Rapid differentiation between bacterial infections and cancer using a near-infrared fluorogenic probe

Harnessing the Potential of a Nitroreductase-Responsive Fluorescent Probe for the Diagnosis of Bacterial Keratitis

Bacterial keratitis, an ocular emergency, is the predominant cause of infectious keratitis. However, diagnostic procedures for it are invasive, time-consuming, and expeditious, thereby limiting effective treatment for the disease in the clinic. It is imperative to develop a timely and convenient method for the noninvasive diagnosis of bacterial keratitis. Fluorescence imaging is a convenient and noninvasive diagnostic method with high sensitivity. In this study, a type of nitroreductase-responsive probe (NTRP), which responds to nitroreductase to generate fluorescence signals, was developed as an activatable fluorescent probe for the imaging diagnosis of bacterial keratitis. Imaging experiments both in vitro and in vivo demonstrated that the probe exhibited "turn-on" fluorescence signals in response to nitroreductase-secreting bacteria within 10 min. Furthermore, the fluorescence intensity reached its highest at 4 or 6 h in vitro and at 30 min in vivo when the excitation wavelength was set at 520 nm. Therefore, the NTRP has the potential to serve as a feasible agent for the rapid and noninvasive in situ fluorescence diagnosis of bacterial keratitis.

Site-Specific Antimicrobial Activity of a Dual-Responsive Ciprofloxacin Prodrug

Bacterial infections create distinctive microenvironments with a unique mix of metabolites and enzymes compared with healthy tissues that can be used to trigger the activation of antibiotic prodrugs. Here, a single and dual prodrug masking the C3 carboxylate and C7 piperazine of the fluoroquinolone, ciprofloxacin, responsive to nitroreductase (NTR) and/or hydrogen sulfide (H2S), was developed. Masking both functional groups reduced the activity of the prodrug against Staphylococcus aureus and Escherichia coli, increasing its minimum inhibitory concentration (MIC) by ∼512-fold (S. aureus) and ∼8000-fold (E. coli strains), while masking a single group only increased the MIC by ∼128-fold. Bacteria subjected to prolonged prodrug exposure did not show any increase in resistance. Triggering assays demonstrated the conversion of prodrugs to ciprofloxacin, and in a murine infection model, responsive prodrugs showed antibacterial activity comparable to that of ciprofloxacin, suggesting in vivo activation of prodrugs. Thus, the potential for site-specific antibiotic treatment with reduced threat of resistance is demonstrated.

Strategies for Constructing Macrocyclic Arene-Based Color-Tunable Supramolecular Luminescent Materials

Over the past decades, supramolecular luminescent materials (SLMs) have attracted considerable attention due to their dynamic noncovalent interactions, versatile functions, and intriguing applications in many research fields. From construction to application, great efforts and progress have been made in color-tunable SLMs in recent years. In order to realize multicolor luminescence, various design strategies have been proposed. Macrocyclic chemistry, one of the brightest jewels in the field of supramolecular chemistry, has played a crucial role in the construction of stimuli-responsive and emission-tunable SLMs. Moreover, the flexible and tunable conformation and multiple noncovalent complexation sites of the macrocyclic arenes (MAs) afford a new opportunity to create such dynamic smart luminescent materials. Inspired by our reported work on the color-tunable supramolecular crystalline assemblies modulated by the conformation of naphth[4]arene, this Concept provides a summary of the latest developments in the construction of color-tunable MA-based SLMs, accompanied by the various construction strategies. The aim is to provide researchers with a new perspective to construct color-tunable SLMs with fascinating functions.

