Intravital microscopy enables direct visualization of dynamic cellular processes within intact tissues, but its application to Mycobacterium tuberculosis (Mtb) has been limited by Biosafety Level 3 (BSL-3) containment requirements and the technical challenges of stabilizing the lung for high-resolution imaging. Here, we present a protocol that combines the thoracic Window for High-Resolution Imaging of the murine Lung (WHRIL) with a genetically defined, triple-auxotrophic Mtb strain (mc 2 7902) approved for use under BSL-2 conditions. We describe the construction of a tdTomato-expressing derivative (mc 2 8471) preparation of bacteria for intravenous infection and intravital imaging in reporter mice. This system enables visualization of rapid bacterial entry into the pulmonary vasculature, subsequent aggregation, and vascular occlusion, dissemination into the lung parenchyma, and macrophage uptake over three days post-infection. This protocol provides the first practical platform for real-time intravital imaging of mycobacteria in the lung and establishes a foundation for mechanistic studies of bacterial physiology, host recognition, and immune-mediated clearance using safe Mtb surrogates. Summary:This protocol describes a biosafety level 2 (BSL-2)-compatible intravital imaging platform for visualizing Mycobacterium tuberculosis (Mtb) in the intact murine lung at single cell resolution. By combining the Window for High-Resolution Imaging of the murine Lung (WHRIL) with a fluorescently labeled, genetically defined triple auxotrophic Mtb strain (mc 2 7902), this approach overcomes long-standing biosafety and technical barriers that have prevented real-time imaging of mycobacterial infection in vivo. The method enables direct visualization of early bacterial localization, aggregation, vascular interactions, and macrophage uptake during the initial hours to days following infection, providing a practical foundation for mechanistic studies of host-pathogen interactions under safe experimental conditions.

Intravital microscopy enables direct visualization of dynamic cellular processes within intact tissues. However, its application to Mycobacterium tuberculosis (Mtb) has been limited by Biosafety Level 3 (BSL-3) containment requirements and the technical challenges of stabilizing the lung for high-resolution imaging. Here, we present a protocol that combines the thoracic Window for High-Resolution Imaging of the murine Lung (WHRIL) with mc27902, a genetically defined triple-auxotrophic Mtb strain approved for use under BSL-2 conditions. We also describe the generation of mc28471, a tdTomato-expressing derivative of mc27902, its preparation for intravenous infection, and application for intravital imaging in reporter mice. This system enables real-time visualization of early infection dynamics, including rapid bacillary entry into the pulmonary vasculature, aggregation, dissemination into the lung parenchyma, and macrophage uptake in the same lung microenvironment over the first 3 days post-infection. By enabling longitudinal imaging of bacilli and host-pathogen interactions at single-cell resolution, this approach overcomes key limitations associated with conventional imaging and high-containment models. This protocol provides a practical BSL-2-compatible platform for real-time intravital imaging of attenuated mycobacteria in the lung and establishes a foundation for mechanistic studies of bacterial physiology, host recognition, and immune-mediated clearance.

The ESX-5 secretion system, uniquely found in slow-growing mycobacteria, is predicted to secrete over 150 proteins across the inner membrane of Mycobacterium tuberculosis (M.tb). Although many of these substrates are believed to promote M.tb virulence, most remain poorly characterized. Here, we use a complete locus deletion strain of ESX-5 in M.tb to examine the molecular changes caused by a broad loss in ESX-5 secretory substrates. We confirmed the selective loss of PE/PPE proteins secreted by ESX-5 into both the culture filtrate (CF) and outer mycomembrane (OMM) fractions of the M.tb ∆esx5 mutant. In examining other ESX systems, we found that ESX-1 substrate levels were increased in both the CF and OMM fractions of the ∆esx5 mutant. Conversely, the ESX-3 locus was transcriptionally repressed upon ESX-5 deletion. We noted that the ∆esx5 mutant had altered morphology in the form of wrinkled distortions of the bacterial surface. Likewise, we identified increased susceptibility of the ∆esx5 mutant to a variety of large (molecular weight >550 g/mol) antimicrobial compounds, suggesting that an intact ESX-5 system is required for M.tb to exclude such molecules. Our findings suggest that removing the ESX-5 system from M.tb fundamentally alters the properties of the mycobacterial OMM and impacts the expression and secretion activity of other ESX systems.

