Preclinical studies and prospective clinical applications for bacteria-targeted imaging: the future is bright

Genetically engineered filamentous phage for bacterial detection using magnetic resonance imaging

Detecting bacterial cells with high specificity in deep tissues is challenging. Optical probes provide specificity, but are limited by the scattering and absorption of light in biological tissues. Conversely, magnetic resonance imaging (MRI) allows unfettered access to deep tissues, but lacks contrast agents for detecting specific bacterial strains. Here, we introduce a biomolecular platform that combines both capabilities by exploiting the modularity of M13 phage to target bacteria with tunable specificity and allow deep-tissue imaging using T1-weighted MRI. We engineered two types of phage probes: one for detecting the phage's natural host, viz., F-pilus expressing E. coli; and the other for detecting a different (F-negative) bacterial target, V. cholerae. We show that these phage sensors generate 3-9-fold stronger T1 relaxation upon recognizing target cells relative to non-target bacteria. We further establish a preliminary proof-of-concept for in vivo applications, by demonstrating that phage-labeled bacteria can be detected in mice using MRI. The framework developed in this study may have potential utility in a broad range of applications, from basic biomedical research to in situ diagnostics, which require methods to detect and track specific bacteria in the context of intact living systems.

Preclinical evaluation of 2-[18F]fluorodeoxysorbitol as a tracer for targeted imaging of Enterobacterales infection

Fluorine-18-fluorodeoxyglucose ([¹⁸F]FDG) positron emission tomography (¹⁸F-FDG-PET) is widely used for the detection of inflammatory and infectious diseases. Although this modality has proven to be a useful diagnostic tool, reliable distinction of bacterial infection from sterile inflammation or even from a malignancy remains challenging. Therefore, there is a need for bacteria-specific tracers for PET imaging that facilitate a reliable distinction of bacterial infection from other pathology. The present study was aimed at exploring the potential of 2-[¹⁸F]-fluorodeoxysorbitol ([¹⁸F]FDS) as a tracer for detection of Enterobacterales infections. Sorbitol is a sugar alcohol that is commonly metabolized by bacteria of the Enterobacterales order, but not by mammalian cells, which makes it an attractive candidate for targeted bacterial imaging. The latter is important in view of the serious clinical implications of infections caused by Enterobacterales. Here we demonstrate that sorbitol-based PET can be applied to detect a broad range of clinical bacterial isolates not only in vitro, but also in blood and ascites samples from patients suffering from Enterobacterales infections. Notably, the possible application of [¹⁸F]FDS is not limited to Enterobacterales since Pseudomonas aeruginosa and Corynebacterium jeikeium also showed substantial uptake of this tracer. We conclude that [¹⁸F]FDS is a promising tracer for PET-imaging of infections caused by a group of bacteria that can cause serious invasive disease.

Synthesis and Bioevaluation of Novel Technetium-99m-Labeled Complexes with Norfloxacin HYNIC Derivatives for Bacterial Infection Imaging

To seek a novel ^99mTc-labeled quinolone derivative for bacterial infection SPECT imaging that aims to lower nontarget organ uptake, a novel norfloxacin 6-hydrazinoicotinamide (HYNIC) derivative (HYNICNF) was designed and synthesized. It was radiolabeled with different coligands, such as tricine, trisodium triphenylphosphine-3,3',3″-trisulfonate (TPPTS), sodium triphenylphosphine-3-monosulfonate (TPPMS), and ethylenediamine-N,N'-diacetic acid (EDDA), to obtain three ^99mTc-labeled norfloxacin HYNIC complexes, namely, [^99mTc]Tc-tricine-TPPTS-HYNICNF, [^99mTc]Tc-tricine-TPPMS-HYNICNF, and [^99mTc]Tc-EDDA-HYNICNF. These complexes were purified (RCP > 95%) and evaluated in vitro and in vivo for targeting bacteria. All three complexes are hydrophilic, maintain good stability, and specifically bind Staphylococcus aureus in vitro. The biodistribution in mice with bacterial infection demonstrated that [^99mTc]Tc-EDDA-HYNICNF showed a higher abscess uptake and lower nontarget organ uptake and was able to distinguish bacterial infection and sterile inflammation. Single photon emission computed tomography (SPECT) image study in bacterial infection mice showed there was a visible accumulation in the infection site, suggesting that [^99mTc]Tc-EDDA-HYNICNF is a potential radiotracer for bacterial infection imaging.

