Luciferase in vivo expression technology: use of recombinant mycobacterial reporter strains to evaluate antimycobacterial activity in mice

Rapid Tuberculosis Diagnosis Using Reporter Enzyme Fluorescence

Tuberculosis is the most frequent cause of death in humans from a single infectious agent. Due to low numbers of bacteria present in sputum during early infection, diagnosis does not usually occur until >3 to 4 months after symptoms develop. We created a new more sensitive diagnostic that can be carried out in 10 min with no processing or technical expertise.

Tuberculosis is the most frequent cause of death in humans from a single infectious agent. Due to low numbers of bacteria present in sputum during early infection, diagnosis does not usually occur until >3 to 4 months after symptoms develop. We created a new more sensitive diagnostic that can be carried out in 10 min with no processing or technical expertise. This assay utilizes the Mycobacterium tuberculosis-specific biomarker BlaC in reporter enzyme fluorescence (REF) that has been optimized for clinical samples, designated REFtb, along with a more specific fluorogenic substrate, CDG-3. We report the first evaluation of clinical specimens with REFtb assays in comparison to the gold standards for tuberculosis diagnosis, culture and smear microscopy. REFtb assays allowed diagnosis of 160 patients from 16 different countries with a sensitivity of 89% for smear-positive, culture-positive samples and 88% for smear-negative, culture-positive samples with a specificity of 82%. The negative predictive value of REFtb for tuberculosis infection is 93%, and the positive predictive value is 79%. Overall, these data point toward the need for larger accuracy studies by third parties using a commercially available REFtb kit to determine whether incorporation of REFtb into the clinical toolbox for suspected tuberculosis patients would improve case identification. If results similar to our own can be obtained by all diagnostic laboratories, REFtb would allow proper treatment of more than 85% of patients that would be missed during their initial visit to a clinic using current diagnostic strategies, reducing the potential for further spread of disease.

Fluorescence & bioluminescence in the quest for imaging, probing & analysis of mycobacterial infections

Mycobacterioses represent a global health problem and rapid diagnostic improvements are urgently required. Mycobacteria-specific fluorescence and bioluminescence phenomena have been found to be useful for a wide range of mycobacteria-focused research. Here, we present a critical survey of the most promising techniques in this field and the potential of new methods under investigation. These approaches include acid-fast staining, intrinsic fluorescence of the coenzyme F₄₂₀, fluorogenic substrates (e.g., β-lactamase-sensitive coumpounds) and recombination of mycobacteria or mycobacteriophages. Probably the most interesting and emerging host-inspecting approach is in vivo imaging. Detection of fluorescence in vivo, however, is complicated by light scattering, light absorption, and autofluorescence, caused by the tissues. Despite this, many of these systems show promise as the foundations for improved rapid analysis and imaging of mycobacterial infections, both in vitro and in vivo.

A comprehensive characterization of PncA polymorphisms that confer resistance to pyrazinamide

Tuberculosis chemotherapy is dependent on the use of the antibiotic pyrazinamide, which is being threatened by emerging drug resistance. Resistance is mediated through mutations in the bacterial gene pncA. Methods for testing pyrazinamide susceptibility are difficult and rarely performed, and this means that the full spectrum of pncA alleles that confer clinical resistance to pyrazinamide is unknown. Here, we performed in vitro saturating mutagenesis of pncA to generate a comprehensive library of PncA polymorphisms resultant from a single-nucleotide polymorphism. We then screened it for pyrazinamide resistance both in vitro and in an infected animal model. We identify over 300 resistance-conferring substitutions. Strikingly, these mutations map throughout the PncA structure and result in either loss of enzymatic activity and/or decrease in protein abundance. Our comprehensive mutational and screening approach should stand as a paradigm for determining resistance mutations and their mechanisms of action.The antibiotic pyrazinamide is central to tuberculosis treatment regimens, globally. Despite its efficacy, resistance to the drug is increasing. Here, Eric Rubin and colleagues characterise the genetic basis of pyrazinamide resistance.

Intravital excitation increases detection sensitivity for pulmonary tuberculosis by whole-body imaging with β-lactamase reporter enzyme fluorescence

Tuberculosis is a pulmonary disease with an especially high mortality rate in immuno-compromised populations, specifically children and HIV positive individuals. The causative agent, Mycobacterium tuberculosis (Mtb), is a very slow growing and difficult organism to work with, making both diagnosis and development of effective treatments cumbersome. We utilize a fiber-optic fluorescence microendoscope integrated with a whole-body imaging system for in vivo Mtb detection. The system exploits an endogenous enzyme of Mtb (β-lactamase, or BlaC) using a BlaC-specific NIR fluorogenic substrate. In the presence of BlaC, this substrate is cleaved and becomes fluorescent. Using intravital illumination of the lung to excite this probe, sensitivity of the optical system increases over trans- and epi-illumination methods of whole-body fluorescence imaging. We demonstrate that integration of these imaging technologies with BlaC-specific fluorescent reporter probe improves the level of detection to ∼100 colony forming units, a 100× increase in sensitivity in comparison to epi-illumination and a 10× increase in sensitivity in comparison to previous work in intravital excitation of tdTomato-expressing Mtb. This lower detection threshold enables the study of early stage bacterial infections with clinical strains of Mtb and longitudinal studies of disease pathogenesis and therapeutic efficacy with multiple time points in a single animal.

Intravital excitation increases detection sensitivity for pulmonary tuberculosis by whole-body imaging with β-lactamase reporter enzyme fluorescence

Tuberculosis is a pulmonary disease with an especially high mortality rate in immuno-compromised populations, specifically children and HIV positive individuals. The causative agent, Mycobacterium tuberculosis (Mtb), is a very slow growing and difficult organism to work with, making both diagnosis and development of effective treatments cumbersome. We utilize a fiber-optic fluorescence microendoscope integrated with a whole-body imaging system for in vivo Mtb detection. The system exploits an endogenous enzyme of Mtb (β-lactamase, or BlaC) using a BlaC-specific NIR fluorogenic substrate. In the presence of BlaC, this substrate is cleaved and becomes fluorescent. Using intravital illumination of the lung to excite this probe, sensitivity of the optical system increases over trans- and epi-illumination methods of whole-body fluorescence imaging. We demonstrate that integration of these imaging technologies with BlaC-specific fluorescent reporter probe improves the level of detection to ∼100 colony forming units, a 100× increase in sensitivity in comparison to epi-illumination and a 10× increase in sensitivity in comparison to previous work in intravital excitation of tdTomato-expressing Mtb. This lower detection threshold enables the study of early stage bacterial infections with clinical strains of Mtb and longitudinal studies of disease pathogenesis and therapeutic efficacy with multiple time points in a single animal.

A Fluorescent Probe for Detecting Mycobacterium tuberculosis and Identifying Genes Critical for Cell Entry

The conventional method for quantitating Mycobacterium tuberculosis (Mtb) in vitro and in vivo relies on bacterial colony forming unit (CFU) enumeration on agar plates. Due to the slow growth rate of Mtb, it takes 3-6 weeks to observe visible colonies on agar plates. Imaging technologies that are capable of quickly quantitating both active and dormant tubercle bacilli in vitro and in vivo would accelerate research toward the development of anti-TB chemotherapies and vaccines. We have developed a fluorescent probe that can directly label the Mtb cell wall components. The fluorescent probe, designated as DLF-1, has a strong affinity to the D-Ala-D-Ala unit of the late peptidoglycan intermediates in the bacterial cell wall. We demonstrate that DLF-1 is capable of detecting Mtb in both the actively replicating and dormant states in vitro at 100 nM without inhibiting bacterial growth. The DLF-1 fluorescence signal correlated well with CFU of the labeled bacteria (R² = 1 and 0.99 for actively replicating and dormant Mtb, respectively). DLF-1 can also quantitate labeled Mtb inside of cells. The utility of DLF-1 probe to quantitate Mtb was successfully applied to identify genes critical for cell invasion. In conclusion, this novel near infrared imaging probe provides a powerful new tool for enumerating Mtb with potential future use in bacterial virulence study.