Tuberculosis (TB), a disease caused by the bacterium Mycobacterium tuberculosis (Mtb), continues to pose a major global health threat, exacerbated by the emergence of drug-resistant strains and the lengthy treatment regimens required for effective management. Bedaquiline (BDQ), a key component in novel regimens for multidrug-resistant (MDR) TB, has demonstrated significant efficacy but is threatened by rising resistance. Our study investigates the potential of vitamin C to enhance BDQ's activity and prevent resistance. We found that combining BDQ with vitamin C sterilized drug-susceptible and MDR Mtb cultures in vitro within 21 days, achieving a 6-log reduction in colony-forming units. This combination also enhanced Mtb killing in infected human macrophages and peripheral blood mononuclear cells. Transcriptomic analysis revealed that the BDQ/vitamin C combination induces widespread metabolic disruption in Mtb, characterized by upregulation of stress response and metal ion homeostasis genes and downregulation of energy metabolism and cell wall biosynthesis genes. Mechanistic studies implicated reactive oxygen species and disrupted copper homeostasis as contributing factors to the sterilization effect. These findings highlight the potential of using vitamin C as an adjunct therapy with BDQ, offering a promising strategy to enhance drug efficacy and mitigate emerging drug resistance during MDR-TB treatment.IMPORTANCETuberculosis (TB) remains a major global health problem, especially as drug-resistant forms become more common and harder to treat. Bedaquiline is one of the most important new drugs for treating these resistant infections, but resistance to bedaquiline is also starting to appear. This study found that the combination of vitamin C and bedaquiline sterilizes Mycobacterium tuberculosis cultures in vitro while potentiating bedaquiline activity in infected human macrophage cells. The combination appears to overwhelm the bacteria by creating stress and disrupting essential functions, like energy production and metal balance. These results suggest that vitamin C, a safe and inexpensive supplement, could be used alongside existing drugs to make treatment faster and more effective while also helping to prevent resistance.

Mycobacterium leprae, the causative agent of leprosy, has never been cultured in vitro, posing significant challenges for genetic manipulation and drug discovery. Current antileprosy drug screening methods relying on microscopic count, radiorespirometry, and qPCR are time consuming and require the use of radioactive elements. We demonstrate mycobacteriophage-mediated introduction of foreign DNA using the broad-host range mycobacteriophage TM4 and the application of the luciferase reporter mycobacteriophage (LRM) for drug screening. Mycobacteriophage infection of M. leprae was shown using TM4 expressing the highly sensitive BRET-nanoluciferase-based reporter, GeNL (TM4::GeNL), which enables luminescence measurement for over 72 h. When M. leprae was exposed to rifampicin, dapsone, and Q203 for 24 and 48 h, followed by TM4::GeNL infection, the luminescence output decreased in a dose-dependent manner, establishing an in vitro two-day screening assay for drugs. We have also electroporated M. leprae with a ColE1-integration proficient plasmid expressing GeNL and shown that the transformed leprosy bacilli could be propagated in mice footpads and detected using an in vivo imaging system (IVIS). These findings introduce powerful genetic tools for M. leprae research enabling in vivo tracking and in vitro viability testing.

Tuberculosis, caused by Mycobacterium tuberculosis ( Mtb ), remains a leading infectious cause of mortality worldwide despite widespread use of the BCG vaccine and the availability of sterilizing pharmacopoeia. Recent research indicates that the intravenous administration of BCG confers sterilizing immunity against Mtb pulmonary challenge in non-human primates. However, while BCG is relatively safe, complications such as disseminated BCGosis have been observed in immunocompromised individuals. Double auxotrophic mutants of Mtb lacking the ability to synthesize leucine and pantothenate are safe and sterilized in immunocompromised mice and SIV-infected Rhesus macaques. We examined how immunization with a Mtb triple auxotrophic strain, mc 2 7902, which cannot synthesize leucine, pantothenate, and arginine, protects immunocompetent mice from a virulent Mtb infection. The route of immunization was a crucial factor for protection with mc 2 7902 with intravenous immunization being 100 times more effective in protecting immunocompetent mice from Mtb challenge when compared to conventional subcutaneous vaccination with BCG. To further increase the safety of the attenuated auxotroph for vaccine purposes, the type VII secretion system Esx1 responsible for BCG attenuation was deleted in mc 2 7902. When tested by prime-boost immunization of immunocompetent mice followed by aerosol challenge with virulent Mtb , mc 2 7902 Δ esx1 provided similar protection to mc 2 7902. This robust protection against Mtb infection conferred by mc 2 7902 and mc 2 7902 Δ esx1 in a mouse model paves the way for new TB vaccine development using highly attenuated, auxotrophic Mtb strains.

