**3. Molecular basis of Mtb resistance to SSC drugs**

The frequency of spontaneous mutations that confer resistance to an individual TB drugs *in vitro* are well known and vary from 1 in 105 (EMB) to 1 in 1010 (RMP) [28].

*Resistance to INH:* INH is a drug precursor that is activated by Mtb catalase-peroxidase enzyme (KatG) to generate a range of highly reactive species [29]*.* Active INH targets essentially enoylacyl carrier protein reductase (InhA enzyme), which is involved in mycolic acid synthesis [29]. Resistance to INH occurs more frequently than for most anti-TB drugs, at a frequency of 1 in 106 bacilli *in vitro* [13]. Clinical isolates of INH-resistant Mtb often lose catalase and peroxidase activities due to KatG S315T mutation [30]. Resistance to INH can also occur through mutations in the promoter region of *inhA*, causing overexpression of InhA, or by mutations at the InhA active site, lowering InhA affinity for INH [31]. *katG* mutation can be associated with *inhA* mutations*,* leading to higher levels of INH resistance [32].

*Resistance to RMP:* RMP interferes with RNA synthesis by binding to the β subunit of myco‐ bacterial RNA polymerase, which is encoded by *rpo*B. Mtb resistance to RMP occurs at a frequency of 10−7 to 10−8 as a result of mutations in *rpo*B. Mutations at positions 531, 526 and 516 in *rpo*B are among the most frequent (96%) in RMP-resistant strains [33].

*Resistance to PZA*: PZA requires conversion to its active form, pyrazinoic acid (POA), by the pyrazinamidase/nicotinamidase enzyme encoded by Mtb *pncA*, which then permeates 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 suggests possible mutations in a putative *pncA* regulatory gene, yet to be identified.

*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 mechanisms of EMB resistance.

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 of multiple drug resistance.