NIRis: A low-cost, versatile imaging system for near-infrared fluorescence detection of phototrophic cell colonies used in research and education

A variety of costly research-grade imaging devices are available for the detection of spectroscopic features. Here we present an affordable, open-source and versatile device, suitable for a range of applications. We provide the files to print the imaging chamber with commonly available 3D printers and instructions to assemble it with easily available hardware. The imager is suitable for rapid sample screening in research, as well as for educational purposes. We provide details and results for an already proven set-up which suits the needs of a research group and students interested in UV-induced near-infrared fluorescence detection of microbial colonies grown on Petri dishes. The fluorescence signal confirms the presence of bacteriochlorophyll a in aerobic anoxygenic phototrophic bacteria (AAPB). The imager allows for the rapid detection and subsequent isolation of AAPB colonies on Petri dishes with diverse environmental samples. To this date, 15 devices have been build and more than 7000 Petri dishes have been analyzed for AAPB, leading to over 1000 new AAPB isolates. Parts can be modified depending on needs and budget. The latest version with automated switches and double band pass filters costs around 350€ in materials and resolves bacterial colonies with diameters of 0.5 mm and larger. The low cost and modular build allow for the integration in high school classes to educate students on light properties, fluorescence and microbiology. Computer-aided design of 3D-printed parts and programming of the employed Raspberry Pi computer could be incorporated in computer sciences classes. Students have been also inspired to do agar art with microbes. The device is currently used in seven different high schools in Finland. Additionally, a science education network of Finnish universities has incorporated it in its program for high school students. Video guides have been produced to facilitate easy operation and accessibility of the device.

Synthetic Peptides with Genetic-Codon-Tailored Affinity for Assembling Tetraspanin CD81 at Cell Interfaces and Inhibiting Cancer Metastasis

Probing biomolecular interactions at cellular interfaces is crucial for understanding and interfering with life processes. Although affinity binders with site specificity for membrane proteins are unparalleled molecular tools, a high demand remains for novel multi-functional ligands. In this study, a synthetic peptide (APQQ) with tight and specific binding to the untargeted extracellular loop of CD81 evolved from a genetically encoded peptide pool. With tailored affinity, APQQ flexibly accesses, site-specifically binds, and forms a complex with CD81, enabling in-situ tracking of the dynamics and activity of this protein in living cells, which has rarely been explored because of the lack of ligands. Furthermore, APQQ triggers the relocalization of CD81 from diffuse to densely clustered at cell junctions and modulates the interplay of membrane proteins at cellular interfaces. Motivated by these, efficient suppression of cancer cell migration, and inhibition of breast cancer metastasis were achieved in vivo.

Non-invasive diagnosis of bacterial and non-bacterial inflammations using a dual-enzyme-responsive fluorescent indicator

Bacterial infections, as the second leading cause of global death, are commonly treated with antibiotics. However, the improper use of antibiotics contributes to the development of bacterial resistance. Therefore, the accurate differentiation between bacterial and non-bacterial inflammations is of utmost importance in the judicious administration of clinical antibiotics and the prevention of bacterial resistance. However, as of now, no fluorescent probes have yet been designed for the relevant assessments. To this end, the present study reports the development of a novel fluorescence probe (CyQ) that exhibits dual-enzyme responsiveness. The designed probe demonstrated excellent sensitivity in detecting NTR and NAD(P)H, which served as critical indicators for bacterial and non-bacterial inflammations. The utilization of CyQ enabled the efficient detection of NTR and NAD(P)H in distinct channels, exhibiting impressive detection limits of 0.26 μg mL-1 for NTR and 5.54 μM for NAD(P)H, respectively. Experimental trials conducted on living cells demonstrated CyQ's ability to differentiate the variations in NTR and NAD(P)H levels between A. baumannii, S. aureus, E. faecium, and P. aeruginosa-infected as well as LPS-stimulated HUVEC cells. Furthermore, in vivo zebrafish experiments demonstrated the efficacy of CyQ in accurately discerning variations in NTR and NAD(P)H levels resulting from bacterial infection or LPS stimulation, thereby facilitating non-invasive detection of both bacterial and non-bacterial inflammations. The outstanding discriminatory ability of CyQ between bacterial and non-bacterial inflammation positions it as a promising clinical diagnostic tool for acute inflammations.

