High-Throughput Screening of the Virulence Regulator VirF: A Novel Antibacterial Target for Shigellosis

Abstract

Shigella flexneri is a human enteropathogen that infects about 165 million people and claims more than 1 million lives per year worldwide. Although shigellosis has been considered a disease of the "Third World," like many other contagious diseases, it does occur in developed countries. The emergence of drug and multidrug-resistant strains of Shigella emphasizes the need for novel antibiotic development. VirF, an AraC-type transcriptional regulator, is responsible for the expression of all downstream virulence factors that control intracellular invasion and cell-to-cell spread of Shigella. Gene knockout studies have validated that inhibition of VirF expression is sufficient to block the normal life cycle of Shigella in the host and thereby increase susceptibility to the host immune system. The authors have developed a high-throughput, cell-based assay to monitor inhibition of VirF using beta-galactosidase as a reporter protein. Using an avirulent strain of Shigella, they have screened libraries containing approximately 42,000 small molecules. Following confirmation and dose-response analysis, they have identified 7 compounds that demonstrate VirF inhibition in vivo >or=55% in comparison with the controls and little general antibacterial activity (measured by cell growth, OD(600)). The authors are in the process of confirming these "hits" in several secondary assays to assess the mechanism of action.