Introduction

Tuberculosis (TB) is possibly a contagious fatal illness that has a destructive effect on the human lungs. Mycobacterium tuberculosis, the TB-causing bacteria, can be spread through airborne droplets from one person to another by sneezes or coughs. In 1882, Robert Koch was awarded the Nobel Prize for this significant discovery. Mycobacterium tuberculosis (MTB) known as an intracellular pathogen and an acid-fast bacillus that has evolved a variety of survival mechanisms to resist of being destroyed by macrophages.1 MTB is one of the most effective pathogens, able to living for decades in the host without creating any signs or symptoms of TB.2 According to the latest evaluation, MTB has infected around 25% of the world’s population. The World Health Organization (WHO) estimated the TB global outbreak peaked between 2003 and 2019, and to be declining slowly and that 1.6 million people being killed of tuberculosis in 2019,3,4 whilst TB was labelled a worldwide health emergency by the World Health Organization in 1993. M. leprae, M. avium and M. kansasii are among species of the pathogenic Mycobacterium. M. kansasii and M. avium do not result in tuberculosis or leprosy but do cause pulmonary infection and are hence known as non-TB mycobacterium.

TB can persist as a latent or active infection, whilst the former is symptomless and does not cause for disease transmission and the latter one is severe and contagious. Some studies have shown that persons with latent tuberculosis have a 10% probability of developing active tuberculosis if they have a weak immune system. Hence, it is also necessary for diagnosing latent tuberculosis and preventing it from becoming active.5 Primarily Mycobacterium mostly affects the respiratory system; however, it has the ability to attack any part of the body, such as bones, bloodstream (miliary tuberculosis), lymphatic system, kidney, joints, genitourinary system and central nervous system (meningitis).6 Success or failure rate in the treatment of TB depends on various factors such as (a) smoking, (b) malnutrition, (c) patient adherence to treatment, (d) insufficient monitoring by clinical staff, and (e) and coexisting diseases like HIV.

The main issue with the current TB chemotherapy is the longer period for the medication to be administered orally or given intravenously, the systemic blood circulation distributes it throughout the body, and most of the molecules fail to reach to the target destinations; therefore most of it remains in the body, creating toxicities and destructive impacts. Drugs have low plasma life with a high clearance rate, which can limit their efficacy.7 To overcome the problems of the anti-tuberculosis medications and reestablish a better efficacy of TB therapy, a new generation of TB treatments must be developed, especially to overcome the tuberculosis clinical management.8

Some major challenges to the current TB therapies include (a) extended treatment duration and (b) continual and common varied drug doses, which often leads to patients’ poor or noncompliance with existing therapy. The patient’s poor complying is the predominant related cause in the re-emergence of the illness, as well as the multidrug resistance (MDR) TB and further serious condition known as extensively drug-resistant (XDR) TB.9 Furthermore, MDR-TB and XDR-TB are on the rise in emergent nations, placing medical researchers to the challenge as well as becoming a serious threat globally.

AIDS, which provides a favorable ground for MTB development in an immunocompromised host, is another key factor in the rising prevalence of tuberculosis.10 Spreading strains of MDR and XDR, as well as a lack of effective treatment strategies, emphasize the need for novel and efficacious anti-TB drugs to conquer issues of shorter treatment durations, drug resistance, and greater compliance.11 Nanotechnology-based treatment, or nanomedicine, has been studied in recent decades as a means of substituting the conventional practice of delivery of antibiotics or other pharmaceuticals using drug-encapsulated nanoparticles (NPs). Drug delivery via nanomedicine technology provides the advantages of a controlled-, slow-, and sustained-release from biodegradable nanoparticles.12 Nanoparticles with sizes defined as nanometers (<100 nm) are commonly employed as nanocarriers in drug delivery techniques. Drugs can be embedded onto the nanoparticles or loaded in a matrix of the nanocarriers for therapeutic reasons.12 Nanocarriers can be any biocompatible and biodegradable nanomaterials like polymers, which can be either solid, lipids, natural, or synthetic, etc.13 They penetrate cells more efficiently and successfully than the larger-sized molecules, making them a potential route for the target delivery system. The nanocarriers are engineered to allow for persistent, controlled- and slow-release of drugs from the matrix. In addition, nanomedicine has advantages over conventional tuberculosis therapy approaches owing to the targeted treatment of anti-tubercular drug-loaded nanocarriers, particularly into the lungs, where primary TB is grown. This innovation could well circumvent anti-TB drug metabolism by escaping the first-pass metabolism system, with fast permeation and therapeutic results owing to the alveolar large surface area, avoid non-targeted distribution by using theranostic and diagnostic capabilities, reducing cytotoxic activity (liver and renal toxicity), and promoting patients’ compliance towards the therapy.14 Despite the high deposition efficiency and consistency of drug delivery to targeted tissues, this delivery system may penetrate straight into the deep tissues of the lungs and alveolar macrophages and provide its therapeutic impact.15 Despite the targeted delivery, controlled and site-specific aerosolized target delivery in nanomedicine, which exhibits greater absorption into the alveoli as well as better macrophage uptake, the absence of medication persistence inside the lung’s alveoli inhibits clinical translation of this therapeutic alternative. As a result, delivery of intracellular nanomedicine that particularly targets medication release or distribution in infected cells can conquer all the therapeutic hurdles linked to conventional TB therapy.