Benzoic Acid-Inducible Gene Expression in Mycobacteria

Conditional expression is a powerful tool to investigate the role of bacterial genes. Here, we adapt the Pseudomonas putida-derived positively regulated XylS/Pm expression system to control inducible gene expression in Mycobacterium smegmatis and Mycobacterium tuberculosis, the causative agent of human tuberculosis. By making simple changes to a Gram-negative broad-host-range XylS/Pm-regulated gene expression vector, we prove that it is possible to adapt this well-studied expression system to non-Gram-negative species. With the benzoic acid-derived inducer m-toluate, we achieve a robust, time- and dose-dependent reversible induction of Pm-mediated expression in mycobacteria, with low background expression levels. XylS/Pm is thus an important addition to existing mycobacterial expression tools, especially when low basal expression is of particular importance.

Bioluminescence for assessing drug potency against nonreplicating Mycobacterium tuberculosis

Targeting dormant Mycobacterium tuberculosis represents a challenge to antituberculosis drug discovery programs. We previously reported and validated the use of the streptomycin (STR)-dependent M. tuberculosis 18b strain as a tool for assessing drug potency against nonreplicating bacteria both in vitro and in vivo. In this study, we generated a luminescent 18b strain, named 18b-Lux, by transforming the bacteria with a vector expressing the luxCDABE operon from Photorhabdus luminescens. Luciferase expression was demonstrated under replicating conditions, and, more importantly, luminescence levels significantly above background were detected following STR removal. The sensitivity of STR-starved 18b-Lux to approved and candidate antituberculosis therapeutic agents was evaluated by means of a luciferase assay in a 96-well format. Results mirrored the data obtained with the standard resazurin reduction microplate assay, and the luminescence readout allowed time course assessments of drug efficacy in vitro. Specifically, we proved that bedaquiline, the rifamycins, and sutezolid displayed time-dependent activity against dormant bacteria, while pyrazinamide and SQ109 showed bactericidal effects at the highest concentrations tested. Overall, we established the optimal conditions for an inexpensive, simple, and very sensitive assay with great potential for future applications.

Using riboswitches to regulate gene expression and define gene function in mycobacteria

Mycobacteria include both environmental species and many pathogenic species such as Mycobacterium tuberculosis, an intracellular pathogen that is the causative agent of tuberculosis in humans. Inducible gene expression is a powerful tool for examining gene function and essentiality, both in in vitro culture and in host cell infections. The theophylline-inducible artificial riboswitch has recently emerged as an alternative to protein repressor-based systems. The riboswitch is translationally regulated and is combined with a mycobacterial promoter that provides transcriptional control. We here provide methods used by our laboratory to characterize the riboswitch response to theophylline in reporter strains, recombinant organisms containing riboswitch-regulated endogenous genes, and in host cell infections. These protocols should facilitate the application of both existing and novel artificial riboswitches to the exploration of gene function in mycobacteria.

Real-time bioluminescence imaging of mixed mycobacterial infections

Molecular analysis of infectious processes in bacteria normally involves construction of isogenic mutants that can then be compared to wild type in an animal model. Pathogenesis and antimicrobial studies are complicated by variability between animals and the need to sacrifice individual animals at specific time points. Live animal imaging allows real-time analysis of infections without the need to sacrifice animals, allowing quantitative data to be collected at multiple time points in all organs simultaneously. However, imaging has not previously allowed simultaneous imaging of both mutant and wild type strains of mycobacteria in the same animal. We address this problem by using both firefly (Photinus pyralis) and click beetle (Pyrophorus plagiophthalamus) red luciferases, which emit distinct bioluminescent spectra, allowing simultaneous imaging of two different mycobacterial strains during infection. We also demonstrate that these same bioluminescence reporters can be used to evaluate therapeutic efficacy in real-time, greatly facilitating our ability to screen novel antibiotics as they are developed. Due to the slow growth rate of mycobacteria, novel imaging technologies are a pressing need, since they can they can impact the rate of development of new therapeutics as well as improving our understanding of virulence mechanisms and the evaluation of novel vaccine candidates.

Simple and rapid method to determine antimycobacterial potency of compounds by using autoluminescent Mycobacterium tuberculosis

A major obstacle in the process of discovery of drugs against Mycobacterium tuberculosis is its extremely slow growth rate and long generation time (∼20 to 24 h). Consequently, determination of MICs and minimum bactericidal concentrations (MBCs) of potential drug candidates using current methods requires 7 days (resazurin-based MIC assay [REMA]) and 1 month (CFU enumeration), respectively. We employed a synthetic luciferase operon optimized for expression in high-GC-content bacteria and adapted it for use in mycobacteria. Using luminescence-based readouts, we were able to determine the MICs and bactericidal activities of approved tuberculosis (TB) drugs, which correlated well with currently used methods. Although luminescence-based readouts have been used previously to determine the MICs and bactericidal activities of approved TB drugs, in this study we adapted this assay to carry out a pilot screen using a library of 1,114 compounds belonging to diverse chemical scaffolds. We found that MICs derived from a 3-day luminescence assay matched well with REMA-based MIC values. To determine the bactericidal potencies of compounds, a 1:10 dilution of the cultures from the MIC plate was carried out on day 7, and the bactericidal concentrations determined based on time to positivity in 2 weeks were found to be comparable with MBC values determined by the conventional CFU approach. Thus, the luminescent mycobacterium-based approach not only is very simple and inexpensive but also allowed us to generate the information in half the time required by conventional methods.

Rapid, serial, non-invasive assessment of drug efficacy in mice with autoluminescent Mycobacterium ulcerans infection

Background

Buruli ulcer (BU) caused by Mycobacterium ulcerans is the world's third most common mycobacterial infection. There is no vaccine against BU and surgery is needed for patients with large ulcers. Although recent experience indicates combination chemotherapy with streptomycin and rifampin improves cure rates, the utility of this regimen is limited by the 2-month duration of therapy, potential toxicity and required parenteral administration of streptomycin, and drug-drug interactions caused by rifampin. Discovery and development of drugs for BU is greatly hampered by the slow growth rate of M. ulcerans, requiring up to 3 months of incubation on solid media to produce colonies. Surrogate markers for evaluating antimicrobial activity in real-time which can be measured serially and non-invasively in infected footpads of live mice would accelerate pre-clinical evaluation of new drugs to treat BU. Previously, we developed bioluminescent M. ulcerans strains, demonstrating proof of concept for measuring luminescence as a surrogate marker for viable M. ulcerans in vitro and in vivo. However, the requirement of exogenous substrate limited the utility of such strains, especially for in vivo experiments.

Methodology/Principal Finding

For this study, we engineered M. ulcerans strains that express the entire luxCDABE operon and therefore are autoluminescent due to endogenous substrate production. The selected reporter strain displayed a growth rate and virulence similar to the wild-type parent strain and enabled rapid, real-time monitoring of in vitro and in vivo drug activity, including serial, non-invasive assessments in live mice, producing results which correlated closely with colony-forming unit (CFU) counts for a panel of drugs with various mechanisms of action.

Conclusions/Significance

Our results indicate that autoluminescent reporter strains of M. ulcerans are exceptional tools for pre-clinical evaluation of new drugs to treat BU due to their potential to drastically reduce the time, effort, animals, compound, and costs required to evaluate drug activity.

