Antibacterial New Target Discovery: Sentinel Examples, Strategies, and Surveying Success

Unrealized targets in the discovery of antibiotics for Gram-negative bacterial infections

Advances in areas that include genomics, systems biology, protein structure determination and artificial intelligence provide new opportunities for target-based antibacterial drug discovery. The selection of a 'good' new target for direct-acting antibacterial compounds is the first decision, for which multiple criteria must be explored, integrated and re-evaluated as drug discovery programmes progress. Criteria include essentiality of the target for bacterial survival, its conservation across different strains of the same species, bacterial species and growth conditions (which determines the spectrum of activity of a potential antibiotic) and the level of homology with human genes (which influences the potential for selective inhibition). Additionally, a bacterial target should have the potential to bind to drug-like molecules, and its subcellular location will govern the need for inhibitors to penetrate one or two bacterial membranes, which is a key challenge in targeting Gram-negative bacteria. The risk of the emergence of target-based drug resistance for drugs with single targets also requires consideration. This Review describes promising but as-yet-unrealized targets for antibacterial drugs against Gram-negative bacteria and examples of cognate inhibitors, and highlights lessons learned from past drug discovery programmes.

β-Lactams against the Fortress of the Gram-Positive Staphylococcus aureus Bacterium

The biological diversity of the unicellular bacteria-whether assessed by shape, food, metabolism, or ecological niche-surely rivals (if not exceeds) that of the multicellular eukaryotes. The relationship between bacteria whose ecological niche is the eukaryote, and the eukaryote, is often symbiosis or stasis. Some bacteria, however, seek advantage in this relationship. One of the most successful-to the disadvantage of the eukaryote-is the small (less than 1 μm diameter) and nearly spherical Staphylococcus aureus bacterium. For decades, successful clinical control of its infection has been accomplished using β-lactam antibiotics such as the penicillins and the cephalosporins. Over these same decades S. aureus has perfected resistance mechanisms against these antibiotics, which are then countered by new generations of β-lactam structure. This review addresses the current breadth of biochemical and microbiological efforts to preserve the future of the β-lactam antibiotics through a better understanding of how S. aureus protects the enzyme targets of the β-lactams, the penicillin-binding proteins. The penicillin-binding proteins are essential enzyme catalysts for the biosynthesis of the cell wall, and understanding how this cell wall is integrated into the protective cell envelope of the bacterium may identify new antibacterials and new adjuvants that preserve the efficacy of the β-lactams.

Improper Coordination of BamA and BamD Results in Bam Complex Jamming by a Lipoprotein Substrate

The β-barrel assembly machinery, the Bam complex, is central to the biogenesis of integral outer membrane proteins (OMPs) as well as OMP-dependent surface-exposed lipoproteins, such as regulator of capsule synthesis protein F (RcsF). Previous genetic analysis established the model that nonessential components BamE and BamB have overlapping, redundant functions to enhance the kinetics of the highly conserved BamA/BamD core. Here we report that BamE plays a specialized nonredundant role in the Bam complex required for surface exposure of RcsF. We show that the lack of bamE, but not bamB, completely abolishes assembly of RcsF/OMP complexes and establish that the inability to assemble RcsF/OMP complexes is a molecular reason underlying all synthetic lethal interactions of ΔbamE Our genetic analysis and biochemical cross-linking suggest that RcsF accumulates on BamA when BamA cannot engage with BamD because of its limited availability or the incompatible conformation. The role of BamE is to promote proper coordination of RcsF-bound BamA with BamD to complete OMP assembly around RcsF. We show that in the absence of BamE, RcsF is stalled on BamA, thus blocking its function, and we identify the lipoprotein RcsF as a bona fide jamming substrate of the Bam complex.IMPORTANCE The β-barrel assembly machinery, the Bam complex, consists of five components, BamA to -E, among which BamA and BamD are highly conserved and essential. The nonessential components are believed to play redundant roles simply by improving the rate of β-barrel folding. Here we show that BamE contributes a specific and nonoverlapping function to the Bam complex. BamE coordinates BamA and BamD to form a complex between the lipoprotein RcsF and its partner outer membrane β-barrel protein, allowing RcsF to reach the cell surface. In the absence of BamE, RcsF accumulates on BamA, thus blocking the activity of the Bam complex. As the Bam complex is a major antibiotic target in Gram-negative bacteria, the discovery that a lipoprotein can act as a jamming substrate may open the door for development of novel Bam complex inhibitors.