistic insights and current understanding of microbial electron transfer, including both direct and mediated electron transfer. Second, we summarize progress on approaches to spatiotemporal characterization of secreted factors, including both metabolites and signaling molecules, which can be used to discern how natural or external factors can alter metabolic states of bacterial cells and change either their individual or collective behavior. Finally, we address in situ methods of single-cell characterization, which can uncover how heterogeneity in cell behavior is reflected in the behavior and properties of collections of bacteria, e.g. bacterial communities. Recent advances in (spectro)electrochemical characterization of bacteria have yielded important new insights both at the ensemble and the single-entity levels, which are furthering our understanding of bacterial behavior. These insights, in turn, promise to benefit applications ranging from biosensors to the use of bacteria in bacteria-based bioenergy generation and storage.

Detection of Pseudomonas aeruginosa quorum sensing molecules at an electrified liquid|liquid micro-interface through facilitated proton transfer

Miniaturization of electrochemical detection methods for point-of-care-devices is ideal for their integration and use within healthcare environments. Simultaneously, the prolific pathogenic bacteria Pseudomonas aeruginosa poses a serious health risk to patients with compromised immune systems. Recognizing these two factors, a proof-of-concept electrochemical method employing a micro-interface between water and oil (w/o) held at the tip of a pulled borosilicate glass capillary is presented. This method targets small molecules produced by P. aeruginosa colonies as signalling factors that control colony growth in a pseudo-multicellular process known as quorum sensing (QS). The QS molecules of interest are 4-hydroxy-2-heptylquinoline (HHQ) and 2-heptyl-3,4-dihydroxyquinoline (PQS, Pseudomonas quinolone signal). Hydrophobic HHQ and PQS molecules, dissolved in the oil phase, were observed electrochemically to facilitate proton transfer across the w/o interface. This interfacial complexation can be exploited as a facile electrochemical detection method for P. aeruginosa and is advantageous as it does not depend on the redox activity of HHQ/PQS. Interestingly, the limit-of-linearity is reached as [H+] ≈ [ligand]. Density functional theory calculations were performed to determine the proton affinities and gas-phase basicities of HHQ/PQS, as well as elucidate the likely site of stepwise protonation within each molecule.

Biological and clinical significance of quorum sensing alkylquinolones: current analytical and bioanalytical methods for their quantification

Quorum sensing (QS) is a sophisticated bacterial communication system which plays a key role in the virulence and biofilm formation of many pathogens. The Pseudomonas aeruginosa QS network consists of four sets of connected systems (las, rlh, pqs and iqs) hierarchically organized. The pqs system involves characteristic autoinducers (AI), most of them sharing an alkylquinolone (AQ) structure, and is able to carry out several relevant biological functions besides its main signalling activity. Their role in bacterial physiology and pathogenicity has been widely studied. Indeed, the presence of these metabolites in several body fluids and infected tissues has pointed to their potential value as biomarkers of infection. In this review, we summarize the most recent findings about the biological implications and the clinical significance of the main P. aeruginosa AQs. These findings have encouraged the development of analytical and bioanalytical techniques addressed to assess the role of these metabolites in bacterial growth and survival, during pathogenesis or as biomarkers of infections. The availability of highly sensitive reliable analytical methods suitable for clinical analysis would allow getting knowledge about pathogenesis and disease prognosis or progression, supporting clinicians on the decision-making process for the management of these infections and guiding them on the application of more effective and appropriate treatments. The benefits from the implementation of the point-of-care (PoC)-type testing in infectious disease diagnostics, which are seen to improve patient outcomes by promoting earlier therapeutic interventions, are also discussed.