Imaging sensitive and drug-resistant bacterial infection with [11C]-trimethoprim

BACKGROUND

Several molecular imaging strategies can identify bacterial infections in humans. PET affords the potential for sensitive infection detection deep within the body. Among PET-based approaches, antibiotic-based radiotracers, which often target key bacterial-specific enzymes, have considerable promise. One question for antibiotic radiotracers is whether antimicrobial resistance (AMR) reduces specific accumulation within bacteria, diminishing the predictive value of the diagnostic test.

METHODS

Using a PET radiotracer based on the antibiotic trimethoprim (TMP), [¹¹C]-TMP, we performed in vitro uptake studies in susceptible and drug-resistant bacterial strains and whole-genome sequencing (WGS) in selected strains to identify TMP resistance mechanisms. Next, we queried the NCBI database of annotated bacterial genomes for WT and resistant dihydrofolate reductase (DHFR) genes. Finally, we initiated a first-in-human protocol of [¹¹C]-TMP in patients infected with both TMP-sensitive and TMP-resistant organisms to demonstrate the clinical feasibility of the tool.

RESULTS

We observed robust [¹¹C]-TMP uptake in our panel of TMP-sensitive and -resistant bacteria, noting relatively variable and decreased uptake in a few strains of P. aeruginosa and E. coli. WGS showed that the vast majority of clinically relevant bacteria harbor a WT copy of DHFR, targetable by [¹¹C]-TMP, and that despite the AMR, these strains should be “imageable.” Clinical imaging of patients with [¹¹C]-TMP demonstrated focal radiotracer uptake in areas of infectious lesions.

CONCLUSION

This work highlights an approach to imaging bacterial infection in patients, which could affect our understanding of bacterial pathogenesis as well as our ability to better diagnose infections and monitor response to therapy.

TRIAL REGISTRATION

ClinicalTrials.gov NCT03424525.

FUNDING

Institute for Translational Medicine and Therapeutics, Burroughs Wellcome Fund, NIH Office of the Director Early Independence Award (DP5-OD26386), and University of Pennsylvania NIH T32 Radiology Research Training Grant (5T32EB004311-12).

Bacteria-targeted fluorescence imaging of extracted osteosynthesis devices for rapid visualization of fracture-related infections

PURPOSE

Fracture-related infection (FRI) is a serious complication in orthopedic trauma surgery worldwide. Especially, the distinction of infection from sterile inflammation and the detection of low-grade infection are highly challenging. The objective of the present study was to obtain proof-of-principle for the use of bacteria-targeted fluorescence imaging to detect FRI on extracted osteosynthesis devices as a step-up towards real-time image-guided trauma surgery.

METHODS

Extracted osteosynthesis devices from 13 patients, who needed revision surgery after fracture treatment, were incubated with a near-infrared fluorescent tracer composed of the antibiotic vancomycin and the fluorophore IRDye800CW (i.e., vanco-800CW). Subsequently, the devices were imaged, and vanco-800CW fluorescence signals were correlated to the results of microbiological culturing and to bacterial growth upon replica plating of the imaged devices on blood agar.

RESULTS

Importantly, compared to culturing, the bacteria-targeted fluorescence imaging of extracted osteosynthesis devices with vanco-800CW allows for a prompt diagnosis of FRI, reducing the time-to-result from days to less than 30 min. Moreover, bacteria-targeted imaging can provide surgeons with real-time visual information on the presence and extent of infection.

CONCLUSION

Here, we present the first clinical application of fluorescence imaging for the detection of FRI. We conclude that imaging with vanco-800CW can provide early, accurate, and real-time visual diagnostic information on FRI in the clinical setting, even in the case of low-grade infections.

