**2. Standard treatment for drug-susceptible TB**

Symptoms associated with active TB are generally defined as loss of weight and energy, poor appetite, fever, a productive cough, and night sweats. Although highly suggestive of TB, such symptoms might easily be assigned to another disease. Therefore accurate diagnosis is important before initiating drug therapy. The current standard laboratory test consists on the analysis of 3 sputum specimens for acid-fast bacilli smears and culture, with nucleic acid amplification performed on at least 1 specimen [10].

In 1994, the WHO introduced the DOTS (Directly Observed Treatment, Short-course) as a major plan to control TB globally [11]. The DOTS strategy focuses on five main points of action: 1) government commitment to control TB, 2) diagnosis based on sputum-smear microscopy tests done on patients with TB symptoms, 3) direct observation short-course chemotherapy treatments, 4) a continuous supply of drugs, and 5) standardized reporting and recording of cases and treatment outcomes [11]. The standard short course (SSC) treatment recommended by WHO [12] consists of 2 months of intensive phase of daily oral administration of isoniazid (INH), rifampicin (RMP), pyrazinamide (PZA) and ethambutol (EMB) followed by 4 months continuous phase of daily INH and RMP alone.

INH is only active against growing tubercle bacilli [13]. RMP is active against both growing and stationary phase bacilli with low metabolic activity and is associated with high sterilizing activity *in vivo* [14]. PZA plays a unique role in shortening TB treatment from the previous 12 months to 6 months because it kills the persistent Mtb population in the lung [15]. EMB is active against growing Mtb but has no effect on dormant bacilli. The combination of drugs acting at different stages of the Mtb life cycle during SSC therapy has been successful in TB treatment in most endemic countries when patients adhere to a fairly strict daily regimen. SSC therapy causes minor or no side effects and is affordable, costing less than \$40 for a full course of treatment. Side effects, if they occur, are manageable and usually do not result in the interruption of the treatment.

Aproximately 90% of people infected with Mtb develop an efficient immune response that successfully contains the infection but unfortunately without killing all the bacteria. Surviving bacteria persist in the lung as non-replicative (i.e. dormant) organisms [16]. In this stage of latent TB infection (LTBI), people do not exhibit TB symptoms and cannot pass the infection on to other individuals. However, in weakened immune system conditions (old age, HIV infection or therapeutic immunosupression), dormant bacteria revert into dividing organisms leading to TB reactivation [16].

LTBI is highly suspected in individuals previously exposed to those with known active TB, which would include people in hospitals, homeless shelters and prisons, or people having recently traveled to countries where TB is highly endemic. The stage of clinical latency is of surpassing importance for TB control as most cases of active TB arise from the vast reservoir of the latently infected population [17]. In fact, it estimated that the infection reactivates and cause active TB in approximately 5 to 10% of latently infected persons [18].

**Figure 1.** Misdiagnosis and mismanagement can result in only fraction of TB patients getting correct diagnosis, appro‐

Symptoms associated with active TB are generally defined as loss of weight and energy, poor appetite, fever, a productive cough, and night sweats. Although highly suggestive of TB, such symptoms might easily be assigned to another disease. Therefore accurate diagnosis is important before initiating drug therapy. The current standard laboratory test consists on the analysis of 3 sputum specimens for acid-fast bacilli smears and culture, with nucleic acid

In 1994, the WHO introduced the DOTS (Directly Observed Treatment, Short-course) as a major plan to control TB globally [11]. The DOTS strategy focuses on five main points of action: 1) government commitment to control TB, 2) diagnosis based on sputum-smear microscopy tests done on patients with TB symptoms, 3) direct observation short-course chemotherapy treatments, 4) a continuous supply of drugs, and 5) standardized reporting and recording of cases and treatment outcomes [11]. The standard short course (SSC) treatment recommended by WHO [12] consists of 2 months of intensive phase of daily oral administration of isoniazid (INH), rifampicin (RMP), pyrazinamide (PZA) and ethambutol (EMB) followed by 4 months

INH is only active against growing tubercle bacilli [13]. RMP is active against both growing and stationary phase bacilli with low metabolic activity and is associated with high sterilizing activity *in vivo* [14]. PZA plays a unique role in shortening TB treatment from the previous 12

priate therapy, and positive outcomes. Reprinted from Ref. 9 with permission from Dr. Madhukar Pai.

**2. Standard treatment for drug-susceptible TB**

204 Tuberculosis - Current Issues in Diagnosis and Management

amplification performed on at least 1 specimen [10].

continuous phase of daily INH and RMP alone.

Purified protein derivative (PPD) skin test (also known as the Mantoux test) is the major diagnostic tool used to identify LTBI patients. A positive skin reaction to the PPD test reflects a local cellular immune response, which is interpreted as recent or remote exposure to the TB bacterium. However, despite its usefulness and simplicity, the PPD test have a low predictive value since false-positive reactions can occur as a result of previous BCG vaccination or sensitization to environmental mycobacteria [19,20,21]. In fact, the skin test uses a crude mix of Mtb antigens shared among many mycobacterial species. As a replacement for the PPD test, new interferon-gamma release assays (IGRAs) have been recently developed and shown to be more accurate for LTB diagnosis [22]. IGRAs measure *ex-vivo* production of IFN-gamma by circulating T cells in whole blood in response to more specific Mtb antigens such as ESAT6, CFP10 and TB7.7.