The discovery and development of new drugs to improve the treatment of BU is greatly hampered by the slow growth rate of the organism, which requires up to 3 months to form countable colonies on solid media. Here, we engineered a virulent reporter strain of M. ulcerans with intrinsic bioluminescence to enable serial, real-time, non-invasive in vivo monitoring of viable bacterial counts and demonstrate its utility for high-throughput in vitro and in vivo screening of antibiotic efficacy using a panel of anti-mycobacterial drugs with various mechanisms of action. We show: 1) that a drug's in vitro activity against M. ulcerans is detectable in real time after as little as 2 days of exposure, and 2) that a drug's in vivo activity against M. ulcerans is detectable in as little as 1 week through rapid, serial, non-invasive assessment in live mice performed with a benchtop luminometer. This method promises dramatic reductions in time and effort as well as requirements for animals, compounds and other supplies. We believe such a strain is capable of transforming drug discovery and development efforts for BU.

Effects of ascent to high altitude on human antimycobacterial immunity

BACKGROUND

Tuberculosis infection, disease and mortality are all less common at high than low altitude and ascent to high altitude was historically recommended for treatment. The immunological and mycobacterial mechanisms underlying the association between altitude and tuberculosis are unclear. We studied the effects of altitude on mycobacteria and antimycobacterial immunity.

METHODS

Antimycobacterial immunity was assayed in 15 healthy adults residing at low altitude before and after they ascended to 3400 meters; and in 47 long-term high-altitude residents. Antimycobacterial immunity was assessed as the extent to which participants' whole blood supported or restricted growth of genetically modified luminescent Bacille Calmette-Guérin (BCG) mycobacteria during 96 hours incubation. We developed a simplified whole blood assay that could be used by a technician in a low-technology setting. We used this to compare mycobacterial growth in participants' whole blood versus positive-control culture broth and versus negative-control plasma.

RESULTS

Measurements of mycobacterial luminescence predicted the number of mycobacterial colonies cultured six weeks later. At low altitude, mycobacteria grew in blood at similar rates to positive-control culture broth whereas ascent to high altitude was associated with restriction (p ≤ 0.002) of mycobacterial growth to be 4-times less than in culture broth. At low altitude, mycobacteria grew in blood 25-times more than negative-control plasma whereas ascent to high altitude was associated with restriction (p ≤ 0.01) of mycobacterial growth to be only 6-times more than in plasma. There was no evidence of differences in antimycobacterial immunity at high altitude between people who had recently ascended to high altitude versus long-term high-altitude residents.

CONCLUSIONS

An assay of luminescent mycobacterial growth in whole blood was adapted and found to be feasible in low-resource settings. This demonstrated that ascent to or residence at high altitude was associated with decreased mycobacterial growth in whole blood relative to controls, consistent with altitude-related augmentation of antimycobacterial cellular immunity.

Development of a new generation of vectors for gene expression, gene replacement, and protein-protein interaction studies in mycobacteria

Escherichia coli-mycobacterium shuttle vectors are important tools for gene expression and gene replacement in mycobacteria. However, most of the currently available vectors are limited in their use because of the lack of extended multiple cloning sites (MCSs) and convenience of appending an epitope tag(s) to the cloned open reading frames (ORFs). Here we report a new series of vectors that allow for the constitutive and regulatable expression of proteins, appended with peptide tag sequences at their N and C termini, respectively. The applicability of these vectors is demonstrated by the constitutive and induced expression of the Mycobacterium tuberculosis pknK gene, coding for protein kinase K, a serine-threonine protein kinase. Furthermore, a suicide plasmid with expanded MCS for creating gene replacements, a plasmid for chromosomal integrations at the commonly used L5 attB site, and a hypoxia-responsive vector, for expression of a gene(s) under hypoxic conditions that mimic latency, have also been created. Additionally, we have created a vector for the coexpression of two proteins controlled by two independent promoters, with each protein being in fusion with a different tag. The shuttle vectors developed in the present study are excellent tools for the analysis of gene function in mycobacteria and are a valuable addition to the existing repertoire of vectors for mycobacterial research.

Potential use of nitrate reductase as a biomarker for the identification of active and dormant inhibitors of Mycobacterium tuberculosis in a THP1 infection model

The development of a macrophage-based, antitubercular high-throughput screening system could expedite discovery programs for identifying novel inhibitors. In this study, the kinetics of nitrate reduction (NR) by Mycobacterium tuberculosis during growth in Thp1 macrophages was found to be almost parallel to viable bacilli count. NR in the culture medium containing 50 mM of nitrate was found to be optimum on the fifth day after infection with M. tuberculosis. The signal-to-noise (S/N) ratio and Z-factor obtained from this macrophage-based assay were 5.4 and 0.965, respectively, which confirms the robustness of the assay protocol. The protocol was further validated by using standard antitubercular inhibitors such as rifampicin, isoniazid, streptomycin, ethambutol, and pyrazinamide, added at their IC(90) value, on the day of infection. These inhibitors were not able to kill the bacilli when added to the culture on the fifth day after infection. Interestingly, pentachlorophenol and rifampicin killed the bacilli immediately after addition on the fifth day of infection. Altogether, this assay protocol using M. tuberculosis-infected Thp-1 macrophages provides a novel, cost-efficient, robust, and easy-to-perform screening platform for the identification of both active and hypoxic stage-specific inhibitors against tuberculosis.

φ(2)GFP10, a high-intensity fluorophage, enables detection and rapid drug susceptibility testing of Mycobacterium tuberculosis directly from sputum samples

The difficulty of diagnosing active tuberculosis (TB) and lack of rapid drug susceptibility testing (DST) at the point of care remain critical obstacles to TB control. This report describes a high-intensity mycobacterium-specific-fluorophage (φ(2)GFP10) that for the first time allows direct visualization of Mycobacterium tuberculosis in clinical sputum samples. Engineered features distinguishing φ(2)GFP10 from previous reporter phages include an improved vector backbone with increased cloning capacity and superior expression of fluorescent reporter genes through use of an efficient phage promoter. φ(2)GFP10 produces a 100-fold increase in fluorescence per cell compared to existing reporter phages. DST for isoniazid and oxofloxacin, carried out in cultured samples, was complete within 36 h. Use of φ(2)GFP10 detected M. tuberculosis in clinical sputum samples collected from TB patients. DST for rifampin and kanamycin from sputum samples yielded results after 12 h of incubation with φ(2)GFP10. Fluorophage φ(2)GFP10 has potential for clinical development as a rapid, sensitive, and inexpensive point-of-care diagnostic tool for M. tuberculosis infection and for rapid DST.

φ(2)GFP10, a high-intensity fluorophage, enables detection and rapid drug susceptibility testing of Mycobacterium tuberculosis directly from sputum samples

The difficulty of diagnosing active tuberculosis (TB) and lack of rapid drug susceptibility testing (DST) at the point of care remain critical obstacles to TB control. This report describes a high-intensity mycobacterium-specific-fluorophage (φ(2)GFP10) that for the first time allows direct visualization of Mycobacterium tuberculosis in clinical sputum samples. Engineered features distinguishing φ(2)GFP10 from previous reporter phages include an improved vector backbone with increased cloning capacity and superior expression of fluorescent reporter genes through use of an efficient phage promoter. φ(2)GFP10 produces a 100-fold increase in fluorescence per cell compared to existing reporter phages. DST for isoniazid and oxofloxacin, carried out in cultured samples, was complete within 36 h. Use of φ(2)GFP10 detected M. tuberculosis in clinical sputum samples collected from TB patients. DST for rifampin and kanamycin from sputum samples yielded results after 12 h of incubation with φ(2)GFP10. Fluorophage φ(2)GFP10 has potential for clinical development as a rapid, sensitive, and inexpensive point-of-care diagnostic tool for M. tuberculosis infection and for rapid DST.