Applications of smartphone-based near-infrared (NIR) imaging, measurement, and spectroscopy technologies to point-of-care (POC) diagnostics

The role of point-of-care (POC) diagnostics is important in public health. With the support of smartphones, POC diagnostic technologies can be greatly improved. This opportunity has arisen from not only the large number and fast spread of cell-phones across the world but also their improved imaging/diagnostic functions. As a tool, the smartphone is regarded as part of a compact, portable, and low-cost system for real-time POC, even in areas with few resources. By combining near-infrared (NIR) imaging, measurement, and spectroscopy techniques, pathogens can be detected with high sensitivity. The whole process is rapid, accurate, and low-cost, and will set the future trend for POC diagnostics. In this review, the development of smartphone-based NIR fluorescent imaging technology was described, and the quality and potential of POC applications were discussed.

Comparison of two fluorescent probes in preclinical non-invasive imaging and image-guided debridement surgery of Staphylococcal biofilm implant infections

Implant-associated infections are challenging to diagnose and treat. Fluorescent probes have been heralded as a technologic advancement that can improve our ability to non-invasively identify infecting organisms, as well as guide the inexact procedure of surgical debridement. This study’s purpose was to compare two fluorescent probes for their ability to localize Staphylococcus aureus biofilm infections on spinal implants utilizing noninvasive optical imaging, then assessing the broader applicability of the more successful probe in other infection animal models. This was followed by real-time, fluorescence image-guided surgery to facilitate debridement of infected tissue. The two probe candidates, a labelled antibiotic that targets peptidoglycan (Vanco-800CW), and the other, a labelled antibody targeting the immunodominant Staphylococcal antigen A (1D9-680), were injected into mice with spine implant infections. Mice were then imaged noninvasively with near infrared fluorescent imaging at wavelengths corresponding to the two probe candidates. Both probes localized to the infection, with the 1D9-680 probe showing greater fidelity over time. The 1D9-680 probe was then tested in mouse models of shoulder implant and allograft infection, demonstrating its broader applicability. Finally, an image-guided surgery system which superimposes fluorescent signals over analog, real-time, tissue images was employed to facilitate debridement of fluorescent-labelled bacteria.

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

The reliable differentiation between bacterial infections and other pathologies is crucial for both diagnostics and therapeutic approaches. To accommodate such needs, we herein report the development of an activatable near-infrared fluorescent probe 1 that could be applied in the ultrafast, ultrasensitive and specific detection of nitroreductase (NTR) activity in bacterial pathogens both in vitro and in vivo. Upon reaction with NTR, the nitro-group of the para-nitro phenyl sulfonic moiety present in probe 1 was reduced to an amino-group, resulting in a near-infrared fluorescence turn-on of the latent cyanine 7 fluorophore. Probe 1 was capable of rapid and real-time quantitative detection of 0–150 ng mL⁻¹ NTR with a limit of detection as low as 0.67 ng mL⁻¹in vitro. In addition, probe 1 exhibited an outstanding performance of ultrafast measurements and suitable selectivity toward NTR to accurately sense intracellular basal NTR in ESKAPE bacterial pathogens. Most remarkably, probe 1 was capable of noninvasively identifying bacterial infection sites without showing any significantly increased signal of tumour sites in the same animal within 30 min.

A new nitroreductase-responsive near-infrared fluorogenic probe can specifically image live bacteria in mouse models and does not accumulate at sites of inflammation or tumor.

Radiochemical Approaches to Imaging Bacterial Infections: Intracellular versus Extracellular Targets

The discovery of penicillin began the age of antibiotics, which was a turning point in human healthcare. However, to this day, microbial infections are still a concern throughout the world, and the rise of multidrug-resistant organisms is an increasing challenge. To combat this threat, diagnostic imaging tools could be used to verify the causative organism and curb inappropriate use of antimicrobial drugs. Nuclear imaging offers the sensitivity needed to detect small numbers of bacteria in situ. Among nuclear imaging tools, radiolabeled antibiotics traditionally have lacked the sensitivity or specificity necessary to diagnose bacterial infections accurately. One reason for the lack of success is that the antibiotics were often chelated to a radiometal. This was done without addressing the ramifications of how the radiolabeling would impact probe entry to the bacterial cell, or the mechanism of binding to an intracellular target. In this review, we approach bacterial infection imaging through the lens of bacterial specific molecular targets, their intracellular or extracellular location, and discuss radiochemistry strategies to guide future probe development.