Visual monitoring of biocatalytic processes using small molecular fluorescent probes: strategies-mechanisms-applications

Real-time monitoring of biocatalytic-based processes is significantly improved and simplified when they can be visualized. Visual monitoring can be achieved by integrating a fluorescent unit with the biocatalyst. Herein, we outline the design strategies of fluorescent probes for monitoring biocatalysis: (1) probes for monitoring biocatalytic transfer: γ-glutamine is linked to the fluorophore as both a recognition group and for intramolecular charge transfer (ICT) inhibition; the probe is initially in an off state and is activated via the transfer of the γ-glutamine group and the release of the free amino group, which results in restoration of the "Donor-π-Acceptor" (D-π-A) system and fluorescence recovery. (2) Probes for monitoring biocatalytic oxidation: a propylamine is connected to the fluorophore as a recognition group, which cages the hydroxyl group, leading to the inhibition of ICT; propylamine is oxidized and subsequently β-elimination occurs, resulting in exposure of the hydroxyl group and fluorescence recovery. (3) Probes for monitoring biocatalytic reduction: a nitro group attached to a fluorophore as a fluorescence quenching group, this is converted to an amino group by catalytic reduction, resulting in fluorescence recovery. (4) Probes for monitoring biocatalytic hydrolysis: β-D-galactopyranoside or phosphate acts as a recognition group attached to hydroxyl groups of the fluorophore; the subsequent biocatalytic hydrolysis reaction releases the hydroxyl group resulting in fluorescence recovery. Following these 4 mechanisms, fluorophores including cyanine, coumarin, rhodamine, and Nile-red, have been used to develop systems for monitoring biocatalytic reactions. We anticipate that these strategies will result in systems able to rapidly diagnose and facilitate the treatment of serious diseases.

Endogenous CO imaging in bacterial pneumonia with a NIR fluorescent probe

Bacterial pneumonia is a serious respiratory illness that poses a great threat to human life. Rapid and precise diagnosis of bacterial pneumonia is crucial for symptomatic clinical treatment. Endogenous carbon monoxide (CO) is regarded as a significant indicator of bacterial pneumonia; herein, we developed a near-infrared (NIR) probe for fluorescence and photoacoustic (PA) dual-mode imaging of endogenous CO in bacterial pneumonia. NO2-BODIPY could rapidly and specifically react with CO to produce strong NIR fluorescence as well as ratiometric PA signals. NO2-BODIPY has outstanding features including fast response, fluorescence/PA dual mode signals, good specificity, and a low limit of detection (LOD = 20.3 nM), which enables it to image endogenous CO in cells and bacterial pneumonia mice with high sensitivity and high contrast ratio. In particular, NO2-BODIPY has two-photon excited (1340 nm, σ1 = 1671 GM) NIR fluorescence and has been utilized to image endogenous CO in bacterial pneumonia mice with deep tissue penetration. NO2-BODIPY has been demonstrated a good capability of fluorescence/PA dual-mode imaging of CO in bacterial pneumonia mice, providing a precise manner to diagnose bacterial pneumonia.

A novel "on-off-on" near-infrared fluorescent probe for Cu2+ and S2- continuous detection based on dicyanoisoflurone derivatives, and its application in bacterial imaging