Nanomedicine has received a lot of attention in the past few years as a viable replacement for conventional therapeutic applications for tuberculosis treatment due to its greater efficacy, the capability to deliver multiple therapeutic payloads at the same time, improve selectivity, the strategies to endure multiple drug resistance and thus increased drug strength. Nanomedicine eliminates side effects and toxicity of the drugs by delivering the medication to the target tissue in a particular way that avoids off-target release, whereas its nano-sized diameters increase the permeability and solubility of the drug loaded in the nanocarrier. Furthermore, this enhances the stability of drug by isolating it from other external factors, and it enhances the drug residence by preventing the drug from passing away, which results in greater therapeutic effects.16

This review will discuss different types of Mycobacterium tuberculosis infections, conventional diagnostics and treatments. In addition, research, development and innovation using nanotechnology approaches for the treatment of TB will also be discussed.

Genus of Mycobacterium

The famous scientist Robert Koch, a Nobel Laureate in Medicine in 1905 discovered and in 1882 isolated this microbe. Discovery and diagnosis of the cause factor of (MTB), on March 24, 1882, paved the way both for a preventive plan based on laboratory germ attenuation and an effective searching for something like a pharmaceutical therapy.17Mycobacterium is the main genus in the Mycobacteriaceae family and belongs to the Actinomycetales order, which now contains about 100 identified genus or proposed species, as well as variety of saprophytic microorganisms and pathogenic of the warm-blooded animals. The presence of mycolic acids and acid-fastness are defining features of this genus. Mycobacteria are aerobic, slow-growing, rod-shaped, slender and free-living in soil and water and non-spore-forming microorganisms. The generation time of these bacteria is about 20 h, but identification and isolation may take up to 6 weeks. Moreover, they are acid-alcohol-fast microorganisms that resist decolorization when stained with strong mineral acids and acidified alcohol. This acid-fastness feature, caused by waxy cell walls, is critical for detecting mycobacteria.18 Since the rest of bacteria with Gram-positive features (such as Rhodococcus, Corynebacterium, or Nocardia) are frequently acid-fast; therefore, the staining process must be done carefully.

The MTB complex includes Mycobacterium tuberculosis, Mycobacterium bovis BCG, Mycobacterium pinnipedii, Mycobacterium africanum, Mycobacterium bovis, Mycobacterium caprae and Mycobacterium microti. These bacteria cause tuberculosis, which is defined by tubercles development as well as the caseous necrosis in tissues. The tubercle bacilli are derived from tuberculous individuals. M. tuberculosis is perpetuated by humans, while M. bovis is perpetuated by cattle, bison, and deer; and M. avium is perpetuated by chickens. M. tuberculosis, M. bovis, and M. avium can infect and transmit to wild mammals and birds, and these animals can be potential infection sources for the domestic animals.19

The MTB cell wall’s structural details and its interactions with anti-TB drugs are shown in Figure 1. Rifampin (RIF) and isoniazid (INH) are the two major anti-TB drugs with recognized mechanisms of resistance. Rifampin is used as the first-line drug in the treatment regimens. Rifampin functions by limiting synthesis of the RNA, commonly known as transcription, and eventually induce cell death. Since Rifampin resistance is usually coupled with Isoniazid resistance, resistance to this antibiotic is used to predict and estimate the multidrug-resistance TB.20

Figure 1 Cell wall structure of MTB and its interaction with the anti-TB drugs.

Notes: Adapted from Tuberculosis drugs and mechanisms of action. National Institute of Allergy and Infectious Diseases. https://www.niaid.nih.gov/diseases-conditions/tbdrugs. Accessed March 1, 2023.21

Mycobacteria are gram-positive bacteria that are immobile, slow-growing, and have a high genomic G+C content (61–71%). Mycobacteria is called acid-fast because of their staining properties under a microscope, that are mediated with mycolic acid inside the cell wall, and this is one of the reasons for mycobacteria’s hardiness.

There are three types of Mycobacteria complex,19 and this will be discussed below.

Nontuberculous Mycobacteria (NTM)

Biochemical differentiation of mycobacteria from the rest of similar genera was carried out by analysis of the mycolic acids or fatty acids. Mycobacteria have mycolic acid with chemo-type IV within their wall.22 Nelson staining is one of the most prevalent approaches and at the same time the easiest technique for the primary detection of mycobacteria. Runyon in the 1950s divided mycobacteria into four categories based on pigment production and growth rate. To some extent, the Runyon classification of nontuberculous mycobacteria is based on the rate of growth: photo-chromogen, non-chromogen, scoto-chromogen and fast-growing. Fast-growing mycobacteria formed in solid media grow in less than 7 days, while most non-tuberculosis (NTM) generations belong in this category. Other generations in the Runyon category were considered slow-growing kinds. It takes more than 7 days for these species to grow in solid media, and the bacilli of Mycobacterium tuberculosis complex and several non-tuberculosis species are classified in this group. It is reported that many slow-growing species were closely associated with disease in humans and animals. Unlike slow-growing species, most fast-growing species were less pathogenic to humans and were considered opportunistic pathogens.23

Mycobacterium tuberculosis

These nine species are the main group of the Mycobacterium which causes tuberculosis: Tuberculosis sensu stricto, M. africanum, M. canetti, M. bovis, M. caprae, M. microti, M. pinnipedii, M. mungi and M. orygis. Slow-growing Mycobacterium tuberculosis should be incubated in culture media for almost 3 to 8 weeks. It takes about 10 to 20 days for the Bacillus colonies to appear in a small, dry, scaly solid medium. Mycobacteria generation time is 14 to 15 hours and in some sources up to 18 hours. Owing to the existence of wax and fat in the bacterial wall, the transfer of nutrients into the cell is slow; therefore, the growth of this bacterium is slow.24

Mycobacterium leprae

Mycobacterium leprae is the pathogen that causes leprosy, an infectious disease which affects skin and nerves. Despite effective therapies in previous decades, the spread of the disease continues unabated, and the transmission channels are still unknown. Despite the widespread opinion which incurable M. leprae-infected individuals are the primary transmission source, little evidence suggests that environmental sources might serve as a reservoir. It was also looked at if M. leprae DNA may be found in soil in leprosy-endemic areas or places with potential animal reservoirs like armadillos and red squirrels.25