An Immunochemical Approach to Quantify and Assess the Potential Value of the Pseudomonas Quinolone Signal as a Biomarker of Infection

Quorum sensing (QS) is a bacterial cell density-based communication system using low molecular weight signals called autoinducers (AIs). Identification and quantification of these molecules could provide valuable information related to the stage of colonization or infection as well as the stage of the disease. With this scenario, we report here for the first time the development of antibodies against the PQS (pseudomonas quinolone signal), the main signaling molecule from the pqs QS system of Pseudomonas aeruginosa, and the development of a microplate-based enzyme-linked immunosorbent assay (ELISA) able of quantifying this molecule in complex biological media in the low nanometer range (LOD, 0.36 ± 0.14 nM in culture broth media). Moreover, the PQS ELISA here reported has been found to be robust and reliable, providing accurate results in culture media. The technique allowed us to follow up the PQS profile of the release of bacterial clinical isolates obtained from patients of different disease status. A clear correlation was found between the PQS immunoreactivity equivalents and the chronic or acute infection conditions, which supports the reported differences on virulence and behavior of these bacterial strains due to their adaptation capability to the host environment. The results obtained point to the potential of the PQS as a biomarker of infection and to the value of the antibodies and the technology developed for improving diagnosis and management of P. aeruginosa infections based on the precise identification of the pathogen, appropriate stratification of the patients according to their disease status, and knowledge of the disease progression.

Changes in toxin production of environmental Pseudomonas aeruginosa isolates exposed to sub-inhibitory concentrations of three common antibiotics

Pseudomonas aeruginosa is an environmental pathogen that can cause severe infections in immunocompromised patients. P. aeruginosa infections are typically treated with multiple antibiotics including tobramycin, ciprofloxacin, and meropenem. However, antibiotics do not always entirely clear the bacteria from the infection site, where they may remain virulent. This is because the effective antibiotic concentration and diffusion in vitro may differ from the in vivo environment in patients. Therefore, it is important to understand the effect of non-lethal sub-inhibitory antibiotic concentrations on bacterial phenotype. Here, we investigate if sub-inhibitory antimicrobial concentrations cause alterations in bacterial virulence factor production using pyocyanin as a model toxin. We tested this using the aforementioned antibiotics on 10 environmental P. aeruginosa strains. Using on-the-spot electrochemical screening, we were able to directly quantify changes in production of pyocyanin in a measurement time of 17 seconds. Upon selecting 3 representative strains to further test the effects of sub-minimum inhibitory concentration (MICs), we found that pyocyanin production changed significantly when the bacteria were exposed to 10-fold MIC of the 3 antibiotics tested, and this was strain specific. A series of biologically relevant measured pyocyanin concentrations were also used to assess the effects of increased virulence on a culture of epithelial cells. We found a decreased viability of the epithelial cells when incubated with biologically relevant pyocyanin concentrations. This suggests that the antibiotic-induced virulence also is a value worth being enclosed in regular testing of pathogens.

Redox cycling-based detection of phenazine metabolites secreted from Pseudomonas aeruginosa in nanopore electrode arrays

The opportunistic pathogen Pseudomonas aeruginosa (P. aeruginosa) produces several redox-active phenazine metabolites, including pyocyanin (PYO) and phenazine-1-carboxamide (PCN), which are electron carrier molecules that also aid in virulence. In particular, PYO is an exclusive metabolite produced by P. aeruginosa, which acts as a virulence factor in hospital-acquired infections and is therefore a good biomarker for identifying early stage colonization by this pathogen. Here, we describe the use of nanopore electrode arrays (NEAs) exhibiting metal-insulator-metal ring electrode architectures for enhanced detection of these phenazine metabolites. The size of the nanopores allows phenazine metabolites to freely diffuse into the interior and access the working electrodes, while the bacteria are excluded. Consequently, highly efficient redox cycling reactions in the NEAs can be accessed by free diffusion unhindered by the presence of bacteria. This strategy yields low limits of detection, i.e. 10.5 and 20.7 nM for PYO and PCN, respectively, values far below single molecule pore occupancy, e.g. at 10.5 nM 〈npore〉∼ 0.082 per nanopore - a limit which reflects the extraordinary signal amplification in the NEAs. Furthermore, experiments that compared results from minimal medium and rich medium show that P. aeruginosa produces the same types of phenazine metabolites even though growth rates and phenazine production patterns differ in these two media. The NEA measurement strategy developed here should be useful as a diagnostic for pathogens generally and for understanding metabolism in clinically important microbial communities.