We have successfully synthesized a near-infrared fluorescent probe for the continuous detection of copper and sulfur ions. The probe has good selectivity and anti-interference ability against Cu2+ and S2-. The results show that after adding Cu2+ to the DL solution of the near-infrared fluorescent probe, Cu2+ forms a [DL + Cu2+] complex with the probe, which leads to fluorescence quenching due to the paramagnetism of Cu2+. The probe can be used for the quantitative detection of Cu2+ with a detection limit of 1.26 × 10-9 M. According to the Job's plot curve the binding stoichiometry between DL and Cu2+ is 1 : 1. Subsequently, S2- was added to the [DL + Cu2+] solution, because the precipitation dissolution equilibrium constant of CuS was Ksp = 1.27 × 10-36, so the binding capacity between Cu2+ and S2- was stronger, CuS precipitation was formed, and red fluorescence was re-released, and the quantitative detection of S2- was realized, and the detection limit was 3.50 × 10-8 M. Through bacterial imaging experiments, we found that the probe can accomplish the fluorescence imaging experiments of Staphylococcus aureus, indicating that the probe has good biopenetration and biocompatibility, and has application prospects in bioimaging and environmental monitoring. In addition, the probe DL has good suitability for Cu2+ and S2- detection in real samples.

In Situ Photoacoustic Visualization of Pneumonia Induced by MRSA and Specific Identifying Tumor-Homing Bacteria

Optical imaging holds great promise for monitoring bacterial infectious processes and drug resistance with high temporal-spatial resolution. Currently, the diagnosis of deep-seated bacterial infections in vivo with fluorescence imaging, including near-infrared (NIR) fluorescence imaging technology, remains a significant challenge due to its limited tissue penetration depth. In this study, we developed a highly specific targeting probe, Cy7-Neo-NO2, by conjugating a bacterial 16S rRNA-targeted moiety, neomycin, with a bacterial nitroreductase (NTR)-activated NIR photoacoustic (PA) scaffold using our previously developed caged photoinduced electron transfer (a-PeT) approach. This conjugation effectively resolved probe aggregation issues in physiological conditions and substantially enhanced its reactivity toward bacterial NTR. Notably, Cy7-Neo-NO2 enabled the first in situ photoacoustic imaging of pneumonia induced by methicillin-resistant Staphylococcus aureus (MRSA), as well as the detection of bacteria within tumors. Furthermore, upon NIR irradiation, Cy7-Neo-NO2 successfully inhibited MRSA growth through a synergistic effect combining photothermal therapy and photodynamic therapy. Our results provided an effective tool for obtaining exceptional PA agents for accurate diagnosis, therapeutic evaluation of deep-seated bacterial infections in vivo, and intratumoral bacteria-specific recognition.

Molecularly Targeted Fluorescent Sensors for Visualizing and Tracking Cellular Senescence

Specific identification and monitoring of senescent cells are essential for the in-depth understanding and regulation of senescence-related life processes and diseases. Fluorescent sensors providing real-time and in situ information with spatiotemporal resolution are unparalleled tools and have contributed greatly to this field. This review focuses on the recent progress in fluorescent sensors for molecularly targeted imaging and real-time tracking of cellular senescence. The molecular design, sensing mechanisms, and biological activities of the sensors are discussed. The sensors are categorized by the types of markers and targeting ligands. Accordingly, their molecular recognition and fluorescent performance towards senescence biomarkers are summarized. Finally, the perspective and challenges in this field are discussed, which are expected to assist future design of next-generation sensors for monitoring cellular senescence.

Exploring antibiotic resistance with chemical tools

Antibiotic resistance is an enormous problem that is accountable for over a million deaths annually, with numbers expected to significantly increase over the coming decades. Although some of the underlying causes leading up to antibiotic resistance are well understood, many of the molecular processes involved remain elusive. To better appreciate at a molecular level how resistance emerges, customized chemical biology tools can offer a solution. This Feature Article attempts to provide an overview of the wide variety of tools that have been developed over the last decade, by highlighting some of the more illustrative examples. These include the use of fluorescent, photoaffinity and activatable antibiotics and bacterial components to start to unravel the molecular mechanisms involved in resistance. The antibiotic crisis is an eminent global threat and requires the continuous development of creative chemical tools to dissect and ultimately counteract resistance.

