The VirAB ABC Transporter Is Required for VirR Regulation of Listeria monocytogenes Virulence and Resistance to Nisin

Listeria monocytogenes is a Gram-positive intracellular pathogen that causes a severe invasive disease. Upon infecting a host cell, L. monocytogenes up-regulates the transcription of numerous factors necessary for productive infection. VirR is the response regulator component of a two-component regulatory system in L. monocytogenes. In this report, we have identified the putative ABC transporter encoded by genes lmo1746-lmo1747 as necessary for VirR function. We have designated lmo1746-lmo1747 virAB. We constructed an in-frame deletion of virAB and determined that the Delta virAB mutant exhibited reduced transcription of VirR-regulated genes. The Delta virAB mutant also showed defects in in vitro plaque formation and in vivo virulence that were similar to those of a Delta virR deletion mutant. Since VirR is important for innate resistance to antimicrobial agents, we determined the MICs of nisin and bacitracin for Delta virAB bacteria. We found that VirAB expression was necessary for nisin resistance but was dispensable for resistance to bacitracin. This result suggested a VirAB-independent mechanism of VirR regulation in response to bacitracin. Lastly, we found that the Delta virR and Delta virAB mutants had no deficiency in growth in broth culture, intracellular replication, or production of the ActA surface protein, which facilitates actin-based motility and cell-to-cell spread. However, the Delta virR and Delta virAB mutants produced shorter actin tails during intracellular infection, which suggested that these mutants have a reduced ability to move and spread via actin-based motility. These findings have demonstrated that L. monocytogenes VirAB functions in a pathway with VirR to regulate the expression of genes necessary for virulence and resistance to antimicrobial agents.