SigV Mediates Lysozyme Resistance in Enterococcus Faecalis Via RsiV and PgdA

Enterococcus faecalis is a gut commensal but transitions to a pathogenic state as a consequence of intestinal dysbiosis and/or the presence of indwelling medical devices, causing a wide range of infections. One of the unique features of E. faecalis is its ability to display high level resistance to lysozyme, an important host defense of the innate immune response. Lysozyme resistance in E. faecalis is known to be mediated by the xtraytoplasmic unction (ECF) sigma factor SigV. PgdA and RsiV expression is directly regulated by SigV, but and mutants display nominal changes in lysozyme resistance, suggesting that additional gene products in the SigV regulon contribute to lysozyme resistance. Using transcriptome sequencing (RNA-seq) analysis, we compared the transcriptional profile of the parental strain to that of an isogenic mutant and show that apart from , only and expression was induced upon lysozyme exposure. The combined deletion mutant of both and rendered E. faecalis sensitive to lysozyme at a level comparable to that of the mutant, highlighting the limited SigV regulon. Several additional genes were also induced upon lysozyme exposure, but in a SigV-independent fashion. Overexpression of from a SigV-independent promoter restored lysozyme resistance in a deletion mutant and also induced cell chaining. Overexpression of from a SigV-independent promoter only partially restored lysozyme resistance in a mutant. Overall, we provide evidence for a simple adaptation to lysozyme stress, in which SigV controls the expression of and , and that both gene products contribute to lysozyme resistance. Enterococcus faecalis causes health care-associated infections and displays resistance to a variety of antibiotics and molecules of the innate immune system. SigV has been shown to play an important role in enterococcal lysozyme resistance. Even though several proteins have been implicated in enterococcal lysozyme resistance, a complete SigV-dependent regulon has not been functionally characterized as being responsible for the dramatic increase in lysozyme susceptibility displayed by a mutant. Using RNA-seq, we have identified the SigV regulon to be comprised of two gene loci, - and . Deletion of both and renders E. faecalis susceptible to lysozyme on par with a mutant. We also demonstrate that overproduction of and contributes to lysozyme resistance in susceptible strains.