Pathogenesis of Tuberculosis

Tuberculosis is disseminated by MTB complex-containing droplet nuclei in the air and is 1–5 μm in diameter. Once people cough, sneeze, talk or sing while suffering from pulmonary or laryngeal TB, these droplet nuclei develop. Aerosol therapies, bronchoscopy aerosolization and lesions or tissue manipulation and sputum induction, or secretion processing in the hospital or laboratory might all result in them.26 Droplet nuclei contain two to three MTB organisms and are so tiny that they may be kept airborne for a long time by air currents found in any indoor area.27 They are tiny enough to go into the lungs’ alveoli, where the germs replicate.28 Although TB patients create bigger particles holding more bacilli, these particles are not recognized to be efficient transmitters for contagious infection since they are unable to remain in airborne and cannot penetrate the alveoli if inhaled. Organisms deposited on healthy mucosa or skin do not affect tissue. Once particles of a larger size are inhaled, these collide with the airway or upper respiratory tract wall, are caught in the mucous blanket and transported into the oropharynx, where they are digested or expectorated.29 The chance of MTB transmission is determined by the four parameters listed below.

The quantity of organisms released into the air. Organism concentration in the air, which is defined by the space’s volume and ventilation. Time the exposed person is exposed to contaminated air. Immunological condition of the person who was exposed.

The droplet nucleus is transferred via the bronchial tubes and implanted in a bronchiole or respiratory alveolus immediately after inhalation. There is no question that an inhaled tubercle bacillus causes an infection in the lungs; nevertheless, the virulence of the bacterium as well as the ability of the microbicides alveolar macrophage that ingests it plays a significant role. The bacillus has a chance to multiply inside the alveolar macrophage if it survives the initial barrier. The tubercle bacillus replicates gradually in the macrophage, dividing every 25 to 32 hours. There are no known endotoxins or exotoxins in Mycobacterium TB; hence, there is no immediate host response to infection. The microorganisms develop for 2 to 12 weeks till they exceed a population of 103 to 104, that triggers a cellular immune response,30 It can be recognized by a TB skin test response.31 Before developing the cellular immunity, tubercle bacilli transmit from the lymphatics to the hilar lymph nodes and subsequently via the circulation to more distant regions.

Many tissues and organs are resistant to the bacilli’s subsequent multiplication. Mycobacteria are frequently found in the bone marrow, liver, and spleen, however uncontrolled growth of the bacteria in these places seems to be an exception. Sediment of organisms inside the upper lungs, brain, bones, and kidneys could find a suitable environment to grow, and numerous bacterial divisions may arise before particular cellular immunity develops and limits duplication. In individuals with flawless cell-mediated immunity, accumulations of activated T-cells and macrophages from granulomas restrict the multiplication and spread of the organism. Antibodies towards M. tuberculosis are shaped; however, they do not appear protective.32 The organisms are often concentrated in the centre of the granuloma, which commonly turns necrotic.33 Duplication of M. tuberculosis is stopped in most people with normal immune systems as soon as cell-mediated immunity forms, however, if a few numbers of live bacilli remain in the granuloma. Although a main complex could often be seen on a chest radiograph, the majority of pulmonary TB infections are clinically and radiographically undetectable.34 The most frequent sign that an infection with M. tuberculosis has occurred is indeed a positive tuberculin skin test result.

People who have latent TB infection but not the active ones, are not contagious, making it impossible to spread the organism. According to the studies, approximately 10% of those with TB infections who do not receive preventative treatment can spread active tuberculosis and the risk of infection is still significantly high during 2 years of infection, this is the time that half of cases will occur.35 Additionally, diseases like silicosis, diabetes mellitus, and illnesses related to immunosuppression, such HIV infection, along with corticosteroids and other immunosuppressive drugs, are likely to reduce the host’s capabilities to respond to the organism. In these conditions, the risk for developing and spreading TB is risen, particularly during the first 2 years of life.36Figure 2 indicates the stages of bacteria that enter the lung tissue, which are generally digested by alveolar macrophages; however, several bacteria grow inside macrophages and once the macrophages die, they are eventually released. Living macrophages (carrying M. tuberculosis) can transfer the bacteria into tissues and organs via the lymphatic system or even the blood, resulting in extra pulmonary TB. Extra pulmonary TB infects the lymph nodes, meninges, kidneys, spine, and bones about 15–20%. Extra-pulmonary TB has an atypical clinical appearance, which is frequently stimulated by inflammation or neoplasia.37

Figure 2 Pathogenesis of tuberculosis.

Types of TB Infections

Tuberculosis infections and tuberculosis illness are two different things. Whenever a person is exposed to someone who has tuberculosis and breathes in the TB germs, this person is recognized as having TB infection. The majority of people with strong immune systems are able to stop the infection at that time and avoid developing TB. Some people with tuberculosis infection (positive TB skin test but normal chest X-ray) are not ill and is not infectious to others.