Supramolecular Host-Guest Strategy for the Accelerating Detection of Nitroreductase

Identifying nitroreductase (NTR) with fluorescent techniques has become a research hotspot, due to its good sensitivity and selectivity toward the early-stage cancer diagnosis and monitoring. Herein, a host-guest reporter (NAQA⊂Zn-MPPB) is successfully achieved by encapsulating the NTR probe NAQA into a new NADH-functioned metal-organic cage Zn-MPPB, which makes the reporter for ultrafast detection of NTR within dozens of seconds in solution. The host-guest strategy fuses the Zn-MPPB and NAQA to form a pseudomolecule material, which changes the reaction process of NTR and NAQA from a double substrates mechanism to a single substrate one, and accelerates the reduction efficiency of NAQA. This advantage make the new host-guest reporter exhibit a linear relationship between emission changes and NTR concentration, and it shows better sensitively toward NTR than that of NAQA. Additionally, the positively charged water-soluble metal-organic cage can encapsulate NAQA in the cavity, promote it to dissolve in an aqueous environment, and facilitate their accumulation into tumor cells. As expected, such host-guest reporter displays a fast and high efficiently imaging capability toward NTR in tumor cells and tumor-bearing mice, and flow cytometry assay is conducted to corroborate the capability as well, implying the considerably potential of host-guest strategy for early tumor diagnosis and treatment.

Polymyxin-based fluorescent probes to combat Gram-negative antimicrobial resistance

Reliable diagnostic approaches especially those targeting critical Gram-negative bacteria are urgently needed for the prevention of antimicrobial resistance. Polymyxin B (PMB) which specifically targets the outer membrane of Gram-negative bacteria is the last-line antibiotic against life-threatening multidrug-resistant Gram-negative bacteria. However, increasing number of studies have reported the spread of PMB-resistant strains. With the aim to specifically detect Gram-negative bacteria and potentially reduce the irrational use of antibiotics, we herein rationally designed two Gram-negative bacteria specific fluorescent probes based on our previous activity-toxicity optimization of PMB. The in vitro probe PMS-Dns showed fast and selective labeling of Gram-negative pathogens in complex biological cultures. Subsequently, we constructed the caged in vivo fluorescent probe PMS-Cy-NO₂ by conjugating bacterial nitroreductase (NTR)-activatable positive charged hydrophobic near-infrared (NIR) fluorophore with polymyxin scaffold. Significantly, PMS-Cy-NO₂ exhibited excellent Gram-negative bacterial detection capability with the differentiation between Gram-positive and Gram-negative in a mouse skin infection model.

Diagnosis of Bacterial Plant Diseases via a Nitroreductase-Activated Fluorescent Sensor

Plant diseases caused by bacteria have become one of the serious problems that threaten human food security, which led to the remarkable reduction of agricultural yields and economic loss. Nitroreductase (NTR), as an important biomarker, is highly expressed in bacteria, and the level of NTR is closely related to the progression of pathogen infection. Therefore, the design of small-molecule fluorescent sensors targeting NTR is of great significance for the detection and diagnosis of plant pathogenic bacteria. In this study, a new fluorescent sensor targeting NTR was discovered and then successfully applied to the imaging of zebrafish and pathogenic bacteria. Most importantly, the developed sensor achieved the real-time diagnosis of Brassica napus L. infected with bacteria, which provides a promising tool for examining the temporal and spatial infection of plant pathogens in precision agriculture.

Precise Monitoring and Assessing Treatment Response of Sepsis-Induced Acute Lung Hypoxia with a Nitroreductase-Activated Golgi-Targetable Fluorescent Probe