2018 international consensus meeting on musculoskeletal infection: Summary from the biofilm workgroup and consensus on biofilm related musculoskeletal infections

Biofilm-associated implant-related bone and joint infections are clinically important due to the extensive morbidity, cost of care and socioeconomic burden that they cause. Research in the field of biofilms has expanded in the past two decades, however, there is still an immense knowledge gap related to many clinical challenges of these biofilm-associated infections. This subject was assigned to the Biofilm Workgroup during the second International Consensus Meeting on Musculoskeletal Infection held in Philadelphia USA (ICM 2018) (https://icmphilly.com). The main objective of the Biofilm Workgroup was to prepare a consensus document based on a review of the literature, prepared responses, discussion, and vote on thirteen biofilm related questions. The Workgroup commenced discussing and refining responses prepared before the meeting on day one using Delphi methodology, followed by a tally of responses using an anonymized voting system on the second day of ICM 2018. The Working group derived consensus on information about biofilms deemed relevant to clinical practice, pertaining to: (1) surface modifications to prevent/inhibit biofilm formation; (2) therapies to prevent and treat biofilm infections; (3) polymicrobial biofilms; (4) diagnostics to detect active and dormant biofilm in patients; (5) methods to establish minimal biofilm eradication concentration for biofilm bacteria; and (6) novel anti-infectives that are effective against biofilm bacteria. It was also noted that biomedical research funding agencies and the pharmaceutical industry should recognize these areas as priorities. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

Towards clinical application of non-invasive imaging to detect bacterial infections

In vivo imaging technologies offer a great potential for the diagnosis of difficult-to-treat bacterial infections. A major limitation of conventional imaging modalities is the lack of specificity to distinguish the site of bacterial infection from sterile inflammation. Targeted approaches like antibiotics linked to imaging tracers for detection of various bacterial pathogens or species-specific antibodies combined with anatomical imaging modalities are currently being evaluated to overcome this problem. Considering the recent progress in optical and targeted imaging that may accelerate preclinical development programs, clinical implementation of in vivo imaging modalities to detect bacterial infection foci becomes realistic in the future.

Preclinical evaluation of potential infection-imaging probe [68 Ga]Ga-DOTA-K-A9 in sterile and infectious inflammation

The development of bacteria-specific infection radiotracers is of considerable interest to improve diagnostic accuracy and enabling therapy monitoring. The aim of this study was to determine if the previously reported radiolabelled 1,4,7,10-tetraazacyclododecane-N,N',N″,N‴-tetraacetic acid (DOTA) conjugated peptide [68 Ga]Ga-DOTA-K-A9 could detect a staphylococcal infection in vivo and distinguish it from aseptic inflammation. An optimized [68 Ga]Ga-DOTA-K-A9 synthesis omitting the use of acetone was developed, yielding 93 ± 0.9% radiochemical purity. The in vivo infection binding specificity of [68 Ga]Ga-DOTA-K-A9 was evaluated by micro positron emission tomography/magnetic resonance imaging of 15 mice with either subcutaneous Staphylococcus aureus infection or turpentine-induced inflammation and compared with 2-deoxy-2-[18 F]fluoro-D-glucose ([18 F]FDG). The scans showed that [68 Ga]Ga-DOTA-K-A9 accumulated in all the infected mice at injected doses ≥3.6 MBq. However, the tracer was not found to be selective towards infection, since the [68 Ga]Ga-DOTA-K-A9 also accumulated in mice with inflammation. In a concurrent in vitro binding evaluation performed with a 5-carboxytetramethylrhodamine (TAMRA) fluorescence analogue of the peptide, TAMRA-K-A9, the microscopy results suggested that TAMRA-K-A9 bound to an intracellular epitope and therefore preferentially targeted dead bacteria. Thus, the [68 Ga]Ga-DOTA-K-A9 uptake observed in vivo is presumably a combination of local hyperemia, vascular leakiness and/or binding to an epitope present in dead bacteria.