Electrochemical detection of redox molecules secreted by Pseudomonas aeruginosa - Part 1: Electrochemical signatures of different strains

During infections, fast identification of the microorganisms is critical to improve patient treatment and to better manage antibiotics use. Electrochemistry exhibits several advantages for rapid diagnostic: it enables easy, cheap and in situ analysis of redox molecules in most liquids. In this work, several culture supernatants of different Pseudomonas aeruginosa strains (including PAO1 and its isogenic mutants PAO1ΔpqsA, PA14, PAK and CHA) were analyzed by square wave voltammetry on glassy carbon electrode during the bacterial growth. The obtained voltamograms shown complex traces exhibiting numerous redox peaks with potential repartitions and current amplitudes depending on the studied bacterium and/or growth time. Among them, some peaks were clearly associated to the well-known redox toxin Pyocyanin (PYO) and the autoinducer Pseudomonas Quinolone Signal (PQS). Other peaks were observed that are not yet attributed to known secreted species. Each complex electrochemical response (number of peaks, peak potential and amplitude) can be interpreted as a fingerprint or "ID-card" of the studied strain that may be implemented for fast bacteria strain identification.

High-Throughput Immunochemical Method to Assess the 2-Heptyl-4-quinolone Quorum Sensing Molecule as a Potential Biomarker of Pseudomonas aeruginosa Infections

Bacterial quorum sensing (QS) is being contemplated as a promising target for developing innovative diagnostic and therapeutic strategies. Here we report for the first time the development of antibodies against 2-heptyl-4-quinolone (HHQ), a signaling molecule from the pqs QS system of Pseudomonas aeruginosa, involved in the production of important virulent factors and biofilm formation. The antibodies produced were used to develop an immunochemical diagnostic approach to assess the potential of this molecule as a biomarker of P. aeruginosa infection. The ELISA developed is able to reach a detectability in the low nM range (IC₅₀ = 4.59 ± 0.29 nM and LOD = 0.34 ± 0.13 nM), even in complex biological samples such as Müeller Hinton (MH) culture media. The ELISA developed is robust and reproducible and has been found to be specific to HHQ, with little interference from other related alkylquinolones from the pqs QS system. The ELISA has been used to analyze the HHQ production kinetics of P. aeruginosa clinical isolates grown in MH media, pointing to its potential as a biomarker of infection and at the possibility to use the technology developed to obtain additional information about the disease stage.

Electrochemical Detection of Pyocyanin as a Biomarker for Pseudomonas aeruginosa: A Focused Review

Pseudomonas aeruginosa (PA) is a pathogen that is recognized for its advanced antibiotic resistance and its association with serious diseases such as ventilator-associated pneumonia and cystic fibrosis. The ability to rapidly detect the presence of pathogenic bacteria in patient samples is crucial for the immediate eradication of the infection. Pyocyanin is one of PA’s virulence factors used to establish infections. Pyocyanin promotes virulence by interfering in numerous cellular functions in host cells due to its redox-activity. Fortunately, the redox-active nature of pyocyanin makes it ideal for detection with simple electrochemical techniques without sample pretreatment or sensor functionalization. The previous decade has seen an increased interest in the electrochemical detection of pyocyanin either as an indicator of the presence of PA in samples or as a tool for quantifying PA virulence. This review provides the first overview of the advances in electrochemical detection of pyocyanin and offers an input regarding the future directions in the field.

Antimicrobial Properties of the Polyaniline Composites against Pseudomonas aeruginosa and Klebsiella pneumoniae

CuO, TiO2, or SiO2 was decorated on polyaniline (PANI) by a sonochemical method, and their antimicrobial properties were investigated for two common Gram-negative pathogens: Pseudomonas aeruginosa (PA) and Klebsiella pneumoniae (KP). Without PANI, CuO, TiO2, or SiO2 with a concentration of 220 µg/mL exhibited no antimicrobial activities. In contrast, PANI-CuO and PANI-TiO2 (1 mg/mL, each) completely suppressed the PA growth after 6 h of exposure, compared to 12 h for the PANI-SiO2 at the same concentration. The damage caused by PANI-SiO2 to KP was less effective, compared to that of PANI-TiO2 with the eradication time of 12 h versus 6 h, respectively. This bacterium was not affected by PANI-CuO. All the composites bind tightly to the negative groups of bacteria cell walls to compromise their regular activities, leading to the damage of the cell wall envelope and eventual cell lysis.