Phthiocerol dimycocerosate (PDIM) is an essential virulence lipid of Mycobacterium tuberculosis. In vitro culturing rapidly selects for spontaneous PDIM-negative mutants that have attenuated virulence and increased cell wall permeability, thus impacting the relevance of experimental findings. PDIM loss can also reduce the efficacy of the BCG Pasteur vaccine. Here we show that vancomycin susceptibility can rapidly screen for M. tuberculosis PDIM production. We find that metabolic deficiency of methylmalonyl-CoA impedes the growth of PDIM-producing bacilli, selecting for PDIM-negative variants. Supplementation with odd-chain fatty acids, cholesterol or vitamin B12 restores PDIM-positive bacterial growth. Specifically, we show that propionate supplementation enhances PDIM-producing bacterial growth and selects against PDIM-negative mutants, analogous to in vivo conditions. Our study provides a simple approach to screen for and maintain PDIM production, and reveals how discrepancies between the host and in vitro nutrient environments can attenuate bacterial pathogenicity.

Results from clinical strains and knockouts of the H37Rv and CDC1551 laboratory strains demonstrated that ndh (Rv1854c) is not a resistance-conferring gene for isoniazid, ethionamide, delamanid, or pretomanid in Mycobacterium tuberculosis. This difference in the susceptibility to NAD-adduct-forming drugs compared with other mycobacteria may be driven by differences in the absolute intrabacterial NADH concentration.

Background Drug-resistant tuberculosis is a growing public health threat, and early characterization of the resistance phenotype is essential for guiding treatment and mitigating the high mortality associated with the disease. However, the slow growth rate of Mycobacterium tuberculosis, the causative agent of tuberculosis, necessitates several weeks for conventional culture-dependent drug susceptibility testing (DST). In addition, there are no widely available molecular diagnostic assays for evaluating resistance to newer tuberculosis drugs or drugs with complex resistance mechanisms.Methods We have developed a luciferase-based reporter mycobacteriophage assay that can determine drug resistance within 48 hours. We engineered the TM4 mycobacteriophage to express green enhanced nanoluciferase (GeNL) cassette and optimized DST for bedaquiline, pretomanid, linezolid, clofazimine, and rifampicin using clinical M. tuberculosis isolates.Results To assess the feasibility of this assay, we conducted a proof-of-principle study using 53 clinical M. tuberculosis isolates. TM4::GeNL phage DST effectively distinguished between sensitive and resistant isolates for bedaquiline and rifampicin at a concentration of 0.125 mu g/mL. Optimal differentiation between sensitive and resistant isolates for pretomanid, clofazimine, and linezolid was achieved at concentrations of 0.5 mu g/mL, 0.25 mu g/mL, and 1 mu g/mL, respectively. Additionally, TM4::GeNL DST identified low-level rifampicin resistance in clinical isolates even though they were classified as sensitive by Mycobacteria Growth Indicator Tube DST.Conclusions TM4::GeNL reporter phage DST offers a rapid method to identify M. tuberculosis drug resistance, including resistance to newer tuberculosis drugs. We present a rapid, phage-based phenotypic drug susceptibility testing method for Mycobacterium tuberculosis. This method enables the determination of susceptibility to drugs in BPaL regimen within 48 hours, making it potentially valuable for implementation in low- and middle-income countries.