Although LTBI is symptom-free and non contagious, many countries have adopted its treatment in order to reduce the risk of infection progression to active TB and the spread of the disease to the general population. Six to 9 month treatment with INH alone was proven to be effective and safe [10]. Unfortunately, if LTBI results from exposure to a person with MDRor TB XDR-TB, preventive treatment options are very limited or may not be possible. In both active and latent TB cases, it is crucial that health care providers make every effort to ensure that infected persons complete the entire course of treatment. They must explain clearly the benefit of the treatment and also possible side effects (or drug interactions). Additionally, They should identify potential barriers to the course of treatment, which will help to establish an efficient plan to ensure adherence.

Understanding the mechanisms of TB latency is crucial to development of better control strategies. Infection with Mtb occurs initially in alveolar macrophage, in which the bacteria replicate and induce cytokines that initiate the inflammatory response in the lungs, leading ultimately to the formation of granuloma [23]. Granuloma is defined as an immune structure consisting of connective tissue, lymphocytes and activated macrophages, which has a central necrotic core containing extracellular bacteria. Within the granuloma the bacterium is exposed to multiple stresses that include, among others, hypoxic, nutrient limiting, oxidative, nitrosa‐ tive and acidic conditions [24,25], which trigger a genetic program controlled by the transcrip‐ tion factor DosR [25]. The later regulates the development of a quiescent physiological state, which maintains viability of non-dividing bacteria for extended periods of time. The granu‐ loma contains the infection and prevents its spread to other organs [26]. However, dormant bacteria are capable of reactivation controlled by Rpf (resuscitation promoting factor) genes, which is associated with reversal of the non-replicating state into a metabolically active growing and dividing bacteria [27]. Thus, life-long immunity is not gained by a first episode of active TB disease and the disease may develop again at a later stage, either through relapse with the same strain or reinfection with a new strain.

through the membrane, disrupts bacterial membrane potential and affects membrane trans‐ port [34]. PZA resistance is linked to defective pyrazinamidase/nicotinamidase activity, which results from mutations that might occur at different regions (3-17, 61-85 and 132-142) of *pncA* [34]. While most PZA-resistant strains (72–97%) have *pncA* mutations, some do not have *pncA* mutations but rather express defective pyrazinamidase/nicotinamidase activity [13], which

Management of Drug-Resistant TB http://dx.doi.org/10.5772/55531 207

*Resistance to EMB*: Arabinosyl transferase, encoded by *embB,* an enzyme involved in the synthesis of cell wall arabinogalactan, has been proposed as the target of EMB in Mtb [35]. Mutation to EMB resistance occurs at a frequency of 10−5 [13]. The *embB* codon 306 mutation account for only 68% EMB resistant strains [36], suggesting that there may be other mecha‐ nisms of EMB resistance. Therefore, further studies are needed to identify potential new

Because the mutations described above are unlinked, the probability of developing bacillary resistance to 4 drugs used simultaneously is unlikely. Clinical drug-resistant TB is definitely the result of genetic mutation amplification through mismanagement of the TB disease. This includes intermittent therapy due to irregular drug supply, inappropriate drug prescriptions and most importantly poor patient adherence to treatment [37]. Sequential accumulation of mutations in different genes involved in individual drug resistance results in the emergence

Conventional culturing of the etiologic agent combined with drug susceptibility testing (DST) is the 'gold standard' for diagnosing drug resistant TB in order to initiate adequate treatment. However, this approach is rarely used because it requires 3 to 4 months to produce results. Indeed, only 7% of all MDR-TB cases are detected globally [1]. Hence, the deficiency in tools for rapid DST is associated with inadequate treatment regimens, which tragically increase transmission and spread of drug resistant TB, especially in HIV-infected individuals [38]. This alarming situation stimulated the development of a great number of rapid culture- and molecular-based methods that are currently being evaluated in TB diagnosis laboratories. The Nitrate Reductase Assay (NRA) is based on detection of nitrate reduction into nitrite by Mtb organisms capable of growth in the presence of the test antibiotic [39]. Whereas the Microscopic Observation of Drug Susceptibility (MODS) uses inverted microscope to detect the formation of cord-like structure by Mtb isolates resistant to the test drug [40]. The commercial Mycobac‐ terium Growth Indicator Tube 960 (MGIT 960) is a drug-containing culture system based on the fluorescence detection of resistant bacteria [41]. The Genotype MTBDR*plus* is a molecular line-probe assay that detects simultaneously mutations in the rpoB gene that confers resistance to RMP as well as mutations in the katG gene and the inhA promoter, which are associated with resistance to INH [42]. The Alamar blue and resazurin assays are liquid-based colori‐ metric tests [43]; a color change in wells containing drug-exposed bacteria reflects resistance*.* The MTT assay relays on the ability of drug-resistant (viable) bacteria to cleave the tetrazolium

suggests possible mutations in a putative *pncA* regulatory gene, yet to be identified.

mechanisms of EMB resistance.

of multiple drug resistance.

**4. Diagnosis of multidrug resistant tuberculosis**

Deciphering the molecular basis of dormancy and reactivation is therefore necessary for developing more efficient TB therapies. Adjuncts of agents that would block transitions between active growth, dormancy, and resuscitation or kill effectively dormant bacteria can significantly enhance the efficacy of current treatments for latent infection. Such agents would also shorten the treatment duration of active TB.