Pathogen-Specific Bacterial Imaging in Nuclear Medicine

When serious infections are suspected, patients are often treated empirically with broad-spectrum antibiotics while awaiting results that provide information on the bacterial class and species causing the infection, as well as drug susceptibilities. For deep-seated infections, these traditional diagnostic techniques often rely on tissue biopsies to obtain clinical samples which can be expensive, dangerous, and has the potential of sampling bias. Moreover, these procedures and results can take several days and may not always provide reliable information. This combination of time and effort required for proper antibiotic selection has become a barrier leading to indiscriminate broad-spectrum antibiotic use. Exposure to nosocomial infections and indiscriminate use of broad-spectrum antibiotics are responsible for promoting bacterial drug-resistance leading to substantial morbidity and mortality, especially in hospitalized and immunosuppressed patients. Therefore, early diagnosis of infection and targeted antibiotic treatments are urgently needed to reduce morbidity and mortality caused by bacterial infections worldwide. Reliable pathogen-specific bacterial imaging techniques have the potential to provide early diagnosis and guide antibiotic treatments.

Ex Vivo Tracer Efficacy in Optical Imaging of Staphylococcus Aureus Nuclease Activity

The key to effective treatment of bacterial infections is a swift and reliable diagnosis. Current clinical standards of bacterial diagnosis are slow and laborious. There are several anatomical imaging modalities that can detect inflammation, but none can distinguish between bacterial and sterile inflammation. Novel tracers such as smart activatable fluorescent probes represent a promising development that allow fast and specific testing without the use of ionizing radiation. Previously, a smart activatable probe was developed that is a substrate for the micrococcal nuclease as produced by Staphylococcus aureus. In the present study, the function of this probe was validated. Practical applicability in terms of sensitivity was assessed by incubation of the probe with 26 clinical S. aureus isolates, and probe specificity was verified by incubation with 30 clinical isolates and laboratory strains of various bacterial pathogens. The results show that the nuclease-specific probe was activated by all tested S. aureus isolates and laboratory strains with a threshold of ~106-107 cells/mL. The probe was also activated by certain opportunistic staphylococci. We therefore propose that the studied nuclease probe represents a significant step forward to address the need for a rapid, practical, and precise method to detect infections caused by S. aureus.

Gadolinium-Labeled Aminoglycoside and Its Potential Application as a Bacteria-Targeting Magnetic Resonance Imaging Contrast Agent

Magnetic resonance imaging (MRI) is a powerful diagnostic technique that can penetrate deep into tissue providing excellent spatial resolution without the need for ionizing radiation or harmful radionuclides. However, diagnosing bacterial infections in vivo with clinical MRI is severely hampered by the lack of contrast agents with high relaxivity, targeting capabilities, and bacterial penetration and specificity. Here, we report the development of the first gadolinium (Gd)-based bacteria-specific targeting MRI contrast agent, probe 1, by conjugating neomycin, an aminoglycoside antibiotic, with Dotarem (Gd-DOTA, an FDA approved T₁-weighted MRI contrast agent). The T₁ relaxivity of probe 1 was found to be comparable to that of Gd-DOTA; additionally, probe 1-treated bacteria generated a significantly brighter T₁-weighted MR signal than Gd-DOTA-treated bacteria. More importantly, in vitro cellular studies and preliminary in vivo MRI demonstrated probe 1 exhibits the ability to efficiently target bacteria over macrophage-like cells, indicating its great potential for high-resolution imaging of bacterial infections in vivo.

Photonic monitoring of treatment during infection and sepsis: development of new detection strategies and potential clinical applications

Despite the strong decline in the infection-associated mortality since the development of the first antibiotics, infectious diseases are still a major cause of death in the world. With the rising number of antibiotic-resistant pathogens, the incidence of deaths caused by infections may increase strongly in the future. Survival rates in sepsis, which occurs when body response to infections becomes uncontrolled, are still very poor if an adequate therapy is not initiated immediately. Therefore, approaches to monitor the treatment efficacy are crucially needed to adapt therapeutic strategies according to the patient's response. An increasing number of photonic technologies are being considered for diagnostic purpose and monitoring of therapeutic response; however many of these strategies have not been introduced into clinical routine, yet. Here, we review photonic strategies to monitor response to treatment in patients with infectious disease, sepsis, and septic shock. We also include some selected approaches for the development of new drugs in animal models as well as new monitoring strategies which might be applicable to evaluate treatment response in humans in the future. Figure Label-free probing of blood properties using photonics.