Emerging Technologies for the Electrochemical Detection of Bacteria

Infections are a huge economic liability to the health care system, although real-time detection can allow early treatment protocols to avoid some of this cost and patient morbidity and mortality. Pseudomonas aeruginosa (PA) is a drug-resistant gram-negative bacterium found ubiquitously in clinical settings, accounting for up to 27% of hospital acquired infections. PA secretes a vast array of molecules, ranging from secondary metabolites to quorum sensing molecules, of which many can be exploited to monitor bacterial presence. In addition to electrochemical immunoassays to sense bacteria via antigen-antibody interactions, PA pertains a distinct redox-active virulence factor called pyocyanin (PYO), allowing a direct electrochemical detection of the bacteria. There has been a surge of publications relating to the electrochemical tracing of PA via a myriad of novel biosensing techniques, materials, and methodologies. In addition to indirect methods, research approaches where PYO has been sensitively detected using surface modified electrodes are reviewed and compared with conventional PA-sensing methodologies. This review aims at presenting indirect and direct electrochemical methods currently developed using various surface modified electrodes, materials, and electrochemical configurations on their electrocatalytic effects on sensing of PA and in particular PYO.

Interfacing gene circuits with microelectronics through engineered population dynamics

While there has been impressive progress connecting bacterial behavior with electrodes, an attractive observation to facilitate advances in synthetic biology is that the growth of a bacterial colony can be determined from impedance changes over time. Here, we interface synthetic biology with microelectronics through engineered population dynamics that regulate the accumulation of charged metabolites. We demonstrate electrical detection of the bacterial response to heavy metals via a population control circuit. We then implement this approach to a synchronized genetic oscillator where we obtain an oscillatory impedance profile from engineered bacteria. We lastly miniaturize an array of electrodes to form "bacterial integrated circuits" and demonstrate its applicability as an interface with genetic circuits. This approach paves the way for new advances in synthetic biology, analytical chemistry, and microelectronic technologies.

Electrochemical sensors for rapid diagnosis of pathogens in real time

Microbial infections remain the principal cause for high morbidity and mortality rates. While approximately 1400 human pathogens have been recognized, the majority of healthcare-associated infectious diseases are caused by only a few opportunistic pathogens (e.g., Pseudomonas aeruginosa, Staphylococcus aureus, Escherichia coli), which are associated with increased antibiotic and antimicrobial resistance. Rapid detection, reliable identification and real-time monitoring of these pathogens remain not only a scientific problem but also a practical challenge of vast importance, especially in tailoring effective treatment strategies. Although the development of vaccinations and antibacterial drug treatments are the leading research, progress, and implementation of early warning, quantitative systems indicative of confirming pathogen presence are necessary. Over the years, various approaches, such as conventional culturing, straining, molecular methods (e.g., polymerase chain reaction and immunological assays), microscopy-based and mass spectrometry techniques, have been employed to identify and quantify pathogenic agents. While being sensitive in some cases, these procedures are costly, time-consuming, mostly qualitative, and are indirect detection methods. A great challenge is therefore to develop rapid, highly sensitive, specific devices with adequate figures of merit to corroborate the presence of microbes and enable dynamic real-time measurements of metabolism. As an alternative, electrochemical sensor platforms have been developed as powerful quantitative tools for label-free detection of infection-related biomarkers with high sensitivity. This minireview is focused on the latest electrochemical-based approaches for pathogen sensing, putting them into the context of standard sensing methods, such as cell culturing, mass spectrometry, and fluorescent-based approaches. Description of the latest, impactful electrochemical sensors for pathogen detection will be presented. Recent breakthroughs will be highlighted, including the use of micro- and nano-electrode arrays for real-time detection of bacteria in polymicrobial infections and microfluidic devices for pathogen separation analysis. We will conclude with perspectives and outlooks to understand shortcomings in designing future sensing schemes. The need for high sensitivity and selectivity, low-cost implementation, fast detection, and screening increases provides an impetus for further development in electrochemical detectors for microorganisms and biologically relevant targets.