The Lantibiotic Lacticin 3147 Prevents Systemic Spread of Staphylococcus aureus in a Murine Infection Model

The objective of this study was to investigate the in vivo activity of the lantibiotic lacticin 3147 against the luminescent Staphylococcus aureus strain Xen 29 using a murine model. Female BALB/c mice (7 weeks old, 17 g) were divided into groups (n = 5) and infected with the Xen 29 strain via the intraperitoneal route at a dose of 1 × 10(6) cfu/animal. After 1.5 hr, the animals were treated subcutaneously with doses of phosphate-buffered saline (PBS; negative control) or lacticin 3147. Luminescent imaging was carried 3 and 5 hours postinfection. Mice were then sacrificed, and the levels of S. aureus Xen 29 in the liver, spleen, and kidneys were quantified. Notably, photoluminescence and culture-based analysis both revealed that lacticin 3147 successfully controlled the systemic spread of S. aureus in mice thus indicating that lacticin 3147 has potential as a chemotherapeutic agent for in vivo applications.

Autoluminescent Mycobacterium tuberculosis for rapid, real-time, non-invasive assessment of drug and vaccine efficacy

Preclinical efforts to discover and develop new drugs and vaccines for tuberculosis are hampered by the reliance on colony-forming unit (CFU) counts as primary outcomes for in vivo efficacy studies and the slow growth of Mycobacterium tuberculosis. The utility of bioluminescent M. tuberculosis reporter strains for real-time in vitro and ex vivo assessment of drug and vaccine activity has been demonstrated but a simple, non-invasive, real-time surrogate marker to replace CFU counts for real-time evaluation of drug and vaccine efficacy in vivo has not been described. We describe the development of a fully virulent and stable autoluminescent strain of M. tuberculosis and proof-of-concept experiments demonstrating its utility for in vivo bioluminescence imaging to assess the efficacy of new drugs and vaccines for tuberculosis in a mouse model. Relative light unit (RLU) counts paralleled CFU counts during the active phase of bacterial growth, with a lower limit of detection of approximately 10⁶ CFU in live, anesthetized mice. Experiments distinguishing active from inactive anti-tuberculosis drugs and bacteriostatic drug effects from bactericidal effects were completed in less than 5 days. The ability of a recombinant BCG vaccine to limit bacterial growth was demonstrated within 3 weeks. Use of this autoluminescent reporter strain has the potential to drastically reduce the time, effort, animals and costs consumed in the evaluation of drug activity in vitro and the in vivo assessment of drug and vaccine efficacy.

Rapid measurement of antituberculosis drug activity in vitro and in macrophages using bioluminescence

Objectives

Tuberculosis drug development is hampered by the slow growth of Mycobacterium tuberculosis. Bioluminescence, light produced by an enzymatic reaction, constitutes a rapid and highly sensitive measurement of cell metabolic function that can be used as an indirect marker of cell viability in drug screening assays. The aim of this work was to validate and standardize the use of luminescent M. tuberculosis strains to test the activity of antibacterial drugs in vitro and inside macrophages in a 96-well format.

Methods

We have used strains that express the bacterial lux operon and therefore do not require exogenous substrate to produce light, as well as strains expressing the firefly luciferase that need luciferin substrate. Results were compared with those obtained using the resazurin reduction assay and cfu plating.

Results

Using bioluminescence we were able to reduce the time required to measure the MIC and bactericidal concentrations of antimicrobials to just 3 and 6 days, respectively. Furthermore, antibacterial activity against intracellular mycobacteria was detected within 2 days post-infection. Results were comparable to those obtained by conventional methods.

Conclusions

We have developed a simple and rapid method for screening antimycobacterial drugs in culture and in macrophages. The use of autoluminescent bacteria also facilitates the determination of growth and inhibition kinetics. The method is cost-effective, can easily be adapted to a larger scale and is amenable to automation. Current efforts are directed towards applying this technology to drug screening in vivo.

Lessons from seven decades of antituberculosis drug discovery

Despite massive global efforts tuberculosis rates continue to climb and drug-resistance rates are rising to alarming levels. Discovering new agents for treating this bacterial pathogen poses unique challenges, but these challenges have been faced throughout the entire modern history of research into anti-infectives. This review looks back at every decade since the 1940s and summarizes the most important drugs developed during each decade highlighting the lessons learned during these successful medicinal chemistry programs. Looking forward we must accelerate the integration of these past lessons with the impressive advances that have been made in the basic understanding of the biology of this disease.

Downregulation of Rv0189c, encoding a dihydroxyacid dehydratase, affects growth of Mycobacterium tuberculosis in vitro and in mice

Dihydroxyacid dehydratase (DHAD), a key enzyme involved in branched-chain amino acid (BCAA) biosynthesis, catalyses the synthesis of 2-ketoacids from dihydroxyacids. In Mycobacterium tuberculosis, DHAD is encoded by gene Rv0189c, and it shares 40 % amino acid sequence identity and conserved motifs with DHAD of Escherichia coli encoded by ilvD. In this study, Rv0189c was overexpressed in E. coli and the resultant protein was characterized as a homodimer (∼155 kDa). Functional characterization of Rv0189c was established by biochemical testing and by genetic complementation of an intron-disrupted ilvD-auxotrophic mutant of E. coli to prototrophy. Growth of M. tuberculosis, E. coli BL21(DE3) and recombinant E. coli BL21(DE3) ΔilvD carrying Rv0189c was inhibited by transient nitric oxide (NO) exposure in minimal medium but growth was restored if the medium was supplemented with BCAA (isoleucine, leucine and valine). This suggested that inactivation of Rv0189c by NO probably inhibited bacterial growth. The role of Rv0189c in M. tuberculosis was elucidated by antisense and sense RNA constructs. Growth of M. tuberculosis transformed with a plasmid encoding antisense mRNA was markedly poor in the lungs of infected mice and in Middlebrook 7H9 broth compared to that of sense and vector-alone transformants, but growth was normal when the medium was supplemented with BCAA. Upregulation of Rv0189c was observed during the early exponential phase of growth, under acid stress and ex vivo, suggesting that Rv0189c has a role in the survival of M. tuberculosis during normal and stress conditions. It may be concluded that the DHAD encoded by Rv0189c is essential for the survival of M. tuberculosis and could be a potential drug/vaccine target, as it is absent in mammals.

Reporter enzyme fluorescence (REF) imaging and quantification of tuberculosis in live animals

The slow growth rate of Mycobacterium tuberculosis hinders research progress, since estimating the bacterial numbers present in all experiments normally relies on determination of colony forming units on agar plates. M. tuberculosis colonies can take as long as four to six weeks to become visible. Whole animal imaging is an emerging technology that has broad applications in all areas of biological sciences, including monitoring infections. Imaging allows bacterial numbers to be determined in real-time for each infected animal, individually, which allows inter-animal variability to be observed and controlled for. Reporter enzyme fluorescence (REF) utilizes custom substrates that allow production of a fluorescent product after cleavage by a bacterial enzyme. In our recently published studies, we demonstrate that the enzyme β-lactamase, a naturally occurring enzyme expressed by M. tuberculosis, can be used for REF. The resulting imaging system is the first that allows non-invasive detection of natural M. tuberculosis strains directly in pulmonary infected living animals. Use of REF for M. tuberculosis infected mice allows detection of ~10⁴ CFU in the lungs, which is very sensitive. This system also displays promise for allowing rapid evaluation of differences in virulence strains and efficacy of therapeutics and vaccines. This system could be developed into a diagnostic tool for tuberculosis through the use of REF to identify infected tissues or other diagnostic specimens.