Sepsis-induced acute lung injury (ALI) is mostly attributed to an outbreak of reactive oxygen species (ROS), which makes leukocytes infiltrate into the lung and results in lung hypoxia. Nitroreductase (NTR) is significantly upregulated under hypoxia, which is commonly regarded as a potential biomarker for assessing sepsis-induced acute lung hypoxia. Increasing evidence shows that NTR in the Golgi apparatus could be induced in sepsis-induced ALI. Meanwhile, the prolyl hydroxylase (PHD) inhibitor (dimethyloxalylglycine, DMOG) attenuated sepsis-induced ALI through further increasing the level of Golgi NTR by improving hypoxia inducible factor-1α (HIF-1α) activity, but as yet, no Golgi-targetable probe has been developed for monitoring and assessing treatment response of sepsis-induced ALI. Herein, we report a Golgi-targetable probe, Gol-NTR, for monitoring and assessing treatment response of sepsis-induced ALI through mapping the generation of NTR. The probe displayed high sensitivity with a low detection limit of 54.8 ng/mL and good selectivity to NTR. In addition, due to the excellent characteristics of Golgi-targetable, Gol-NTR was successfully applied in mapping the change of Golgi NTR in cells and zebrafish caused by various stimuli. Most importantly, the production of Golgi NTR in the sepsis-induced ALI and the PHD inhibitor (DMOG) against sepsis-induced ALI were visualized and precisely assessed for the first time with the assistance of Gol-NTR. The results demonstrated the practicability of Gol-NTR for the precise monitoring and assessing of the personalized treatment response of sepsis-induced ALI.

Membrane dual-targeting probes: A promising strategy for fluorescence-guided prostate cancer surgery and lymph node metastases detection

Fluorescence-guided surgery (FGS) with tumor-targeted imaging agents, particularly those using the near-infrared wavelength, has emerged as a real-time technique to highlight the tumor location and margins during a surgical procedure. For accurate visualization of prostate cancer (PCa) boundary and lymphatic metastasis, we developed a new approach involving an efficient self-quenched near-infrared fluorescence probe, Cy-KUE-OA, with dual PCa-membrane affinity. Cy-KUE-OA specifically targeted the prostate-specific membrane antigen (PSMA), anchored into the phospholipids of the cell membrane of PCa cells and consequently showed a strong Cy7-de-quenching effect. This dual-membrane-targeting probe allowed us to detect PSMA-expressing PCa cells both in vitro and in vivo and enabled clear visualization of the tumor boundary during fluorescence-guided laparoscopic surgery in PCa mouse models. Furthermore, the high PCa preference of Cy-KUE-OA was confirmed on surgically resected patient specimens of healthy tissues, PCa, and lymph node metastases. Taken together, our results serve as a bridge between preclinical and clinical research in FGS of PCa and lay a solid foundation for further clinical research.

Structure-Activity Studies of Nitroreductase-Responsive Near-Infrared Heptamethine Cyanine Fluorescent Probes

Two new classes of near-infrared molecular probes were prepared and shown to exhibit "turn on" fluorescence when cleaved by the nitroreductase enzyme, a well-known biomarker of cell hypoxia. The fluorescent probes are heptamethine cyanine dyes with a central 4'-carboxylic ester group on the heptamethine chain that is converted by a self-immolative fragmentation mechanism to a 4'-caboxylate group that greatly enhances the fluorescence brightness. Each compound was prepared by ring opening of a Zincke salt. The chemical structures have either terminal benzoindolinenes or propargyloxy auxochromes, which provide favorable red-shifted absorption/emission wavelengths and a hyperchromic effect that enhances the photon output when excited by 808 nm light. A fluorescent probe with terminal propargyloxy-indolenines exhibited less self-aggregation and was rapidly activated by nitroreductase with large "turn on" fluorescence; thus, it is the preferred choice for translation towards in vivo applications.

