Discovery of a small molecule that blocks wall teichoic acid biosynthesis in Staphylococcus aureus

Both Gram-positive and Gram-negative bacteria contain bactoprenol-dependent biosynthetic pathways expressing non-essential cell surface polysaccharides that function as virulence factors. Although these polymers are not required for bacterial viability in vitro, genes in many of the biosynthetic pathways are conditionally essential: they cannot be deleted except in strains incapable of initiating polymer synthesis. We report a cell-based, pathway-specific strategy to screen for small molecule inhibitors of conditionally essential enzymes. The screen identifies molecules that prevent the growth of a wildtype bacterial strain but do not affect the growth of a mutant strain incapable of initiating polymer synthesis. We have applied this approach to discover inhibitors of wall teichoic acid (WTA) biosynthesis in Staphylococcus aureus. WTAs are anionic cell surface polysaccharides required for host colonization that have been suggested as targets for new antimicrobials. We have identified a small molecule, 7-chloro-N,Ndiethyl-3-(phenylsulfonyl)-[1,2,3]triazolo[1,5-a]quinolin-5-amine (1835F03), that inhibits the growth of a panel of S. aureus strains (MIC = 1 - 3 μg mL^-1), including clinical methicillin-resistant S. aureus (MRSA) isolates. Using a combination of biochemistry and genetics, we have identified the molecular target as TarG, the transmembrane component of the ABC transporter that exports WTAs to the cell surface. We also show that preventing the completion of WTA biosynthesis once it has been initiated triggers growth arrest. The discovery of 1835F03 validates our chemical genetics strategy for identifying inhibitors of conditionally essential enzymes, and the strategy should be applicable to many other bactoprenol-dependent biosynthetic pathways in the pursuit of novel antibacterials and probes of bacterial stress responses.