Imaging tuberculosis with endogenous beta-lactamase reporter enzyme fluorescence in live mice

The slow growth rate and genetic intractability of tubercle bacilli has hindered progress toward understanding tuberculosis, one of the most frequent causes of death worldwide. We overcame this roadblock through development of near-infrared (NIR) fluorogenic substrates for beta-lactamase, an enzyme expressed by tubercle bacilli, but not by their eukaryotic hosts, to allow real-time imaging of pulmonary infections and rapid quantification of bacteria in living animals by a strategy called reporter enzyme fluorescence (REF). This strategy has a detection limit of 6 +/- 2 x 10(2) colony-forming units (CFU) of bacteria with the NIR substrate CNIR5 in only 24 h of incubation in vitro, and as few as 10(4) CFU in the lungs of live mice. REF can also be used to differentiate infected from uninfected macrophages by using confocal microscopy and fluorescence activated cell sorting. Mycobacterium tuberculosis and the bacillus Calmette-Guérin can be tracked directly in the lungs of living mice without sacrificing the animals. Therapeutic efficacy can also be evaluated through loss of REF signal within 24 h posttreatment by using in vitro whole-bacteria assays directly in living mice. We expect that rapid quantification of bacteria within tissues of a living host and in the laboratory is potentially transformative for tuberculosis virulence studies, evaluation of therapeutics, and efficacy of vaccine candidates. This is a unique use of an endogenous bacterial enzyme probe to detect and image tubercle bacilli that demonstrates REF is likely to be useful for the study of many bacterial infections.

Mycobacterium tuberculosis CYP125A1, a steroid C27 monooxygenase that detoxifies intracellularly generated cholest-4-en-3-one

The infectivity and persistence of Mycobacterium tuberculosis requires the utilization of host cell cholesterol. We have examined the specific role of cytochrome P450 CYP125A1 in the cholesterol degradation pathway using genetic, biochemical and high-resolution mass spectrometric approaches. The analysis of lipid profiles from cells grown on cholesterol revealed that CYP125A1 is required to incorporate the cholesterol side-chain carbon atoms into cellular lipids, as evidenced by an increase in the mass of the methyl-branched phthiocerol dimycocerosates. We observed that cholesterol-exposed cells lacking CYP125A1 accumulate cholest-4-en-3-one, suggesting that this is a physiological substrate for this enzyme. Reconstitution of enzymatic activity with spinach ferredoxin and ferredoxin reductase revealed that recombinant CYP125A1 indeed binds both cholest-4-en-3-one and cholesterol, efficiently hydroxylates both of them at C-27, and then further oxidizes 27-hydroxycholest-4-en-3-one to cholest-4-en-3-one-27-oic acid. We determined the X-ray structure of cholest-4-en-3-one-bound CYP125A1 at a resolution of 1.58 A. CYP125A1 is essential for growth of CDC1551 in media containing cholesterol or cholest-4-en-3-one. In its absence, the latter compound is toxic for both CDC1551 and H37Rv when added with glycerol as a second carbon source. CYP125A1 is a key enzyme in cholesterol metabolism and plays a crucial role in circumventing the deleterious effect of cholest-4-en-3-one.

Optimisation of bioluminescent reporters for use with mycobacteria

Background

Mycobacterium tuberculosis, the causative agent of tuberculosis, still represents a major public health threat in many countries. Bioluminescence, the production of light by luciferase-catalyzed reactions, is a versatile reporter technology with multiple applications both in vitro and in vivo. In vivo bioluminescence imaging (BLI) represents one of its most outstanding uses by allowing the non-invasive localization of luciferase-expressing cells within a live animal. Despite the extensive use of luminescent reporters in mycobacteria, the resultant luminescent strains have not been fully applied to BLI.

Methodology/Principal Findings

One of the main obstacles to the use of bioluminescence for in vivo imaging is the achievement of reporter protein expression levels high enough to obtain a signal that can be detected externally. Therefore, as a first step in the application of this technology to the study of mycobacterial infection in vivo, we have optimised the use of firefly, Gaussia and bacterial luciferases in mycobacteria using a combination of vectors, promoters, and codon-optimised genes. We report for the first time the functional expression of the whole bacterial lux operon in Mycobacterium tuberculosis and M. smegmatis thus allowing the development of auto-luminescent mycobacteria. We demonstrate that the Gaussia luciferase is secreted from bacterial cells and that this secretion does not require a signal sequence. Finally we prove that the signal produced by recombinant mycobacteria expressing either the firefly or bacterial luciferases can be non-invasively detected in the lungs of infected mice by bioluminescence imaging.

Conclusions/Significance

While much work remains to be done, the finding that both firefly and bacterial luciferases can be detected non-invasively in live mice is an important first step to using these reporters to study the pathogenesis of M. tuberculosis and other mycobacterial species in vivo. Furthermore, the development of auto-luminescent mycobacteria has enormous ramifications for high throughput mycobacterial drug screening assays which are currently carried out either in a destructive manner using LuxAB or the firefly luciferase.

The chemical biology of new drugs in the development for tuberculosis

With the worldwide emergence of multidrug-resistant (MDR) and extensively drug-resistant (XDR) strains of Mycobacterium tuberculosis (Mtb), there are serious concerns about the continued ability to contain this disease. We discuss the most promising new drugs in late-stage development that might be useful in treating MDR and XDR forms of the disease. These agents have novel mechanisms of action that are not targeted by the standard drugs used presently to treat susceptible strains.

Rapid assessment of antibacterial activity against Mycobacterium ulcerans by using recombinant luminescent strains

Mycobacterium ulcerans causes Buruli ulcer, an emerging infectious disease for which antimicrobial therapy has only recently proven to be beneficial. The discovery and development of new drugs against M. ulcerans are severely impeded by its very slow growth. Recombinant bioluminescent strains have proven useful in drug development for other mycobacterial infections, but the ability of such strains to discriminate bacteriostatic from bactericidal activity has not been well demonstrated. We engineered recombinant M. ulcerans strains to express luxAB from Vibrio harveyi. In drug susceptibility tests employing a wide range of antimicrobial agents and concentrations, the relative light unit (RLU) count measured in real time was a reliable surrogate marker for CFU counts available 3 months later, indicating utility for the rapid determination of drug susceptibility and discrimination of bacteriostatic and bactericidal effects. A second important finding of this study is that the addition of subinhibitory concentrations of the ATP-binding cassette transporter inhibitor reserpine increases the susceptibility of M. ulcerans to tetracycline and erythromycin, indicating that drug efflux may explain at least part of the intrinsic resistance of M. ulcerans to these agents.

Real-time fluorescence monitoring of phenothiazinium photosensitizers and their anti-mycobacterial photodynamic activity against Mycobacterium bovis BCG in in vitro and in vivo models of localized infection

An objective was to explore the photodynamic activity of two cationic photosensitizers (PS) (benzo[a]phenothiazinium chloride and benzo[a]phenoselenazinium chloride) against Mycobacterium bovis BCG both in vitro and in a murine model of BCG-granuloma. The hypothesis being tested in this study was that cationic molecules could best interact with the negatively charged membrane of BCG as a model for mycobacterial infection. Cells in culture were incubated with various concentrations of PS and subsequently illuminated using a 635 nm diode laser. Dark- and light-induced killing profiles were generated as a function of fluence and dye concentration. In vivo, local injection of the PS into subcutaneous Mycobacterium-induced granuloma sites in murine model was followed by red light illumination of the same area. A special microscope was fabricated for real-time in vivo fluorescent microscopy to monitor EtNBS delivery to subcutaneous murine granulomata. Both PS demonstrated good in vitro antimycobacterial photodynamic activity with varying degrees of toxicity under dark conditions. Real time in vivo monitoring of benzophenothiazine chloride in the mouse model indicated that this fluorescent photosensitizer was delivered rapidly to the subcutaneous granuloma site. In vivo, photosensitizer specific dark- and photo-toxicities depended on the structure, concentration of the photosensitizer and the light dose utilized. Cationic phenothiazine photosensitizers are promising candidates for use in anti-mycobacterial PDT for localized diseases such as cutaneous and pulmonary granulomata.

