## 2 Genomics
### Mode of Action
Therefore, the inability to generate spontaneous resistance in vitro is likely due to teixobactin inhibiting multiple non-protein targets. Like many other antimicrobial drugs, teixobactin was isolated from an environmental microorganism. It is therefore possible that bacteria in close proximity to _E. terrae_ in the soil may be resistant to teixobactin, offering a scenario where teixobactin resistance mechanisms could be discovered [36]. While the inability to detect drug resistance in the laboratory does not mirror the complexity of TB clinical disease alongside host pharmacokinetic and pharmacodynamic factors, the concept of resistance-proof antimicrobial drugs is very attractive and could transform the destructive antimicrobial drug to evolution of drug resistance cycle. WGS is also shaping our understanding of drug action and drug resistance through the large-scale sequencing of clinical isolates. Consortia mapping drug-resistance conferring mutations have revealed novel mechanisms of resistance and potential novel modes of action of existing anti-_M.tb_ drugs in patients [37]. The CRyPTIC Consortium and the 100,000 Genomes Project obtained over 10,000 _M.tb_ clinical isolate genomes and associated phenotypes to predict phenotypic drug susceptibility to front line agents from genome sequence. The authors successfully demonstrated that susceptibility could be correlated to the presence or absence of specific antimicrobial drug resistance mutations [37]. This approach also exposed mutations with as yet unexplained roles in drug action. For example, _M.tb_ isolates have been identified with mutations in the mycothi biosynthesis genes _mshA_ and _mshC_ that confer high-level resistance to ethonamide and low-level resistance to isoniaizid [38]. Mycothiol is a key detoxifying and reducing molecule with similar functions to glutathione, which is absent in mycobacteria [39]. Knockout mutants of _mshA_ grew normally in vitro and in immunodeficient mice but were growth defective in immunocompetent mice [40], and mycothiol-deficient mutants were less susceptible to ethonamide and isoniaizid [41]. Ethionamide and isoniaizid are both prodrugs that target the NADH-dependent enoyl-ACP reductase InhA; however, isoniaizid is activated by KatG and ethionamide by EthA [42; 43]. In order to delineate the roles mycothi biosynthetic genes play in isoniazid and ethionamide susceptibility, [PERSON] et al. generated null mutants of each mycothiol biosynthetic gene in _M. smegmatis_[44]. _M. smegmatis_ is useful for studying the action of these two drugs, with their target InhA characterized in this bacterium to have \(>\)95% sequence identity to the _M.tb_ InhA [45]. While the exact role of mycothiol in isoniazid and ethionamide resistance remains to be determined, the authors hypothesized that mycothiol is involved in the activation of isoniazid and ethionamide. _M.tb_ KatG, required for isoniazid activation, likely compensates for loss of mycothiol in some settings, resulting in isoniazid susceptibility and ethionamide drug resistance in mycothiol-deficient isolates [40]. Understanding the action of current _M.tb_ drugs has direct implications for the development of new inhibitors. For example, bedaquiline and clofazimine resistance can be mediated through SNPs in _Rv0678_, a transcriptional repressor of _mmpL5_ and _mmpS5_, which encode an efflux pump of the resistance-modulation-division (RND) family [46; 47]. SNPs in _Rv0678_ were detected in clinical isolates of _M.tb_ with no prior exposure to bedaquiline or clofazimine [48]; meaning that resistance to bedaquiline was already circulating in _M.tb_ isolates before the new drug was introduced to the TB clinic. While the authors could not determine the driver of these mutations, they ruled out rifampicin as a selective pressure. Studies such as these are revealing drug detoxification and efflux pathways that operate across drug class to reduce efficacy of both existing and new antimicrobial drugs. To expand the methods available for understanding drug mechanism of action, [PERSON] et al.