Metal-Organic Framework-Based Nanoheater with Photo-Triggered Cascade Effects for On-Demand Suppression of Cellular Thermoresistance and Synergistic Cancer Therapy

Nanomedicine with stable light-heat conversion and spatiotemporally controllable drug activation is crucial for the success of photothermal therapy (PTT). Herein, a metal-organic framework (MOF)-based nanoheater with light-triggered multi-responsiveness is engineered to in-situ and on-demand sensitize cancer cells to local hyperthermia. Well-dispersed platinum nanoparticles synthesized inside nanospaces of the MOF are employed as the near-infrared (NIR)-harvesting unit with stable and high light-heat conversion performance. A conformation switchable polymer shell is constructed as a secondary light-responding unit to gate the targeted activation of a molecular inhibitor against thermoresistance. By cascade transformation of light stimuli to downstream signals, the nanoheater enables inhibitor release to go with local heating at the same time restricted in lesion sites to maximize efficacy and minimize systemic toxicity. The efficient photothermal conversion and the blockage of cellular heat-protective pathways provide a dual-mode of action which selectively sensitizes cancer cells to hyperthermia in a spatiotemporally controlled manner. With NIR as the remote switch, the MOF-based nanosystem demonstrates localized and boosted PTT efficacy against cancer both in vitro and in vivo. These results present nanosized MOFs as tailorable and versatile platforms for synergistic and precise cancer therapy.

MicroPET imaging of bacterial infection with nitroreductase-specific responsive 18F-labelled nitrogen mustard analogues

PURPOSE

Bacterial infection and antibiotic resistance are serious threats to human health. This study aimed to develop two novel radiotracers, ¹⁸F-NTRP and ¹⁸F-NCRP, that possess a specific nitroreductase (NTR) response to image deep-seated bacterial infections using positron emission tomography (PET). This method can distinguish infection from sterile inflammation.

METHODS

¹⁸F-NTRP and ¹⁸F-NCRP were synthesized via a one-step method; all the steps usually involved in tracer radiosynthesis were successfully adapted in the All-In-One automated module. After the physiochemical properties of ¹⁸F-NTRP and ¹⁸F-NCRP were characterized, their specificity and selectivity for NTR were verified in E. coli and S. aureus. The ex vivo biodistribution of the tracers was evaluated in normal mice. MicroPET-CT imaging was performed in mouse models of bacterial infection and inflammation after the administration of ¹⁸F-NTRP or ¹⁸F-NCRP.

RESULTS

Fully automated radiosynthesis of ¹⁸F-NTRP and ¹⁸F-NCRP was achieved within 90-110 min with overall decay-uncorrected, isolated radiochemical yields of 21.24 ± 4.25% and 11.3 ± 3.78%, respectively. The molar activities of ¹⁸F-NTRP and ¹⁸F-NCRP were 320 ± 40 GBq/μmol and 275 ± 33 GBq/µmol, respectively. In addition, ¹⁸F-NTRP and ¹⁸F-NCRP exhibited high selectivity and specificity for NTR response. PET-CT imaging in bacteria-infected mouse models with ¹⁸F-NTRP or ¹⁸F-NCRP showed significant radioactivity uptake in either E. coli- or S. aureus-infected muscles. The uptake for E. coli-infected muscles, 2.4 ± 0.2%ID/g with ¹⁸F-NTRP and 4.05 ± 0.49%ID/g with ¹⁸F-NCRP, was up to three times greater than that for uninfected control muscles. Furthermore, for both ¹⁸F-NTRP and ¹⁸F-NCRP, the uptake in bacterial infection was 2.6 times higher than that in sterile inflammation, allowing an effective distinction of infection from inflammation.

CONCLUSION

¹⁸F-NTRP and ¹⁸F-NCRP are worth further investigation to verify their potential clinical application for distinguishing bacterial infection from sterile inflammation via their specific NTR responsiveness.