Assessment of in vitro immunity to Mycobacterium tuberculosis in a human peripheral blood infection model using a luciferase reporter construct of M. tuberculosis H37Rv

Protective immune responses to tuberculosis in man are primarily cell-mediated and require the interaction of specific T cells, cytokines and activated macrophages. In the present study, Mycobacterium tuberculosis H37Rv labelled with luciferase reporter enzyme was used to analyse the anti-mycobacterial immunity in man using an in vitro whole blood infection model. Peripheral blood samples obtained from M. bovis bacille Calmette-Guérin (BCG)-vaccinated tuberculin-positive healthy volunteers (n = 23) were cultured with M. tuberculosis H37Rv reporter strain. The growth of bacteria in the whole blood cultures was monitored after 48 and 96 h of infection. The results showed that the growth of M. tuberculosis was significantly inhibited after 96 h (P < 0.029) of culture. Among the cytokines studied, interleukin (IL)-10 and IL-12 were not detected at all, whereas low levels of interferon (IFN)-gamma after 96 h (0.4 IU/ml) and tumour necrosis factor (TNF)-alpha after 48 (135 pg/ml) and 96 h (47 pg/ml) of culture were detected in the supernatants of whole blood infected with M. tuberculosis. The magnitude of bacterial growth correlated directly with the concentration of TNF-alpha detected after 48 h (r = 0.722) and 96 h (r = 0.747) of culture (P <or= 0.0001 and P <or= 0.0001, respectively). However, the addition of monoclonal antibodies specific to TNF-alpha and IFN-gamma to the blood cultures did not alter mycobacterial growth indicating the role of other mechanisms/factors in restricting the growth of M. tuberculosis in whole blood cultures.

Rv3303c of Mycobacterium tuberculosis protects tubercle bacilli against oxidative stress in vivo and contributes to virulence in mice

Ability of Mycobacterium tuberculosis to survive under oxidative stress in vivo is an important aspect of pathogenesis. Rv3303c gene from M. tuberculosis encodes an NAD(P)H quinone reductase. These enzymes have been shown to manage oxidative stress in other pathogenic bacteria. We have hypothesized that Rv3303c protein will remove reactive oxygen species released by the host and hence reduce oxidative stress to M. tuberculosis. rv3303c was PCR cloned and the purified recombinant enzyme reduced superoxide generator menadione. Antisense and sense RNA constructs of rv3303c were electroporated in M. tuberculosis H37Rv. The transformants were characterized by difference in expression of specific mRNA and protein. Antisense transformants were markedly reduced in virulence as compared to sense transformants as judged by several parameters such as weight and survival of infected mice, growth in vivo, colonization and histopathology of lungs. In the presence of menadione, the sense transformant was more resistant to killing in vitro than the antisense transformant. It may be concluded that the rv3303c gene contributes to virulence of M. tuberculosis in vivo and this might be mediated in part by increased resistance to reactive oxygen intermediates thereby enhancing intracellular growth and colonization.

Molecular validation of LpxC as an antibacterial drug target in Pseudomonas aeruginosa

LpxC [UDP-3-O-(R-3-hydroxymyristoyl)-GlcNAc deacetylase] is a metalloamidase that catalyzes the first committed step in the biosynthesis of the lipid A component of lipopolysaccharide. A previous study (H. R. Onishi, B. A. Pelak, L. S. Gerckens, L. L. Silver, F. M. Kahan, M. H. Chen, A. A. Patchett, S. M. Galloway, S. A. Hyland, M. S. Anderson, and C. R. H. Raetz, Science 274:980-982, 1996) identified a series of synthetic LpxC-inhibitory molecules that were bactericidal for Escherichia coli. These molecules did not inhibit the growth of Pseudomonas aeruginosa and were therefore not developed further as antibacterial drugs. The inactivity of the LpxC inhibitors for P. aeruginosa raised the possibility that LpxC activity might not be essential for all gram-negative bacteria. By placing the lpxC gene of P. aeruginosa under tight control of an arabinose-inducible promoter, we demonstrated the essentiality of LpxC activity for P. aeruginosa. It was found that compound L-161,240, the most potent inhibitor from the previous study, was active against a P. aeruginosa construct in which the endogenous lpxC gene was inactivated and in which LpxC activity was supplied by the lpxC gene from E. coli. Conversely, an E. coli construct in which growth was dependent on the P. aeruginosa lpxC gene was resistant to the compound. The differential activities of L-161,240 against the two bacterial species are thus the result primarily of greater potency toward the E. coli enzyme rather than of differences in the intrinsic resistance of the bacteria toward antibacterial compounds due to permeability or efflux. These data validate P. aeruginosa LpxC as a target for novel antibiotic drugs and should help direct the design of inhibitors against clinically important gram-negative bacteria.

Photoinactivation of Mycobacteria in vitro and in a new murine model of localized Mycobacterium bovis BCG-induced granulomatous infection

Treatment of tuberculosis is currently hindered by prolonged antibiotic regimens and the emergence of significant drug resistance. Alternatives and adjuncts to standard antimycobacterial agents are needed. We propose that a direct attack utilizing photosensitizers and light-based treatments may be effective in curtailing Mycobacterium tuberculosis in discrete anatomical sites in the most infectious phase of pulmonary tuberculosis. To demonstrate experimental proof of principle, we have applied established photodynamic therapy (PDT) technology to in vitro cultures and an in vivo mouse model using Mycobacterium bovis BCG. We report here in vitro and in vivo PDT efficacy studies and the use of a three-dimensional collagen gel as a delivery vehicle for BCG, subcutaneously inserted, to induce specifically localized granuloma-like lesions in mice. When a benzoporphyrin derivative was utilized as the photosensitive agent, exposure to light killed extracellular and intracellular BCG in significant numbers. Collagen scaffolds containing BCG inserted in situ in BALB/c mice for 3 months mimicked granulomatous lesions and demonstrated a marked cellular infiltration upon histological examination, with evidence of caseating necrosis and fibrous capsule formation. When 10(5) BCG were present in the in vivo-induced granulomas, a significant reduction in viable mycobacterial cells was demonstrated in PDT-treated granulomas compared to those of controls. We conclude that PDT has potential in the treatment of localized mycobacterial infections, such as pulmonary granulomas and cavities.

Reconstitution of antimycobacterial immune responses in HIV-infected children receiving HAART

OBJECTIVE

Recent epidemiological studies in adults suggest that HAART can prevent the development of tuberculosis in HIV-infected individuals, but the mechanisms are incompletely understood and no data exist in children. We investigated whether changes in mycobacterial-specific immune responses can be demonstrated in children after commencing antiretroviral therapy.

DESIGN

We measured mycobacterial growth in vitro using a novel whole-blood assay employing reporter-gene tagged bacillus Calmette-Guérin (BCG) in a prospective cohort study in the tuberculosis-endemic environment of South Africa. Key cytokines were measured in supernatants collected from the whole-blood assay using cytometric bead array.

PATIENTS

A cohort of 15 BCG-vaccinated HIV-infected children was evaluated prospectively for in-vitro antimycobacterial immune responses before and during the first year of HAART. All children had advanced HIV disease. Nine children completed all study timepoints.

RESULTS

Before HAART, blood from children showed limited ability to restrict the growth of mycobacteria in the functional whole-blood assay. The introduction of HAART was followed by rapid and sustained reconstitution of specific antimycobacterial immune responses, measured as the decreased growth of mycobacteria. IFN-gamma levels in culture supernatants did not reflect this response; however, a decline in TNF-alpha was observed.

CONCLUSION

This is the first study using a functional in-vitro assay to assess the effect of HAART on immune responses to mycobacteria in HIV-infected children. Our in-vitro data mirror the in-vivo observation of decreased susceptibility to tuberculosis in HIV-infected adults receiving antiretroviral agents. This model may be useful for further characterizing immune reconstitution after HAART.

Bioluminescence imaging

Bioluminescence refers to the process of visible light emission in living organisms. Bioluminescence imaging is a powerful methodology that has been developed over the last decade as a tool for molecular imaging of small laboratory animals, enabling the study of ongoing biological processes in vivo. This form of optical imaging is low cost and noninvasive and facilitates real-time analysis of disease processes at the molecular level in living organisms. In this article, we provide a brief introduction to bioluminescence imaging technology and discuss its applications in mouse models of lung inflammation/injury, bacterial pneumonia, and tumor growth and metastasis.