Peptides for optical medical imaging and steps towards therapy

Optical medical imaging is a rapidly growing area of research and development that offers a multitude of healthcare solutions both diagnostically and therapeutically. In this review, some of the most recently described peptide-based optical probes are reviewed with a special emphasis on their in vivo use and potential application in a clinical setting.

Bacterial infection imaging with [18F]fluoropropyl-trimethoprim

There is often overlap in the diagnostic features of common pathologic processes such as infection, sterile inflammation, and cancer both clinically and using conventional imaging techniques. Here, we report the development of a positron emission tomography probe for live bacterial infection based on the small-molecule antibiotic trimethoprim (TMP). [¹⁸F]fluoropropyl-trimethoprim, or [¹⁸F]FPTMP, shows a greater than 100-fold increased uptake in vitro in live bacteria (Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa) relative to controls. In a rodent myositis model, [¹⁸F]FPTMP identified live bacterial infection without demonstrating confounding increased signal in the same animal from other etiologies including chemical inflammation (turpentine) and cancer (breast carcinoma). Additionally, the biodistribution of [¹⁸F]FPTMP in a nonhuman primate shows low background in many important tissues that may be sites of infection such as the lungs and soft tissues. These results suggest that [¹⁸F]FPTMP could be a broadly useful agent for the sensitive and specific imaging of bacterial infection with strong translational potential.

In vitro imaging of bacteria using 18F-fluorodeoxyglucose micro positron emission tomography

Positron emission tomography (PET) with fluorine-18-fluorodeoxyglucose (¹⁸F-FDG) can be applied to detect infection and inflammation. However, it was so far not known to what extent bacterial pathogens may contribute to the PET signal. Therefore, we investigated whether clinical isolates of frequently encountered bacterial pathogens take up ¹⁸F-FDG in vitro, and whether FDG inhibits bacterial growth as previously shown for 2-deoxy-glucose. 22 isolates of Gram-positive and Gram-negative bacterial pathogens implicated in fever and inflammation were incubated with ¹⁸F-FDG and uptake of ¹⁸F-FDG was assessed by gamma-counting and µPET imaging. Possible growth inhibition by FDG was assayed with Staphylococcus aureus and the Gram-positive model bacterium Bacillus subtilis. The results show that all tested isolates accumulated ¹⁸F-FDG actively. Further, ¹⁸F-FDG uptake was hampered in B. subtilis pts mutants impaired in glucose uptake. FDG inhibited growth of S. aureus and B. subtilis only to minor extents, and this effect was abrogated by pts mutations in B. subtilis. These observations imply that bacteria may contribute to the signals observed in FDG-PET infection imaging in vivo. Active bacterial FDG uptake is corroborated by the fact that the B. subtilis phosphotransferase system is needed for ¹⁸F-FDG uptake, while pts mutations protect against growth inhibition by FDG.

A human monoclonal antibody that specifically binds and inhibits the staphylococcal complement inhibitor protein SCIN

Staphylococcus aureus is a serious public health burden causing a wide variety of infections. Earlier detection of such infections could result in faster and more directed therapies that also prevent resistance development. Human monoclonal antibodies (humAbs) are promising tools for diagnosis and therapy owing to their relatively straightforward synthesis, long history of safe clinical use and high target specificity. Here we show that the humAb 6D4, which was obtained from a random screen of B-cells producing antibodies that bind to whole cells of S. aureus, targets the staphylococcal complement inhibitor (SCIN). The epitope recognized by 6D4 was localized to residues 26 to 36 in the N-terminus of SCIN, which overlap with the active site. Accordingly, 6D4 can inhibit SCIN activity as demonstrated through the analysis of C3b deposition on S. aureus cells and complement-induced lysis of rabbit erythrocytes. Importantly, while SCIN is generally regarded as a secreted virulence factor, 6D4 allowed detection of strongly increased SCIN binding to S. aureus cells upon exposure to human serum, relating to the known binding of SCIN to C3 convertases deposited on the staphylococcal cell surface. Lastly, we show that labeling of humAb 6D4 with a near-infrared fluorophore allows one-step detection of SCIN-producing S. aureus cells. Together, our findings show that the newly described humAb 6D4 specifically recognizes S. aureus SCIN, which can potentially be used for detection of human serum-incubated S. aureus strains expressing SCIN.