Small-molecule fluorescent probes: big future for specific bacterial labeling and infection detection

Bacterial infections remain a global healthcare problem that is particularly attributed to the spread of antibiotic resistance and the evolving pathogenicity. Accurate and swift approaches for infection diagnosis are urgently needed to facilitate antibiotic stewardship and effective medical treatment. Direct optical imaging for specific bacterial labeling and infection detection offers an attractive prospect of precisely monitoring the infectious disease status and therapeutic response in real time. This feature article focuses on the recent advances of small-molecule probes developed for fluorescent imaging of bacteria and infection, which covers the probe design, responsive mechanisms and representative applications. In addition, the perspective and challenges to advance small-molecule fluorescent probes in the field of rapid drug-resistant bacterial detection and clinical diagnosis of bacterial infections are discussed. We envision that the continuous advancement and clinical translations of such a technique will have a strong impact on future anti-infective medicine.

Emissive oxidase-like nanozyme based on an organic molecular cage

In this study, we introduced four "claw-like" units of dipicolylamine (DPA) to a tetraphenylethylene (TPE)-based organic molecular cage (DPA-TPE-Cage). Coordinated with Zn²⁺ ions, the obtained ZnDPA-TPE-Cage exhibited aggregation induced emission (AIE) effects and oxidase-like properties, which endowed it with the ability to selectively image and kill Gram-positive bacteria S. aureus efficiently.

Strategies of Detecting Bacteria Using Fluorescence-Based Dyes

Identification of bacterial strains is critical for the theranostics of bacterial infections and the development of antibiotics. Many organic fluorescent probes have been developed to overcome the limitations of conventional detection methods. These probes can detect bacteria with "off-on" fluorescence change, which enables the real-time imaging and quantitative analysis of bacteria in vitro and in vivo. In this review, we outline recent advances in the development of fluorescence-based dyes capable of detecting bacteria. Detection strategies are described, including specific interactions with bacterial cell wall components, bacterial and intracellular enzyme reactions, and peptidoglycan synthesis reactions. These include theranostic probes that allow simultaneous bacterial detection and photodynamic antimicrobial effects. Some examples of other miscellaneous detections in bacteria have also been described. In addition, this review demonstrates the validation of these fluorescent probes using a variety of biological models such as gram-negative and -positive bacteria, antibiotic-resistant bacteria, infected cancer cells, tumor-bearing, and infected mice. Prospects for future research are outlined by presenting the importance of effective in vitro and in vivo detection of bacteria and development of antimicrobial agents.

Developing fluoromodule-based probes for in vivo monitoring the bacterial infections and antibiotic responses

Recently, antibiotic resistant has become a serious public health concern, which warrants new generations of antibiotics to be developed. Pharmacodynamic evaluation is crucial in drug discovery processes. Despite numerous advanced imaging systems are available nowadays, technologies for the sensitive in vivo diagnosis of bacterial infections and direct visualization of drug efficacy are yet to be developed. In this study, we have developed novel near-infrared (NIR) fluorogenic probes. These probes are dark in solution but highly fluorescent when bound to the cognate reporter, fluorogen-activating protein (FAP). We established the in vivo bacterial infection model using FAP_dH6.2 recombinantly expressed E. coli and applied this NIR fluoromodule-based system for diagnosing bacterial infections and monitoring disease progressions and its responses to a type of antibiotics through classic mechanism of membrane lysis. This NIR fluoromodule-based system will discover new information on bacterial infections and identify newer antibacterial entities.

Small-molecule probes for fluorescent detection of cellular hypoxia-related nitroreductase

Nitroreductase is a reductase that catalyzes nitro aromatic compounds to aromatic amines. It effectively reduces nitro to hydroxylamine or amino when in the presence of nicotinamide adenine dinucleotide or nicotinamide adenine dinucleotide phosphate. In terms of tumor, nitroreductase is upregulated in hypoxic tumor cells, and its content is directly related to the degree of hypoxia. Therefore, effective detection of nitroreductase is important not only for the study of cellular hypoxia, but also for the diagnosis and treatment of tumors and related diseases. In this review, we summarized the latest advances in small-molecule fluorescent probes for nitroreductase detection based on different fluorescence mechanisms, with a focus on research conducted between May 2018 and December 2020. The development trends and application prospect in this rapidly developing field were also highlighted.