Medication therapy Tuberculosis can effectively destroy bacteria and stop future TB infections. Nevertheless, if a person with tuberculosis does not take preventative medication, the bacteria can develop and cause active tuberculosis. A persistent cough that lasts 2 or more weeks, pain in the chest, fatigue, and lack of appetite are all symptoms of tuberculosis.38 When a person has active TB disease, the individual may be contagious and cause infection in other people, specifically those with whom they spent the most contact. Anyone with active tuberculosis will need to take various TB drugs for several months to recover and not infect others. Although, after a few weeks of medication, they may no longer be contagious and will be allowed to return to work and normal activities while finishing their TB treatment. People can have active tuberculosis in any organ of the body, but this is only infectious to others when it is in the lungs or larynx.39

Table 1 shows some of the most common signs of active tuberculosis in the lungs: cough with sputum and blood at times, chest pains, weakness, weight loss, fever and night sweats.40 Latent TB occurs when a person is infected with tuberculosis but does not develop an overt disease. They have no symptoms, and their chest X-ray may even be clear. A reaction to a tuberculin skin test (TST) or an interferon-gamma release assay (IGRA) may be the only symptom of this interaction. Yet, there is a continuing possibility that the latent TB infection could escalate to the active TB. Other diseases, such as HIV, or medications that weaken the immune system, enhance the risk of infection as well. Furthermore, miliary tuberculosis is a rare type of active tuberculosis that arises when TB bacteria enter the bloodstream. In this condition, the bacteria rapidly spread throughout the body in tiny nodules and affect numerous organs at the same time. This kind of tuberculosis can be rapidly fatal.41

Table 1 Symptoms Difference Between Latent and Active TB

Classification of Mycobacterium

While TB affects the respiratory system, it may also infect other cells and tissues, for instance the spine, brain, and kidney. TB can be fatal if adequate medical treatment is not taken. The medical community classifies TB into two categories, pulmonary and extrapulmonary, which result in 11 different forms of tuberculosis (Table 2). Four of these are caused by pulmonary TB, whereas the remaining seven are caused by extrapulmonary tuberculosis. Extrapulmonary TB is the most common in those with a weakened immune system.

Table 2 Classification of TB and Their Main Symptoms

The Factor That Influences TB

There are two stages in the possibility of developing active TB after exposure to tuberculosis bacilli, and each stage is impacted by both internal and external risk factors. Furthermore, exogenous factors including bacillary concentration in sputum and contact with an active TB case play an essential role in speeding the development from exposure to infection. The progression of infection to active TB illness is also influenced by endogenous factors. Comorbidities like diabetes, indoor air pollution, alcohol, immunosuppressive medication use, and cigarette smoke, illicit drug use, in addition to well-established risk factors (such as HIV, malnutrition, and young age) have a massive effect on both populations and individuals.61 Socioeconomic status and behavioral aspects have also been shown to enhance infection susceptibility. Certain groups, including health care professionals and indigenous peoples, are also at a greater risk of catching TB. Figure 3 highlights the major risk variables that affect an individual’s risk of getting TB infection. It illustrates the primary features that influence an individual’s risk of contracting infection and disease.62

Figure 3 TB infection risk factors.

Tuberculosis Current Situation and Challenges

Tuberculosis (TB) is among the most deadly and oldest infectious diseases that harmed human skeletons for thousands of years.63 Until 1882, the major cause of TB was unclear. Dr Robert Koch reported the discovery of bacillus Mycobacterium tuberculosis as the major cause of the disease, during the second final decade of the nineteenth century. Every year since then, the 24th of March has been observed as “World Tuberculosis Day”.64 TB is a contagious illness that spreads readily from person to person. The main cause of tuberculosis is coughing, which occurs when a person afflicted with tuberculosis, expel the bacteria into the air. Data from the National vital registration systems indicate that in the late eighteenth century, tuberculosis was known as the leading cause of mortality in various European countries, North America, and other parts of the world. However, throughout the twentieth century, with rapid improvements in nutrition, income, and housing, the rate of TB cases began to decline, and the majority of TB casualties also started to drop, albeit slowly (1–2% each year).65 Since the early 1940s, the number of tuberculosis cases has been declining at a pace of 10% each year. Similarly, the rate of TB mortality has been dropping. One of the primary reasons for the decrease in the cash rate and the fatality rate was the innovation, progress, and administration of effective drug treatments. In several countries, tuberculosis is known as a disease of the past, owing to the low disease prevalence. These countries have less than one mortality per 100,000 people and fewer than 10 fatalities per year. Even though the prevalence of tuberculosis is declining, in some parts of the globe, tuberculosis remains a persistent and never-ending public health problem. With prompt diagnosis and appropriate medication therapy, tuberculosis (TB) can be cured.

In 1993, many international health agencies, including the World Health Organization (WHO) announced a worldwide health emergency. TB fatalities also increased in 2021, rising to 1.7 million from 1.6 million in 2020 (Including 187,000 people with HIV). TB is the second most common infectious cause of death, after COVID-19 (over HIV/AIDS), and the 13th largest cause of death overall. The COVID-19 pandemic had a negative impact on the fight against tuberculosis, undoing years of work in providing critical TB care and reducing disease burden. A (4.5%) rise in 2021 from 2020 is due to the wide range of misdiagnosed and untreated cases that contributed to the first spike in TB mortality in more than a decade.66,67 In addition, 450,000 instances of multidrug- or rifampicin-resistant (MDR/RR)-TB were recorded in 2021, representing a 3% rise in the incidence of drug-resistant tuberculosis from 2020. The excessive number of infections and death are related to a decline in crucial TB care since the pandemic starts in 2020.68

MDR-TB or multidrug-resistant tuberculosis is still a common health concern and a health security risk. The number of people with multidrug- or rifampicin-resistant tuberculosis (MDR/RR-TB) who were discovered and notified in 2019 was 206,030, which was an increase of 10% from 186,883 in 2018. Globally, from 2015 to 2019, the incidence of the TB disease fell by roughly 2% every year, the cumulative reduction was 9%. This was less than halfway to the End TB Strategy milestone of a 20% reduction between 2015 and 2020. TB diagnosis and treatment saved up approximately 60 million lives between 2000 and 2019. Ending the tuberculosis pandemic by 2030 is one of the United Nations Sustainable Development Goals’ health goals (SDGs).69–71