Alternative luciferase for monitoring bacterial cells under adverse conditions

The availability of cloned luciferase genes from fireflies (luc) and from bacteria (luxAB) has led to the widespread use of bioluminescence as a reporter to measure cell viability and gene expression. The most commonly occurring bioluminescence system in nature is the deep-sea imidazolopyrazine bioluminescence system. Coelenterazine is an imidazolopyrazine derivative which, when oxidized by an appropriate luciferase enzyme, produces carbon dioxide, coelenteramide, and light. The luciferase from the marine copepod Gaussia princeps (Gluc) has recently been cloned. We expressed the Gluc gene in Mycobacterium smegmatis using a shuttle vector and compared its performance with that of an existing luxAB reporter. In contrast to luxAB, the Gluc luciferase retained its luminescence output in the stationary phase of growth and exhibited enhanced stability during exposure to low pH, hydrogen peroxide, and high temperature. The work presented here demonstrated the utility of the copepod luciferase bioluminescent reporter as an alternative to bacterial luciferase, particularly for monitoring responses to environmental stress stimuli.

Novel human in vitro system for evaluating antimycobacterial vaccines

Major research efforts are directed towards the development of a better antimycobacterial vaccine. But progress in the field of tuberculosis vaccine development has been hampered by the lack of human in vitro models to assess vaccine immunogenicity and efficacy. New candidate vaccines will have to be evaluated against the existing Mycobacterium bovis BCG "gold standard." It is therefore important to understand the type of immune responses elicited by BCG vaccination to enable comparisons with potential new candidates. We used a novel human in vitro whole-blood model, which measures immune responses to mycobacteria by use of reporter gene-tagged BCG (BCG lux), to study immune responses to BCG vaccination in 50 neonates in a setting in Cape Town, Republic of South Africa, where tuberculosis is endemic. BCG vaccination significantly reduced growth of BCG lux in whole blood (prevaccination median growth ratio [GR], 9.6; range, 1.3 to 24; postvaccination median GR, 3.9; range, 0.6 to 12.2 [P < 0.0001]). Growth of BCG lux was better restricted in vaccinated infants than in unvaccinated age-matched controls (n = 4). BCG vaccination induced significantly higher gamma interferon production in response to BCG lux (P < 0.0001) and to purified protein derivative (P = 0.0001). No significant changes in either growth of BCG lux or cytokine production occurred in an adult control group (n = 6) over the study period. The whole-blood luminescence model detects changes in cellular immune responses to mycobacteria induced by BCG vaccination. It is therefore a useful new tool in studying the immunogenicity of newly developed vaccine candidates prior to large field trials assessing efficacy.

Rapid direct method for monitoring antibiotics in a mouse model of bacterial biofilm infection

We have developed a rapid, continuous method for monitoring the effectiveness of several antibacterial agents in real time, noninvasively, by using a recently described mouse model of chronic biofilm infection (J. L. Kadurugamuwa et al., Infect. Immun. 71:882-890, 2003), which relies on biophotonic imaging of bioluminescent bacteria. To facilitate real-time monitoring of infection, we used a Staphylococcus aureus isolate that was made bioluminescent by inserting a modified lux operon into the bacterial chromosome. This bioluminescent reporter bacterium was used to study the antimicrobial effects of several antibiotics belonging to different molecular families. Treatment with rifampin, tobramycin, and ciprofloxacin was started 7 days after subcutaneous implantation of catheters precolonized with 10(4) CFU of S. aureus. Three different doses of antibiotics were administered twice a day for 4 consecutive days. The number of metabolically active bacteria in untreated mice and the tobramycin- and ciprofloxacin-treated groups remained relatively unchanged over the 4-week observation period, indicating poor efficacies for tobramycin and ciprofloxacin. A rapid dose-dependent decline in metabolic activity in rifampin-treated groups was observed, with almost a 90% reduction after two doses and nearly undetectable levels after three doses. The disappearance of light emission correlated with colony counts. After the final treatment, cell numbers rebounded as a function of concentration in a time-dependent manner. The staphylococci isolated from the catheters of mice treated with rifampin were uniformly resistant to rifampin but retained their in vitro susceptibilities to tobramycin and ciprofloxacin. Since the metabolic activities of viable cells and a postantibiotic effect could be detected directly on the support matrix nondestructively and noninvasively, the methodology is specifically appealing for investigating the effects of antibiotics on biofilms in vivo. Moreover, our study points to the possible use of biophotonic imaging for the detection of the development of resistance to therapeutic agents during treatment of chronic infections in vivo.

Bioluminescent detection of endotoxin effects on HIV-1 LTR-driven transcription in vivo

We investigated the effects of Gram-negative bacterial lipopolysaccharide (LPS) on luciferase expression in transgenic reporter mice in which luciferase expression is driven by the nuclear factor kappaB (NF-kappaB)-dependent portion of the human immunodeficiency virus-1 (HIV-1) long terminal repeat (HIV-1 LTR). Using these mice, we dissected the sources of luciferase activity at the organ level by (a) assessing luciferase activity in organ homogenates, (b) bioluminescence imaging in vivo, and (c) bioluminescence imaging of individual organs ex vivo. Luciferin dosage was a critical determinant of the magnitude of photon emission from these reporter mice. Photon emission increased at doses from 0.5-6 mg of luciferin given by intraperitoneal (IP) injection. The differential between basal and LPS-induced bioluminescence was maximal at 3-6 mg of luciferin. Luciferase expression was highly inducible in lungs, liver, spleen, and kidneys after a single IP injection of LPS, as assessed by luciferase activity measurements in organ homogenates. Luciferase activity was also induced in the forebrain by treatment with IP LPS. In contrast, aerosolized LPS produced a response localized to the lungs as assessed by both bioluminescence and ex vivo luciferase assay measurements. These studies demonstrate the utility of luciferase reporter mice for determining organ-specific gene expression in response to local and systemic stimuli.

Combinatorial lead optimization of [1,2]-diamines based on ethambutol as potential antituberculosis preclinical candidates

Despite relatively modest potency, ethambutol (EMB, (S,S)-[N,N-di-2-amino-1-butanol]ethylenediamine) is a mainstay of contemporary chemotherapy for the treatment of tuberculosis. We have developed a solid-phase synthesis of 1,2-diamine analogues of EMB using a novel acylation-reduction sequence that is compatible with high-throughput 96-well format chemistry. Using this procedure, we have synthesized 63 238 diamine analogues in pools of 10 that are suitable for testing. MIC and a target-based reporter assay were used to direct deconvolution of 2796 individual compounds from these mixtures, and the 69 most potent molecules were resynthesized in milligram quantities for hit confirmation. Purification of these individual active diamine analogues allowed the identification of 26 compounds with activity equal to or greater than EMB. Amines which occurred most frequently in active compounds included many with large hydrophobic moieties, suggesting that optimization was perhaps selecting for the isoprenoid binding site of the arabinosyltransferase target of EMB. N-Geranyl-N'-(2-adamantyl)ethane-1,2-diamine (109), the most active of these diamines, displayed a 14-35-fold improvement in activity in vitro against Mycobacterium tuberculosis, as compared to EMB.

