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

ABSTRACT

Listeria monocytogenes is a Gram-positive intracellular pathogen that causes a severe invasive disease. Upon infecting a host cell, L. monocytogenes upregulates 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 Δ virAB mutant exhibited reduced transcription of VirR-regulated genes. The Δ virAB mutant also showed defects in in vitro plaque formation and in vivo virulence that were similar to those of a Δ virR deletion mutant. Since VirR is important for innate resistance to antimicrobial agents, we determined the MICs of nisin and bacitracin for Δ 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 Δ virR and Δ 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 Δ virR and Δ 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.