This disease is more frequent in males than in women, and in adults rather than in youngsters. According to some researchers, tuberculosis is one of the top 10 deadliest diseases on the planet. In terms of mortality, this disease ranks higher than HIV/AIDS. In 2014–2015, all UN and WHO members agreed to put an end to this pandemic disease. In May 2014, at the World Health Assembly, these states overwhelmingly approved and adopted the UN Sustainable Development Goals (SDGs) and WHO’s End Tuberculosis Strategy. The End TB Strategy’s and tuberculosis-related fatalities chief goal is to eradicate TB endemic by 2030–2035.72

The primary purpose for combining anti-tuberculosis drugs (ATDs) with diverse mechanisms of action is to prevent the growth of drug-resistant strains. However, inconsistencies in medication production, noncompliance with therapy, unsuitable regimens, and poor management all led to the formation of resistant strains. Multidrug-resistant (MDR) Mtb strains are those that are resistant to the essential antibiotics INH and RIF, whilst extensively drug-resistant (XDR) Mtb strains are those that are also resistant to a second-line injectable antibiotic (amikacin (AMK), capreomycin, or kanamycin) and fluoroquinolone (such levofloxacin or moxifloxacin (MXF)).73 Furthermore, multiple examples of strains resistant to all first- and second-line TB treatments have been documented in recent years. MDR-TB and XDR-TB treatments duration are almost between 18 and 20 months and need to be taken with second-line drugs, that costs more with higher toxicity effects than in compared to the first line. Injectable drugs additionally make it increasingly challenging to follow treatment plans. Furthermore, TB therapy in HIV patients is worsened by considerable drug–drug interactions between ATDs and antiretroviral treatments.74 The only three ATDs include pretomanid (2019), bedaquiline (BDQ, 2012) and delamanid (2014) authorized towards MDR-/XDR-TB treatment in the past four decades. Different therapies for MDR-/XDR-TB are being tested in clinical trials, such as the NiX-TB experiment. This 6-month all-oral regimen, which the Food and Drug Administration (FDA) lately authorised, combines linezolid, BDQ and pretomanid for the treatment of XDR-TB and other MDR-TB cases.66 It had a successful outcome; however, side effects were recorded. The majority of RR-/MDR-TB cases may be cured with completely oral regimens, according to the most recent WHO recommendations for the treatment of drug-resistant TB, with injectable drugs (particularly AMK) only being used in cases when other treatments are not possible. These reflect a significant requirement for shorter treatment times and more potent medicines with less toxicity side effects. To overcome the limits of existing TB treatment, nanotechnology provides various advantages.75–77

Current Conventional Methodology for the Diagnosis of Tuberculosis

To identify Mycobacteria, methods ranging from the most basic laboratory tests to the most complicated molecular and non-molecular approaches have been developed. Sputum smear and culture techniques are the most conventional techniques for identifying MTB. These methods are time-consuming, requiring experienced employees to conduct the test and requiring several days or weeks to obtain a result. Most conventional methods are so insensitive that they can only identify 50% of the active MTB. This was found that sensitivity is much lower in HIV co-infected patients or youngsters with a small active colony-forming unit. Diagnosing TB and its drug resistance in the early stages of the infection increases the chances of surviving, and by diagnosing contagious patients at an early stage, we can manage and treat them and try to decrease the number of fatalities.78Table 3 provides a detailed overview of the many conventional methods utilized in tuberculosis diagnosis.

Table 3 Comprehensive Overview of the Various Conventional Technologies Used in Tuberculosis Diagnosis

Current Tuberculosis Drugs

WHO, in 2021, updated the most recent guidance for treating patients with drug-susceptible TB. Based on this guidance, a 6-month treatment plan consisting of four first-line TB medicines, namely isoniazid, rifampicin, ethambutol, and pyrazinamide, is suggested. All four medications are used during the first 2 months of therapy, and two medications are utilized throughout the remaining 4 months of treatment until the treatment is finished. This is a strong recommendation based on data with intermediate certainty.94 This treatment regimen has been extensively accepted across the world, and about 85% who use this procedure will have a satisfactory treatment outcome. The present four-drug treatment program has been in effect over almost three decade and is based on foundational TB therapy research done in the 1980s according to the British Medical Research Council.

Research into shortening the duration of TB treatment has increased during the past several years. As a result, several trials and other research have been conducted to determine whether therapy may be reduced while remaining highly effective. While this research has mostly concentrated on drug-resistant TB (owing to the fact that the typical duration of treatment was 18–24 months until recent years). A recent trial evaluated the effectiveness and safety of two 4-month regimens for the TB drug-susceptible treatment. One of these regimens, which included a fluoroquinolone and a high dosage of rifapentine, passed the non-inferiority test. This trial represents the recommended body of evidence for the forthcoming Guideline Development Group to evaluate (GDG).70Table 4 illustrates the first and second-line TB drugs’ discovery year, primary features, and most often reported adverse effects.95Table 4 includes the first- and second-line drugs for TB treatment.