Tethered molecular redox capacitors for nanoconfinement-assisted electrochemical signal amplification

Nanostructured materials offer the potential to drive future developments and applications of electrochemical devices, but are underutilized because their nanoscale cavities can impose mass transfer limitations that constrain electrochemical signal generation. Here, we report a new signal-generating mechanism that employs a molecular redox capacitor to enable nanostructured electrodes to amplify electrochemical signals even without an enhanced reactant mass transfer. The surface-tethered molecular redox capacitor engages diffusible reactants and products in redox-cycling reactions with the electrode. Such redox-cycling reactions are facilitated by the nanostructure that increases the probabilities of both reactant-electrode and product-redox-capacitor encounters (i.e., the nanoconfinement effect), resulting in substantial signal amplification. Using redox-capacitor-tethered Au nanopillar electrodes, we demonstrate improved sensitivity for measuring pyocyanin (bacterial metabolite). This study paves a new way of using nanostructured materials in electrochemical applications by engineering the reaction pathway within the nanoscale cavities of the materials.

Nanograss sensor for selective detection of Pseudomonas aeruginosa by pyocyanin identification in airway samples

Pyocyanin is a virulence factor solely produced by the pathogen Pseudomonas aeruginosa. Pyocyanin is also a redox active molecule that can be directly detected by electrochemical sensing. A nanograss (NG) based sensor for sensitive quantification of pyocyanin in sputum samples from cystic fibrosis (CF) patients is presented here. The NG sensors were custom made in a cleanroom environment by etching nanograss topography on the electrode surface followed by depositing 200 nm gold. The NG sensors were utilized for amperometric quantification of pyocyanin in spiked hypertonic saline samples, resulting in a linear calibration curve with a R² value of 0.9901 and a limit of detection of 172 nM. The NG sensors were applied in a small pilot test on five airway samples from five CF patients. The NG sensor was capable of identifying P. aeruginosa in the airway samples in 60 s without any sample pretreatment.

The expanding horizon of alkyl quinolone signalling and communication in polycellular interactomes

Population dynamics within natural ecosystems is underpinned by microbial diversity and the heterogeneity of host-microbe and microbe-microbe interactions. Small molecule signals that intersperse between species have been shown to govern many virulence-related processes in established and emerging pathogens. Understanding the capacity of microbes to decode diverse languages and adapt to the presence of 'non-self' cells will provide an important new direction to the understanding of the 'polycellular' interactome. Alkyl quinolones (AQs) have been described in the ESKAPE pathogen Pseudomonas aeruginosa, the primary agent associated with mortality in patients with cystic fibrosis and the third most prevalent nosocomial pathogen worldwide. The role of these molecules in governing the physiology and virulence of P. aeruginosa and other pathogens has received considerable attention, while a role in interspecies and interkingdom communication has recently emerged. Herein we discuss recent advances in our understanding of AQ signalling and communication in the context of microbe-microbe and microbe-host interactions. The integrated knowledge from these systems-based investigations will facilitate the development of new therapeutics based on the AQ framework that serves to disarm the pathogenesis of P. aeruginosa and competing pathogens.

Rapid and highly sensitive detection of pyocyanin biomarker in different Pseudomonas aeruginosa infections using gold nanoparticles modified sensor

