# Current Promising Strategies against Antibiotic-Resistant Bacterial Infections
## 2 Discovery of New Antibiotics
### Identifying New Antibiotics from Small-Molecule Libraries  
A small-molecule library typically contains more than 10,000 synthetically produced chemical compounds, which differ in appendages and in their molecular skeleton [17]. Considering the estimation that the probability for a new molecule to become an available medication for patients is only 1:10,000, employing large chemical libraries can increase the chance of discovering new lead antibiotics (Figure 2). One such molecule (compound 1771) was identified by screening 167,405 compounds from small molecule libraries to possess growth-inhibiting properties against antibiotic-resistant GPBPs (such as MRSA and VRE) but not against gram-negative bacterial pathogens (GNBPs) [18]. The compound 1771 targets lipoteichoic acid (LTA) synthase (LtaS), an important enzyme for the growth of gram-positive bacteria. In addition, two anti-MRSA and -VRE compounds (HSGN-94 and HSGN-189) were recently identified by employing a different chemical library to share an  
Figure 2: New antibiotics are mined by the following three strategies: modification of old antibiotics, screening of small-molecule libraries, and exploration of peculiar places. aryl-substituted 1,3,4-oxadiazolyl unit with compound 1771 [19; 20; 21], confirming that the LTA biosynthesis pathway is an essential target for the development of antibiotics against GPBPs. Moreover, some inhibitors targeting wall teichoic acid (WTA) and peptidoglycan pathways are being developed to combat GPBPs. Single deletion of the first two genes (_tagO_ and _tagA_) of the WTA biosynthetic pathway results in bacteria lacking WTA polymers while still being viable [22]. However, most of the downstream genes in WTA backbone biosynthesis are essential, and their deletion leads to a lethal phenotype, possibly due to the accumulation of toxic intermediates in the cell or depletion of cellular pools of undecaprenyl phosphate, which is indispensable for the peptidoglycan biosynthesis. Notably, several library screening studies reported that TarG (also known as TagG), a subunit of a two-component ABC transporter that facilitates the translocation of WTA precursors across the membrane, could be a druggable target since all identified compounds could block TarG [23; 24; 25]. Unfortunately, TarG inhibitors suffered from a relatively high resistance frequency and their inhibitory activities were inconsistent among different isolates [21; 25; 26]. The peptidoglycan backbone is catalyzed by penicillin-binding proteins (PBPs). The \(\beta\)-lactam antibiotics act by blocking PBPs interaction with the acyl-D-Ala-D-Ala part of the peptidoglycan [27]. MRSA produces a modified penicillin-binding protein called PBP2a, which is insensitive to \(\beta\)-lactam antibiotic binding [28]. The computational screening of 1.2 million drug-like compounds from the ZINC database recently revealed two kinds of PBP2a inhibitors that could protect mice against MRSA and VRSA infection [29; 30; 31]. The outer membrane of gram-negative bacteria consists of lipopolysaccharide (LPS), outer membrane proteins (OMPs), and lipoproteins, all of which are required for bacterial viability and constitute the major obstacle to the effectiveness of current antibiotics [32]. The essential pathways accounting for the transportation of outer membrane components are Lpt, Bam, Lol, and Sec [33]. Studies exploring antibiotic-like inhibitors targeting these pathways through chemical libraries have been conducted. A clinical antibiotic candidate called murepavadin was discovered by iterative cycles of peptidomimetic library synthesis and screening for improved antimicrobial activity [34; 35]. Murepavadin specifically binds to LptD in _P. aeruginosa_ and has recently completed a successfully phase-II clinical trial in patients with life-threatening _Pseudomonas_ lung infections (clinical trial identifier NCT02096328). Two inhibitors, JB-95 and MRL-494, targeting BamA have been discovered by compound screening [36; 37].