Phthiocerol dimycocerosate (PDIM) is an essential virulence lipid of Mycobacterium tuberculosis . In vitro culturing rapidly selects for spontaneous mutations that cause PDIM loss leading to virulence attenuation and increased cell wall permeability. We discovered that PDIM loss is due to a metabolic deficiency of methylmalonyl-CoA that impedes the growth of PDIM-producing bacilli. This can be remedied by supplementation with odd-chain fatty acids, cholesterol, or vitamin B 12 . We developed a much-needed facile and scalable routine assay for PDIM production and show that propionate supplementation enhances the growth of PDIM-producing bacilli and selects against PDIM-negative mutants, analogous to in vivo conditions. Our results solve a major issue in tuberculosis research and exemplify how discrepancies between the host and in vitro nutrient environments can attenuate bacterial pathogenicity.

Mycobacterium tuberculosis variant bovis (MBO) has one of the widest known mammalian host ranges, including humans. Despite the characterization of this pathogen in the 1800s and whole genome sequencing of a UK strain (AF2122) nearly two decades ago, the basis of its host specificity and pathogenicity remains poorly understood. Recent experimental calf infection studies show that MBO strain Ravenel (MBO Ravenel) is attenuated in the cattle host compared to other pathogenic strains of MBO. In the present study, experimental infections were performed to define attenuation. Whole genome sequencing was completed to identify regions of differences (RD) and single nucleotide polymorphisms (SNPs) to explain the observed attenuation. Comparative genomic analysis of MBO Ravenel against three pathogenic strains of MBO (strains AF2122-97, 10-7428, and 95-1315) was performed. Experimental infection studies on five calves each, with either MBO Ravenel or 95-1315, revealed no visible lesions in all five animals in the Ravenel group despite robust IFN-γ responses. Out of 486 polymorphisms in the present analysis, 173 were unique to MBO Ravenel among the strains compared. A high-confidence subset of nine unique SNPs were missense mutations in genes with annotated functions impacting two major MBO survival and virulence pathways: (1) Cell wall synthesis & transport [espH (A103T), mmpL8 (V888I), aftB (H484Y), eccC5 (T507M), rpfB (E263G)], and (2) Lipid metabolism & respiration [mycP1(T125I), pks5 (G455S), fadD29 (N231S), fadE29 (V360G)]. These substitutions likely contribute to the observed attenuation. Results from experimental calf infections and the functional attributions of polymorphic loci on the genome of MBO Ravenel provide new insights into the strain’s genotype-disease phenotype associations.

As the goal of a bacterium is to become bacteria, evolution has imposed continued selections for gene expression. The intracellular pathogen Mycobacterium tuberculosis, the causative agent of tuberculosis, has adopted a fine-tuned response to survive its host’s methods to aggressively eradicate invaders. The development of microarrays and later RNA sequencing has led to a better understanding of biological processes controlling the relationship between host and pathogens. In this study, RNA-seq was performed to detail the transcriptomes of M. tuberculosis grown in various conditions related to stresses endured by M. tuberculosis during host infection and to delineate a general stress response incurring during persisting macrophage stresses. M. tuberculosis was subjected to long-term growth, nutrient starvation, hypoxic and acidic environments. The commonalities between these stresses point to M. tuberculosis maneuvering to exploit propionate metabolism for lipid synthesis or to withstand propionate toxicity whilst in the intracellular environment. While nearly all stresses led to a general shutdown of most biological processes, up-regulation of pathways involved in the synthesis of amino acids, cofactors, and lipids were observed only in hypoxic M. tuberculosis. This data reveals genes and gene cohorts that are specifically or exclusively induced during all of these persisting stresses. Such knowledge could be used to design novel drug targets or to define possible M. tuberculosis vulnerabilities for vaccine development. Furthermore, the disruption of specific functions from this gene set will enhance our understanding of the evolutionary forces that have caused the tubercle bacillus to be a highly successful pathogen.

N-acetylcysteine (NAC) is most commonly used for the treatment of acetaminophen overdose and acetaminophen-induced liver injury. In patients infected with Mycobacterium tuberculosis, the causative agent of tuberculosis (TB), NAC is given to treat hepatotoxicity induced by TB drugs. We had previously shown that cysteine, a derivative of NAC, potentiated the activity of isoniazid, a first-line TB drug, by preventing the emergence of INH resistance and persistence in M. tuberculosis in vitro. Herein, we demonstrate that in vitro, NAC has the same boosting activity with various combinations of first- and second-line TB drugs against drug-susceptible and multidrug-resistant M. tuberculosis strains. Similar to cysteine, NAC increased M. tuberculosis respiration. However, in M. tuberculosis-infected mice, the addition of NAC did not augment the activity of first- or second-line TB drugs. A comparison of the activity of NAC combined with TB drugs in murine and human macrophage cell lines revealed that studies in mice might not be recapitulated during host infection in vivo.

Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis, can enter into a persistent state that confers resistance to antibacterial agents. Many observations suggest that persistent M. tuberculosis cells also evade the antimycobacterial immune mechanisms, thereby reducing the effectiveness of the current tuberculosis vaccine. Understanding the factors that contribute to persistence may enable the rational design of vaccines that stimulate effective immune killing mechanisms against persister cells. Independent mutations targeting the methionine and arginine biosynthetic pathways are bactericidal for M. tuberculosis in mice. However, in this study, we discovered that the addition of leucine and pantothenate auxotrophy altered the bactericidality of methionine auxotrophy. Whereas the leucine/pantothenate/methionine auxotrophic M. tuberculosis strain H37Rv ΔleuCD ΔpanCD ΔmetA was eliminated in immunocompetent mice, this strain persisted in multiple organs of immunodeficient Rag1-/- mice for at least a year. In contrast, the leucine/pantothenate/arginine auxotroph H37Rv ΔleuCD ΔpanCD ΔargB was eliminated in both immunocompetent and immunodeficient Rag1-/- mice. Our results showed that leucine and pantothenate starvation metabolically blocked the sterilization mechanisms of methionine starvation but not those of arginine starvation. These triple-auxotrophic strains should be invaluable tools for unravelling the bacterial and host factors that enable persistence and for vaccine development studies to assess the efficacy of vaccines that boost immune recognition of M. tuberculosis in the persistent state. The sterilization of the ΔleuCD ΔpanCD ΔmetA auxotroph in immunocompetent mice, but not in mice lacking an adaptive immune response, could provide a new system for studying the antimycobacterial killing mechanisms of adaptive immunity.IMPORTANCE The bacterial pathogen Mycobacterium tuberculosis can enter into a persistent state in which M. tuberculosis can evade host immunity, thereby reducing the effectiveness of current tuberculosis vaccines. Understanding the factors that contribute to persistence would enable the rational design of vaccines effective against persisters. We previously generated two attenuated, triple-auxotrophic M. tuberculosis strains that are safe to use in a biosafety level 2 laboratory. Herein, we discovered that the triple-auxotrophic strain H37Rv ΔleuCD ΔpanCD ΔmetA persisted in immunodeficient Rag1-/- mice, which lack adaptive immunity, but not in immunocompetent mice. The conditional persistence of this auxotrophic mutant, which is susceptible to the sterilizing effect of the adaptive immune response over time, provides an important tool to dissect the mycobactericidal effector mechanisms mediated by adaptive immunity. Furthermore, because of its remarkable safety attributes, this auxotrophic mutant can potentially be used to develop a practical human challenge model to facilitate vaccine development.

Four serine/threonine kinases are present in all mycobacteria: PknA, PknB, PknG and PknL. PknA and PknB are essential for growth and replication, PknG regulates metabolism, but little is known about PknL. Inactivation of pknL and adjacent regulator MSMEG_4242 in rough colony M. smegmatis mc2155 produced both smooth and rough colonies. Upon restreaking rough colonies, smooth colonies appeared at a frequency of ~ 1/250. Smooth mutants did not form biofilms, showed increased sliding motility and anomalous lipids on thin-layer chromatography, identified by mass spectrometry as lipooligosaccharides and perhaps also glycopeptidolipids. RNA-seq and Sanger sequencing revealed that all smooth mutants had inactivated lsr2 genes due to mutations and different IS1096 insertions. When complemented with lsr2, the colonies became rough, anomalous lipids disappeared and sliding motility decreased. Smooth mutants showed increased expression of IS1096 transposase TnpA and MSMEG_4727, which encodes a protein similar to PKS5. When MSMEG_4727 was deleted, smooth pknL/MSMEG_4242/lsr2 mutants reverted to rough, formed good biofilms, their motility decreased slightly and their anomalous lipids disappeared. Rough delpknL/del4242 mutants formed poor biofilms and showed decreased, aberrant sliding motility and both phenotypes were complemented with the two deleted genes. Inactivation of lsr2 changes colony morphology from rough to smooth, augments sliding motility and increases expression of MSMEG_4727 and other enzymes synthesizing lipooligosaccharides, apparently preventing biofilm formation. Similar morphological phase changes occur in other mycobacteria, likely reflecting environmental adaptations. PknL and MSMEG_4242 regulate lipid components of the outer cell envelope and their absence selects for lsr2 inactivation. A regulatory, phosphorylation cascade model is proposed.