Successful antibiotic treatment of infections relies on accurate and rapid identification of the infectious agents. Pseudomonas aeruginosa is implicated in a wide range of human infections that mostly become complicated and life threating, especially in immunocompromised and critically ill patients. Conventional microbiological methods take more than three days to obtain accurate results. Pyocyanin is a distinctive electroactive biomarker for Pseudomonas aeruginosa. Here, we have prepared polyaniline/gold nanoparticles decorated ITO electrode and tested it to establish a rapid, diagnostic and highly sensitive pyocyanin sensor in a culture of Pseudomonas aeruginosa clinical isolates with high selectivity for traces of pyocyanin when measured in the existence of different interferences like vitamin C, uric acid, and glucose. The scanning electron microscopy and cyclic voltammetry techniques were used to characterize the morphology and electrical conductivity of the constructed electrode. The determined linear range for pyocyanin detection was from 238 μM to 1.9 μM with a detection limit of 500 nM. Compared to the screen-printed electrode used before, the constructed electrode showed a 4-fold enhanced performance. Furthermore, PANI/Au NPs/ITO modified electrodes have demonstrated the ability to detect pyocyanin directly in Pseudomonas aeruginosa culture without any potential interference with other species.

Electrochemical Surface-Enhanced Raman Spectroscopy of Pyocyanin Secreted by Pseudomonas aeruginosa Communities

Pyocyanin (PYO) is one of many toxins secreted by the opportunistic human pathogenic bacterium Pseudomonas aeruginosa. Direct detection of PYO in biofilms is crucial because PYO can provide important information about infection-related virulence mechanisms in P. aeruginosa. Because PYO is both redox-active and Raman-active, we seek to simultaneously acquire both spectroscopic and redox state information about PYO. The combination of surface-enhanced Raman spectroscopy (SERS) and voltammetry is used here to provide insights into the molecular redox behavior of PYO while controlling the SERS and electrochemical (EC) response of PYO with external stimuli, such as pH and applied potential. Furthermore, PYO secretion from biofilms of different P. aeruginosa strains is compared. Both SERS spectra and EC behavior are observed to change with pH, and several pH-dependent bands are identified in the SERS spectra, which can potentially be used to probe the local environment. Comparison of the voltammetric behavior of wild-type and a PYO-deficient mutant unequivocally identifies PYO as a major component of the secretome. Spectroelectrochemical studies of the PYO standard reveal decreasing SERS intensities of PYO bands under reducing conditions. Extending these experiments to pellicle biofilms shows similar behavior with applied potential, and SERS imaging indicates that secreted PYO is localized in regions approximately the size of P. aeruginosa cells. The in situ spectroelectrochemical biofilm characterization approach developed here suggests that EC-SERS monitoring of secreted molecules can be used diagnostically and correlated with the progress of infection.

Electrochemical Sensing of Biotin Using Nafion-Modified Boron-Doped Diamond Electrode

Nafion formed on the surface of a boron-doped diamond electrode allows for a chemosensing system for biotin. The modified electrode is capable of oxidizing biotin and offers a detection limit of 5 nM, the average normal level of biotin in blood plasma. The developed method was successfully applied to determine biotin in human plasma samples and a popular health product as two popular models.

Electrochemical Probes of Microbial Community Behavior

Advances in next-generation sequencing technology along with decreasing costs now allow the microbial population, or microbiome, of a location to be determined relatively quickly. This research reveals that microbial communities are more diverse and complex than ever imagined. New and specialized instrumentation is required to investigate, with high spatial and temporal resolution, the dynamic biochemical environment that is created by microbes, which allows them to exist in every corner of the Earth. This review describes how electrochemical probes and techniques are being used and optimized to learn about microbial communities. Described approaches include voltammetry, electrochemical impedance spectroscopy, scanning electrochemical microscopy, separation techniques coupled with electrochemical detection, and arrays of complementary metal-oxide-semiconductor circuits. Microbial communities also interact with and influence their surroundings; therefore, the review also includes a discussion of how electrochemical probes optimized for microbial analysis are utilized in healthcare diagnostics and environmental monitoring applications.

