Bacterial virulence determinants can be identified, according to the molecular Koch's postulates, if inactivation of a gene associated with a suspected virulence trait results in a loss in pathogenicity. This approach is commonly used with genetically tractable organisms. However, the current lack of tools for targeted gene disruptions in obligate intracellular microbial pathogens seriously hampers the identification of their virulence factors. Here we demonstrate an approach to studying potential virulence factors of genetically intractable organisms, such as Chlamydia. Heterologous expression of Chlamydia pneumoniae CopN in yeast and mammalian cells resulted in a cell cycle arrest, presumably owing to alterations in the microtubule cytoskeleton. A screen of a small molecule library identified two compounds that alleviated CopN-induced growth inhibition in yeast. These compounds interfered with C. pneumoniae replication in mammalian cells, presumably by 'knocking out' CopN function, revealing an essential role of CopN in the support of C. pneumoniae growth during infection. This work demonstrates the role of a specific chlamydial protein in virulence. The chemical biology approach described here can be used to identify virulence factors, and the reverse chemical genetic strategy can result in the identification of lead compounds for the development of novel therapeutics.