Construction and immunogenicity of recombinant Mycobacterium bovis BCG expressing GP5 and M protein of porcine reproductive respiratory syndrome virus

Mycobacterium bovis BCG was used to express a truncated form of GP5 (lacking the first 30 NH(2)-terminal residues) and M protein of porcine reproductive and respiratory syndrome virus (PRRSV). The PRRSV proteins were expressed in BCG under control of the mycobacterial hsp60 gene promoter either in the mycobacterial cytoplasm (BCGGP5cyt and BCGMcyt) or as MT19-fusion proteins on the mycobacterial surface (BCGGP5surf and BCGMsurf). Mice inoculated with BCGGP5surf and BCGMsurf developed antibodies against the viral proteins at 30 days post-inoculation (dpi) as detected by ELISA and Western blot. By 60 dpi, the animals developed titer of neutralizing antibodies of 8. A PRRSV-specific gamma interferon response was also detected in splenocytes of recombinant BCG-inoculated mice at 60 and 90 dpi. These results indicate that BCG was able to express antigens of PRRSV and elicit an immune response against the viral proteins in mice.

Novel Saccharomyces cerevisiae screen identifies WR99210 analogues that inhibit Mycobacterium tuberculosis dihydrofolate reductase

The ongoing selection of multidrug-resistant strains of Mycobacterium tuberculosis has markedly reduced the effectiveness of the standard treatment regimens. Thus, there is an urgent need for new drugs that are potent inhibitors of M. tuberculosis, that exhibit favorable resistance profiles, and that are well tolerated by patients. One promising drug target for treatment of mycobacterial infections is dihydrofolate reductase (DHFR; EC 1.5.1.3), a key enzyme in folate utilization. DHFR is an important drug target in many pathogens, but it has not been exploited in the search for drugs effective against M. tuberculosis. The triazine DHFR inhibitor WR99210 has been shown to be effective against other mycobacteria. We show here that WR99210 is also a potent inhibitor of M. tuberculosis and Mycobacterium bovis BCG growth in vitro and that resistance to WR99210 occurred less frequently than resistance to either rifampin or isoniazid. Screening of drugs with M. tuberculosis cultures is slow and requires biosafety level 3 facilities and procedures. We have developed an alternative strategy: initial screening in an engineered strain of the budding yeast Saccharomyces cerevisiae that is dependent on the M. tuberculosis DHFR for its growth. Using this system, we have screened 19 compounds related to WR99210 and found that 7 of these related compounds are also potent inhibitors of the M. tuberculosis DHFR. These studies suggest that compounds of this class are excellent potential leads for further development of drugs effective against M. tuberculosis.

Simple fibroblast-based assay for screening of new antimicrobial drugs against Mycobacterium tuberculosis

In this study, we propose a simple and reproducible host-cell-based assay for the screening of antimycobacterial drugs that is suitable for drug discovery. The method evaluates both antimycobacterial activity of the drugs and their cytotoxicity to host cells. The basis of this simple fibroblast-based assay (SFA) is that cells of human lung fibroblast cell line MRC-5, which are highly sensitive to mycobacterial cytotoxicity, are killed by virulent Mycobacterium tuberculosis strain H(37)Rv bacilli in response to the viability of bacilli. Clinically used antimycobacterial drugs inhibited the mycobacterial cytotoxicity to MRC-5 cells in a dose-dependent manner. MICs of isoniazid, streptomycin, rifampin, and ethambutol determined by this SFA (0.428, 1.816, 0.013, and 3.465 microg/ml, respectively) were within 1 log of MICs determined by the broth dilution test (BDT) using Middlebrook 7H9 medium. The MIC of pyrazinamide, which exhibits bactericidal activity only at a high dose by BDT (1,231 microg/ml at pH 6.6 and 492 microg/ml at pH 5.8), was 3.847 microg/ml in the modified method of SFA. On the other hand, sodium azide, a toxic agent for both mammalian cells and bacteria, exhibited cytotoxicity to fibroblasts at a dose lower than that required to inhibit mycobacterial growth. Thus, this fibroblast-based method enabled us to evaluate both antibacterial activity of drugs and their cytotoxicity to human cells within a short period of time.

In vivo monitoring of intracellular ATP levels in Leishmania donovani promastigotes as a rapid method to screen drugs targeting bioenergetic metabolism

A method for the rapid screening of drugs targeting the bioenergetic metabolism of Leishmania spp. was developed. The system is based on the monitoring of changes in the intracellular ATP levels of Leishmania donovani promastigotes that occur in vivo, as assessed by the luminescence produced by parasites transfected with a cytoplasmic form of Phothinus pyralis luciferase and incubated with free-membrane permeable D-luciferin analogue D-luciferin-[1-(4,5-dimethoxy-2-nitrophenyl) ethyl ester]. A significant correlation was obtained between the rapid inhibition of luminescence with parasite proliferation and the dissipation of changes in mitochondrial membrane potential (DeltaPsi(m)) produced by buparvaquone or plumbagin, two leishmanicidal inhibitors of oxidative phosphorylation. To further validate this test, a screen of 14 standard leishmanicidal drugs, using a 50 microM cutoff, was carried out. Despite its semiquantitative properties and restriction to the promastigote stage, this test compares favorably with other bioenergetic parameters with respect to time and cell number requirements for the screening of drugs that affect mitochondrial activity.

In vivo imaging of gene expression:

With the ability to readily engineer genes, create knock-in and knock-out models of human disease, and replace and insert genes in clinical trials of gene therapy, it has become clear that imaging will play a critical role in these fields. Imaging is particularly helpful in recording temporal and spatial resolution of gene expression in vivo, determining vector distribution, and, ultimately, understanding endogenous gene expression during disease development. While endeavors are under way to image targets ranging from DNA to entire phenotypes in vivo, this short review focuses on in vivo imaging of gene expression with magnetic resonance and optical techniques.

Bioluminescent Mycobacterium aurum expressing firefly luciferase for rapid and high throughput screening of antimycobacterial drugs in vitro and in infected macrophages

The slow growth and highly infectious nature of Mycobacterium tuberculosis is a limiting factor in its use as test organism in high throughput screening for inhibitory compounds. To overcome these problems, use of surrogate strains and reporter genes have been considered. In this study, we have investigated the application of a fast growing nonpathogenic M. aurum expressing firefly luciferase in rapid screening of antituberculosis compounds in vitro and in infected macrophages using bioluminescence assay. The assay is based on luminescence determination using luciferin as substrate. Inhibition of bioluminescence was obtained with frontline antimycobacterial drugs like streptomycin, rifampicin, isoniazid, ethambutol, ofloxacin, and sparfloxacin at their reported MICs. Inhibition could be observed as early as 2 h in vitro and within 24 h in infected macrophages. The system can reliably be used in high throughput screening.

A small-molecule nitroimidazopyran drug candidate for the treatment of tuberculosis

Mycobacterium tuberculosis, which causes tuberculosis, is the greatest single infectious cause of mortality worldwide, killing roughly two million people annually. Estimates indicate that one-third of the world population is infected with latent M. tuberculosis. The synergy between tuberculosis and the AIDS epidemic, and the surge of multidrug-resistant clinical isolates of M. tuberculosis have reaffirmed tuberculosis as a primary public health threat. However, new antitubercular drugs with new mechanisms of action have not been developed in over thirty years. Here we report a series of compounds containing a nitroimidazopyran nucleus that possess antitubercular activity. After activation by a mechanism dependent on M. tuberculosis F420 cofactor, nitroimidazopyrans inhibited the synthesis of protein and cell wall lipid. In contrast to current antitubercular drugs, nitroimidazopyrans exhibited bactericidal activity against both replicating and static M. tuberculosis. Lead compound PA-824 showed potent bactericidal activity against multidrugresistant M. tuberculosis and promising oral activity in animal infection models. We conclude that nitroimidazopyrans offer the practical qualities of a small molecule with the potential for the treatment of tuberculosis.

Current tuberculosis vaccine development

Information derived from the complete genome sequence of Mycobacterium tuberculosis makes it possible to develop a range of new vaccine candidates. Strategies currently under investigation include construction of whole cell live attenuated mycobacterial vaccines, as well as the use of individual antigens delivered by a variety of subunit vaccination procedures. Fundamental questions associated with the rational design, preclinical testing, and future application of new tuberculosis vaccines are reviewed.