The heme sensing response regulator HssR in Staphylococcus aureus but not the homologous RR23 in Listeria monocytogenes modulates susceptibility to the antimicrobial peptide plectasin

<p>Abstract</p> <p>Background</p> <p>Host defence peptides (HDPs), also known as antimicrobial peptides (AMPs), have emerged as potential new therapeutics and their antimicrobial spectrum covers a wide range of target organisms. However, the mode of action and the genetics behind the bacterial response to HDPs is incompletely understood and such knowledge is required to evaluate their potential as antimicrobial therapeutics. Plectasin is a recently discovered HDP active against Gram-positive bacteria with the human pathogen, <it>Staphylococcus aureus </it>(<it>S. aureus</it>) being highly susceptible and the food borne pathogen, <it>Listeria monocytogenes </it>(<it>L. monocytogenes</it>) being less sensitive. In the present study we aimed to use transposon mutagenesis to determine the genetic basis for <it>S. aureus </it>and <it>L. monocytogenes </it>susceptibility to plectasin.</p> <p>Results</p> <p>In order to identify genes that provide susceptibility to plectasin we constructed bacterial transposon mutant libraries of <it>S. aureus </it>NCTC8325-4 and <it>L. monocytogenes </it>4446 and screened for increased resistance to the peptide. No resistant mutants arose when <it>L. monocytogenes </it>was screened on plates containing 5 and 10 fold Minimal Inhibitory Concentration (MIC) of plectasin. However, in <it>S. aureus</it>, four mutants with insertion in the heme response regulator (<it>hssR</it>) were 2-4 fold more resistant to plectasin as compared to the wild type. The <it>hssR </it>mutation also enhanced resistance to the plectasin-like defensin eurocin, but not to other classes of HDPs or to other stressors tested. Addition of plectasin did not influence the expression of <it>hssR </it>or <it>hrtA</it>, a gene regulated by HssR. The genome of <it>L. monocytogenes </it>LO28 encodes a putative HssR homologue, RR23 (in <it>L. monocytogenes </it>EGD-e lmo2583) with 48% identity to the <it>S. aureus </it>HssR, but a mutation in the <it>rr23 </it>gene did not change the susceptibility of <it>L. monocytogenes </it>to plectasin.</p> <p>Conclusions</p> <p><it>S. aureus </it>HssR, but not the homologue RR23 from <it>L. monocytogenes</it>, provides susceptibility to the defensins plectasin and eurocin. Our data suggest that a functional difference between response regulators HssR and RR23 is responsible for the difference in plectasin susceptibility observed between <it>S. aureus </it>and <it>L. monocytogenes</it>.</p

The antimicrobial lysine-peptoid hybrid LP5 inhibits DNA replication and induces the SOS response in Staphylococcus aureus

BACKGROUND: The increase in antibiotic resistant bacteria has led to renewed interest in development of alternative antimicrobial compounds such as antimicrobial peptides (AMPs), either naturally-occurring or synthetically-derived. Knowledge of the mode of action (MOA) of synthetic compounds mimicking the function of AMPs is highly valuable both when developing new types of antimicrobials and when predicting resistance development. Despite many functional studies of AMPs, only a few of the synthetic peptides have been studied in detail. RESULTS: We investigated the MOA of the lysine-peptoid hybrid, LP5, which previously has been shown to display antimicrobial activity against Staphylococcus aureus. At concentrations of LP5 above the minimal inhibitory concentration (MIC), the peptoid caused ATP leakage from bacterial cells. However, at concentrations close to the MIC, LP5 inhibited the growth of S. aureus without ATP leakage. Instead, LP5 bound DNA and inhibited macromolecular synthesis. The binding to DNA also led to inhibition of DNA gyrase and topoisomerase IV and caused induction of the SOS response. CONCLUSIONS: Our data demonstrate that LP5 may have a dual mode of action against S. aureus. At MIC concentrations, LP5 binds DNA and inhibits macromolecular synthesis and growth, whereas at concentrations above the MIC, LP5 targets the bacterial membrane leading to disruption of the membrane. These results add new information about the MOA of a new synthetic AMP and aid in the future design of synthetic peptides with increased therapeutic potential

The Two-Component System CesRK Controls the Transcriptional Induction of Cell Envelope-Related Genes in Listeria monocytogenes in Response to Cell Wall-Acting Antibiotics▿ †

The two-component system CesRK of Listeria monocytogenes responds to cell wall-acting antibiotics. We show here that CesRK controls the transcription of several cell envelope-related genes. The CesRK-dependent induction of these genes may be viewed as an attempt by L. monocytogenes to protect itself against the damaging effects of cell wall-acting antibiotics