Table 4 The First- and Second-Line Drugs for TB Treatment

Anti-TB therapy is typically used for a period of 6 months. For the first 2 months, only four drugs, namely pyrazinamide, isoniazid, rifampin, and in some cases, ethambutol are used. The use of rifampin and isoniazid is continued for the next 4 months (Table 4). Sometimes, the six-month regimen is termed as “short-course” anti-TB therapy. These drugs are administered daily. Now the questions remain that why does it take so long to TB disease in patients while the cultures of M. tuberculosis may be destroyed in a matter of mere hours to days? The existing model shows that numerous populations of bacilli must be dealt with in infected individuals: those that are growing rapidly, those growing slowly, and those that sporadically replicate. The cells of these separate populations are killed in two phases. At the first bactericidal phase, rapid multipliers are cleared. During the second sterilizing phase, the remaining multiplying organisms are being killed slowly and sporadically. Streptomycin and isoniazid are two types of drugs that had bactericidal activity when tested in animals. These drugs reduce the infection burden by a magnitude of four to five orders. However, these drugs failed to kill the infections. Thus, despite the fact that these drugs killed the inflections, the multiplying populations of M. tuberculosis remain persistent. These slowly and sporadically multiplying populations of M. tuberculosis can only be cleared with antibiotics possessing “sterilizing” activity. Sometimes, these persisted cells become more phenotypically resistant to anti-TB therapy and therefore can take longer to be cleared. Rifampin is sterilizing antibiotic that can decrease the bacillary counts to zero during the second stage of treatment in human beings. With the use of pyrazinamide during the first 2 months of therapy, the length of therapy is reduced. However, it shows minimal bacterial activity when used alone. Ethambutol is an effective drug for drug-susceptible M. tuberculosis and is normally included in the regimen when it is suspected that a certain population has drug-resistant TB.96

Sometimes treatment of TB diseases takes 6 to 9 months by taking several drugs. Only 10 drugs have been recommended by the United States Food and Drug Administration (FDA). Pyrazinamide (PZA), ethambutol (EMB), rifampin (RIF), and isoniazid (INH) are the first line of the approved drugs. These drugs form the core of treatment regimens.97 TB bacteria that is resistant to at least one of the first-line anti-TB drug, is a kind of TB drug-resistant. TB bacteria that are resistant to more than one anti-TB drug are called multidrug-resistant TB (MDR-TB). This bacteria is at least resistant to both rifampin (RIF) and isoniazid (INH). While another uncommon kind of MDR-TB is resistant to rifampin and isoniazid, fluoroquinolone is also one of at least three injectable intravenous second-line drugs – capreomycin, kanamycin, or amikacin – and is called extensively drug-resistant TB (XDR-TB). The treatment and cure of drug-resistant TB are very complicated. In order to treat this drug-resistant TB, a comprehensive consultation with a TB specialist is required.98

Clinically Approved ATD’s for Therapy and Diagnostic

To treat MTB, first-line drugs INH, PYR, EMB, and RIF have been given orally in combination in recent years. Second-line drugs are also combined with the first-line drugs in this treatment.

A combination of second-line drugs is also used to treat MDR-TB. Some of the medications on this list are polypeptides – enviomycin, viomycin and capreomycin; aminoglycosides – kanamycin and amikacin; thioamides – ethionamide, cycloserine and prothionamide viomycin; fluoroquinolones – moxifloxacin, levofloxacin and ciprofloxacin. Generally, first-line drugs are cheaper than second-line ones. Drugs that are now widely known as the third-line TB therapy are thioridazine, rifabutin, vitamin D, thioacetazone, macrolides, linezolid, arginine, and clarithromycin.99 Third-line drugs may not be as successful as first- and second-lines, though, as their effectiveness has not yet been shown and is still being researched.6

For the treatment of MDR, several medications have been proven in clinical research, while others still require further study. Bedaquiline, rifapentine, linezolid, and pretomanid are a few examples. A brand-new ATD called Bedaquiline (Phase 2 trial) has been approved for use in the management of drug-resistant TB. Due to its core heterocyclic quinolinic nucleus containing amine and alcohol side chains, bedaquiline (BDQ), a member of the diarylquinoline class of compounds, plays an important role in antituberculosis therapy. In addition to inhibiting synthesising mycobacterial ATP, it binds to and disrupts the interface of the a-c subunit, impairing the proton cycle and killing the Mycobacterium.100 A nitroimidazo-oxazine known as pretomanid has anti-replicating and anti-dormant mycobacterial activity by inhibiting the formation of mycolic acid and preventing the formation of cell walls. Furthermore, in anaerobic or hypoxic situation, pretomanid effects on non-replicating bacilli.101 Linezolid (LNZ), an oxazolidinone that has been modified and is currently in phase 2, prevents the production of proteins in mycobacteria. The creation of a 70S initiation complex is prevented by LNZ when it binds to the 50S ribosomal subunit’s 23S rRNA domain V.102 These medications’ efficacy is constrained by their fast release and shorter plasma half-life. Therefore, in order to increase the success rate of TB therapy, pharmacological treatments are required.102Figure 4 includes the nanomedicines that are available on the global markets.

Figure 4 Authorized nanomedicines available on the global markets: (a) Current status of development, (b) indications, and (c) formulation.

The Cortellis Drug Discovery Intelligence (CDDI) database was used to find nanomedicines that have been authorized for use on the market or in clinical studies. In June 2021 recent data was examined and evaluated, which indicated that there are now 563 nanomedicines in various phases of drug clinical trials and 100 nanomedicines on the market (663 totally). The majority of these are in clinical phases I and II in which (33%) for former and (21%) for later. They mostly target the treatment of cancer (53%) and infections (14%) respectively (Figure 4a). Recent advances in nanomedicine have enabled the treatment of more diseases affecting the nervous, immunological and metabolic systems, blood, brain, endocrine inflammatory, ophthalmic, cardiovascular and dermatitis problems among other diseases (Figure 4b). Additionally, they are used in the imaging diagnostics and vaccinations development. Liposomes, or lipid-based nanoparticles, account for the vast majority of nanomedicines on the market or in different phases of clinical development are (33%), next by antibody-drug conjugates (15%), polymer-drug/protein conjugates (10%), and polymers (10%). Viral vectors (8%), inorganic nanoparticles (3%), cell-derived vehicles (4%), emulsions, protein-based nanoparticles (3%), nanocrystals, micelles, and dendrimers are more forms of nanomedicines (Figure 4c).103,104