Eighty years after the introduction of the first antituberculosis (TB) drug, the treatment of drug-susceptible TB remains very cumbersome, requiring the use of four drugs (isoniazid, rifampicin, ethambutol and pyrazinamide) for two months followed by four months on isoniazid and rifampicin. Two of the drugs used in this “short”-course, six-month chemotherapy, isoniazid and ethambutol, target the mycobacterial cell wall. Disruption of the cell wall structure can enhance the entry of other TB drugs, resulting in a more potent chemotherapy. More importantly, inhibition of cell wall components can lead to mycobacterial cell death. The complexity of the mycobacterial cell wall offers numerous opportunities to develop drugs to eradicate Mycobacterium tuberculosis, the causative agent of TB. In the past 20 years, researchers from industrial and academic laboratories have tested new molecules to find the best candidates that will change the face of TB treatment: drugs that will shorten TB treatment and be efficacious against active and latent, as well as drug-resistant TB. Two of these new TB drugs block components of the mycobacterial cell wall and have reached phase 3 clinical trial. This article reviews TB drugs targeting the mycobacterial cell wall in use clinically and those in clinical development.

Large genomic deletions (LGDs) (6 to 63 kbp) were observed in isoniazid-resistant Mycobacterium tuberculosis mutants derived from four M. tuberculosis strains. These LGDs had no growth defect in vitro but could be defective in intracellular growth and showed various sensitivities toward oxidative stress despite lacking katG. The LGD regions comprise 74 genes, mostly of unknown function, that may be important for M. tuberculosis intracellular growth and protection against oxidative stress.

3-(Phenethylamino)demethyl(oxy)aaptamine (1) was re-discovered from the marine sponge of Aaptos sp. as an anti-dormant mycobacterial substance through the bioassay-guided separation. Compound 1 showed potent anti-microbial activity against Mycobacterium bovis BCG with a minimum inhibitory concentration of 0.75 µg/mL under both aerobic conditions and hypoxic conditions inducing dormant state. Compound 1 was also effective against pathogenic M. tuberculosis strains including clinical multidrug-resistant strains. Furthermore, the successful total syntheses of 1 and its analog 3-aminodemethyl(oxy)aaptamine (2) afford sufficient quantities for further biological studies.

Isoniazid (INH) was the first synthesized drug that mediated bactericidal killing of the bacterium Mycobacterium tuberculosis, a major clinical breakthrough. To this day, INH remains a cornerstone of modern tuberculosis (TB) chemotherapy. This review describes the serendipitous discovery of INH, its effectiveness on TB patients, and early studies to discover its mechanisms of bacteriocidal activity. Forty years after its introduction as a TB drug, the development of gene transfer in mycobacteria enabled the discovery of the genes encoding INH resistance, namely, the activator (katG) and the target (inhA) of INH. Further biochemical and x-ray crystallography studies on KatG and InhA proteins and mutants provided comprehensive understanding of INH mode of action and resistance mechanisms. Bacterial cultures can harbor subpopulations that are genetically or phenotypically resistant cells, the latter known as persisters. Treatment of exponentially growing cultures of M. tuberculosis with INH reproducibly kills 99% to 99.9% of cells in 3 days. Importantly, the surviving cells are slowly replicating or non-replicating cells expressing a unique stress response signature: these are the persisters. These persisters can be visualized using dual-reporter mycobacteriophages and their formation prevented using reducing compounds, such as N-acetylcysteine or vitamin C, that enhance M. tuberculosis' respiration. Altogether, this review portrays a detailed molecular analysis of INH killing and resistance mechanisms including persistence. The phenomenon of persistence is clearly the single greatest impediment to TB control, and research aimed at understanding persistence will provide new strategies to improve TB chemotherapy.