Paper-based sensors for rapid detection of virulence factor produced by Pseudomonas aeruginosa

Pyocyanin is a toxin produced by Pseudomonas aeruginosa. Here we describe a novel paper-based electrochemical sensor for pyocyanin detection, manufactured with a simple and inexpensive approach based on electrode printing on paper. The resulting sensors constitute an effective electrochemical method to quantify pyocyanin in bacterial cultures without the conventional time consuming pretreatment of the samples. The electrochemical properties of the paper-based sensors were evaluated by ferri/ferrocyanide as a redox mediator, and showed reliable sensing performance. The paper-based sensors readily allow for the determination of pyocyanin in bacterial cultures with high reproducibility, achieving a limit of detection of 95 nM and a sensitivity of 4.30 μA/μM in standard culture media. Compared to the similar commercial ceramic based sensors, it is a 2.3-fold enhanced performance. The simple in-house fabrication of sensors for pyocyanin quantification allows researchers to understand in vitro adaptation of P. aeruginosa infections via rapid screenings of bacterial cultures that otherwise are expensive and time-consuming.

Miniaturized dispersive liquid-liquid microextraction and MALDI MS using ionic liquid matrices for the detection of bacterial communication molecules and virulence factors

The identification and quantification of molecules involved in bacterial communication are major prerequisites for the understanding of interspecies interactions at the molecular level. We developed a procedure allowing the determination of 2-heptyl-4(1H)-quinolone (HHQ) and 2-heptyl-3-hydroxy-4(1H)-quinolone (PQS) and the virulence factor pyocyanin (PYO) formed by the Gram-negative bacterium Pseudomonas aeruginosa. The method is based on dispersive liquid-liquid microextraction from small supernatant volumes (below 10 μL) followed by quantitative matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS). The use of ionic liquid matrix led to a lowered limit of detection for pyocyanin and, due to suppression of matrix background signals, easy to interpret mass spectra compared to crystalline matrices. Using an isotope-labeled pyocyanin standard synthesized in small-scale synthesis, quantitative analysis spanning approximately one order of magnitude (0.5 to 250 fmol) was feasible. The method was successfully applied to the detection of the signaling molecules PQS and HHQ in cultures of P. aeruginosa strains isolated from sputum of cystic fibrosis patients and allowed a highly sensitive quantification of PYO from these cultures. Hence, the developed method bears the potential to be used for screening purposes in clinical settings and will help to decipher the molecular basis of bacterial communication. Graphical abstract Ionic liquid matrices for the detection and quantification of the toxin pyocyanin and other signaling molecules from P. aeruginosa by MALDI MS.

Rapid Electrochemical Detection of Pseudomonas aeruginosa Signaling Molecules by Boron-Doped Diamond Electrode

As the leading cause of morbidity and mortality of cystic fibrosis (CF) patients, early detection of Pseudomonas aeruginosa (PA) is critical in the clinical management of this pathogen. Herein, we describe rapid and sensitive electroanalytical methods using differential pulse voltammetry (DPV) at a boron-doped diamond (BDD) electrode for the detection of PA signaling biomolecules. Monitoring the production of key signaling molecules in bacterial cultures of P. aeruginosa PA14 over 8 h is described, involving sample pretreatment by liquid-liquid and solid-phase extraction. In addition, direct electrochemical detection approach of PA signaling molecules is also reported in conjunction with hexadecyltrimethylammonium bromide (CTAB) to disrupt the bacterial membrane.

Connecting Biology to Electronics: Molecular Communication via Redox Modality

Biology and electronics are both expert at for accessing, analyzing, and responding to information. Biology uses ions, small molecules, and macromolecules to receive, analyze, store, and transmit information, whereas electronic devices receive input in the form of electromagnetic radiation, process the information using electrons, and then transmit output as electromagnetic waves. Generating the capabilities to connect biology-electronic modalities offers exciting opportunities to shape the future of biosensors, point-of-care medicine, and wearable/implantable devices. Redox reactions offer unique opportunities for bio-device communication that spans the molecular modalities of biology and electrical modality of devices. Here, an approach to search for redox information through an interactive electrochemical probing that is analogous to sonar is adopted. The capabilities of this approach to access global chemical information as well as information of specific redox-active chemical entities are illustrated using recent examples. An example of the use of synthetic biology to recognize external molecular information, process this information through intracellular signal transduction pathways, and generate output responses that can be detected by electrical modalities is also provided. Finally, exciting results in the use of redox reactions to actuate biology are provided to illustrate that synthetic biology offers the potential to guide biological response through electrical cues.