Current Tuberculosis Vaccine

The Bacillus Calmette–Guérin (BCG) vaccine was utilized to combat tuberculosis (TB) since 1921 and is regarded as the most extensively used vaccination in the world.105 However, it is recognized that the BCG protective effectiveness depends on the geographic region in which it is given.106

How BCG’S Response to Mycobacterium tuberculosis

BCG, a live-attenuated Mycobacterium-based immune complex vaccine, promotes immune activation and the local infection at the injection area, where existing dendritic cells (DCs), macrophages and monocytes react to the bacterium.107 BCG vaccine may survive inside dendritic cells for up to 2 weeks,108 producing the immune-polarizing cytokines, in the meantime causing the activation of costimulatory molecules, which are characterized by enhanced expression of the differentiation clusters including CD40, CD80, CD83, and CD86.109 BCG cell wall skeleton immunogenic parts, such as covalently linked mycolic acid, arabinogalactan, and peptidoglycan, are phagocytosed and destroyed by macrophages,107 which may also significantly stimulate an inflammatory response by activating several pattern recognition receptors (PRRs).110,111 While toll-like receptors constitute the first line of defense system against microbes (TLR) 2, signaling is activated by lipids and mycobacterial cell wall lipoglycans like lipomannan (LM). Other mycobacterial proteins, including as heat-shock proteins and glycoproteins, trigger by TLR4 signals.112,113 BCG CpG-DNA has been shown to induce TLR9 intracellular in macrophages and DCs, leading to activation of the cell and the generation of interleukin-12 (IL-12), tumor necrosis factor α (TNF-α), and monocyte chemoattractant protein-1 (MCP-1), a key mediator of the T helper type 1 (Th1) immunological responses.114,115 Similarly, the complement receptors CR3 and CR4, as well as nucleotide-binding oligomerization domain (NOD)-like receptors or even C-type lectins, have all been demonstrated to react with the cell wall components of bacteria and have a crucial role in BCG recognition and internalization.116 Similarly, research on BCG-immunized adults has shown that monocyte progenitors are reprogrammed with stronger PRR expression and enhanced responsiveness to stimuli like TLR agonists. Antigen-presenting cells, including DCs, macrophages, and B lymphocytes white blood cells (B cells), deliver BCG-derived antigen peptides to primary T-cells and major histocompatibility complex (MHC) molecules in the surrounding secondary spleen or lymphoid tissue in order to initiate an adaptive immune response (Figure 5).117,118

Figure 5 BCG vaccination induces an immune response, with MTB-specific effects. (A) BCG is absorbed by DCs or macrophages and could activate the anti-TB immune response when their cell walls perform as PRRs and are connected to a co-stimulatory receptor-ligand. (B) The formation of plasma cells, memory cells, and antigens with particular antibodies is triggered by B cell activation.Memory T and B cells remain in lymph nodes after activation.

After the BCG vaccine is given intravenously, macrophages, monocytes and dendritic cells (DCs) detect the BCG and create a powerful immunological response. DCs or macrophages detect and internalize BCG. BCG’s cell wall and other components operate as PRRs, binding to various ligands, upregulating costimulatory molecules in activated cells, and activating the anti-TB innate immunity. When B-activated DC cells move to lymph nodes, they trigger Th1-type, CD4+ and CD8+ T cells that are specific for mycobacteria and then secrete large quantities of cytokines like IFN-gamma and granzyme. Memory cells, plasma cells, and antigen-specific antibodies are produced all at once by B cells that react to BCG antigens.118

Activated DCs at the BCG vaccination spot move towards the draining lymph nodes, where they release tumor necrosis factor-alpha (TNF-α), Interleukin 6(IL-6), and Interleukin 12 (IL-12), which trigger both of the CD4+ and CD8+ T-cells.119,120 Activated CD4+ and CD8+ T cells produced IFN-γ, which boosts macrophage antimycobacterial activity.121 This demonstrates how people with IFN--γ mutations are more susceptible to TB.122 Neutrophils and activated DCs both works together to boost antigen-specific T-cell responses. According to some researchers, 10 weeks after BCG vaccination, newborns’ peripheral blood may contain a substantial amount of BCG-specific CD4+ and CD8+ T cells. Furthermore, the serum contains higher concentrations of IFN-γ, granzyme, TNF-α, and IL-2.123,124Figure 6 indicates the BCG vaccination results in the formation of trained immunity.

Figure 6 BCG vaccination results in the formation of trained immunity.

BCG vaccination causes monocytes/macrophages to develop trained immunity, resulting in greater epigenetic reprogramming and host defensive abilities. When monocytes are exposed to non-specific infections (respiratory syncytial virus or yellow fever virus), chromatin rearrangement causes them to become “trained”, increasing the efficiency of their innate immune response and triggering the release of proinflammatory cytokines like IL-6, IL-1, and TNF-alpha (Figure 6). However, further research is needed to determine if this “training process” will try and stop SARS or other respiratory syndrome-like coronaviruses.

Conventional TB Drug Therapy Disadvantages

Clinical management in TB therapy is presently seen as one of the most difficult challenges. Drugs are transported all over the body by systemic blood circulation after being taken orally or intravenously, and many molecules aggregate in other parts of the body rather than targeting the right site, triggering disadvantages including nephrotoxicity, hepatotoxicity, ocular toxicity, and ototoxicity. Most TB drugs are administered orally, which leads to pharmacokinetic issues like limited bioavailability and a low therapeutic index. Drug therapy in a conventional route necessitates a lengthy therapy regimen that includes frequent and continuous administration of multiple drugs, leading to lower patient adherence to existing treatments. This is a critical indicator sign of infection recurrence, as well as formation of multidrug-resistant MDR-TB and the more severe extensively drug-resistant XDR-TB.9

In undeveloped countries, the outbreak of MDR-TB is steadily growing. This is a source of a big problem and concern for the health care services.125 Although present anti-TB medications are successful, there seems to be a critical need to find effective and sustainable solutions with producing new short-term regimens that combine novel drugs to tackle the challenges of treatment length, drug resistance and adhere to the treatment.11 As a result, a method will be developed to overcome the therapeutic limitations associated with conventional treatment. Higher bioavailability, effective drug loading, longer retention time, and improved drug encapsulation capacity in body cells, and gradual and consistent drug release might lead to reduced frequency and multiple-drug dosage. Moreover, it was mentioned that when a drug is delivered to the targeted tissue, localised drug exposure is preferable over systemic exposure in order to reduce or indeed prevent systemic side effects. Hence, targeted drug therapies have gained a lot of attention as a way to reduce such systemic adverse effects.126

Advantages and Disadvantages of Nano Drug Delivery System

To overcome the challenges and the side effect of conventional treatment, researchers have brought new novel formulation of nanomedicine which can provide effective therapeutics index. Inhalable nanoparticles are biodegradable and biocompatible, which contain high drug loading capacity this made them to enhanced mucosal cell adhesion, and improved drug delivery to the respiratory system for the treatment of (TB).127 Although nanocarriers based drug delivery agents are achieved high drug loading, high stability, effective drug tolerability, reduce multidrug and extra drug resistance, controlled release, and site-specific delivery, the clinical phases are slow for the benefit of human. Physical and biological factors like pH, protein, phagocytic sequestration, enzyme, shear forces, renal clearance, and aggregation create hostile environments for nanocarriers. The nanocarrier-based medicines have marketed in the pharmaceutical industry at a slow phase.128

For TB drug delivery, the nanocarriers cannot often reach to the deep lung since majority of the drug doses are exhaled. The nanocarriers administer in different routes in TB treatment such oral, topical, intravenous, and pulmonary delivery. These routes have different various limitations, barriers of biological environment, interaction of biological components such as protein corona, loose of nanocarriers before reaching the disease sites, clearance by phagocyte system and so on. The nanocarriers with low water solubility, low bioavailability and low biodistribution exhibit biological and anatomical barriers in the respiratory tract as well as gastrointestinal tract and liver. Beside these barriers, the inhaled therapeutics agents in lung airways exhibit some obstacles in reaching the target TB cells. As a result, the deposition of nanocarriers’ physicochemical properties, particle size, size distribution, density, shape, and electrostatic charge play an important role to reach in deep lung deposition. The composition of the nanocarriers’ inhaled particles should be very small enough to reach the lung TB cells and avoid the deposition in the airways by sedimentation or loss by exhalation. The size and composition become challenging nowadays for the researchers to deliver nanocarriers. To overcome these limitations and achieve the highest efficiency, the nanocarrier characteristics could be tailor-made to target and function effectively for the desired delivery routes.129,130 Nanocarrier based delivery has shown potentially remarkable feature such as biodegradable particle, slow, sustained, controlled release. Although different animal models have been studied to develop the anti-TB therapy based on nanotechnology against M. tuberculosis , unfortunately the clinical management of tuberculosis remains a difficult task and many models do not stand up to the expectations of human TB treatment. Several studies have been conducted by researcher for nanocarrier-based TB therapy. Bhardwaj et al used spray-dried lipid–polymer hybrid nanoparticles for the delivery of isoniazid to TB cells. The formulation was found to be proven to have more uptake by TB cells.131 Recently, Ahmed et al has developed dendrimer-based nanocarrier system to load rifampicin and delivered to TB cells and has found that the drug carrier has high drug loading, negligible toxicity, and extended-release pattern for rifampicin. Although nanocarrier-based systems are more effective than conventional treatments, many studies need to be conducted due to the high necessity of TB effective treatment as well as reduction of the limitations of nanocarrier-based treatment.132

Barriers to Nanoparticle Drug Delivery to the Lung Anatomy

The airway defense system, tight epithelial cell layer, as well as the mucus hypersecretion are the key barriers to NP delivery for chronic lung diseases. However, this will vary depending upon the type of delivery system, respiratory system disease area and the development of the disease. Intranasal, inhaled, or intravenous delivery all were employed to treat pulmonary (TB), even though each brings its own set of challenges. The appropriate route will be determined by the kind of NP, the required bioavailability, as well as the airway target tissue. Targeted nanoparticles towards the lungs must contend with not just the particular structure of the lung but also intravascular, specialized pulmonary, and intracellular barriers.133 In terms of drug delivery, the respiratory tract in lung anatomy is divided into two sections: the top, which includes the nasal cavity, pharynx, whilst the bottom includes the larynx, trachea, bronchi, and alveolar. There are two principal bronchi in this trachea, which branch into the tinier bronchioles inside the lungs, creating air pathways toward the alveoli terminal. The tubes become smaller and split into even more tubes as they pass down from the trachea to the respiratory tract.

There were also reported to be 20 to 23 divisions, all of which led to the alveolus. Although each bronchus or bronchiole has a lower cross-sectional size, the total surface area is bigger due to the huge number of them. As a result, resistance in the terminal bronchioles was reduced. Because of the turbulence, the majority of the resistance is positioned three to four divisions from the trachea. Despite their small size, alveoli contain a significant overall surface area that allows for effective and sufficient gas exchange. Moreover, alveoli contain a single epithelial (400 nm) cell layer that separates the air from the capillaries.134 Rather than being a barrier, this wide surface area offers a great capacity for drug absorption, and thus escape the first-pass metabolism helps to make the pulmonary ro