Characterization of an Autoinducing Peptide Signal Reveals Highly Efficacious Synthetic Inhibitors and Activators of Quorum Sensing and Biofilm Formation in <i>Listeria monocytogenes</i>

Abstract

Bacteria can use chemical signals to assess their local population density in a process called quorum sensing (QS). Many of these bacteria are common pathogens, including Gram-positive bacteria that utilize <i>agr</i> QS systems regulated by macrocyclic autoinducing peptide (AIP) signals. <i>Listeria monocytogenes</i>, an important foodborne pathogen, uses an <i>agr</i> system to regulate a variety of virulence factors and biofilm formation, yet little is known about the specific roles of <i>agr</i> in <i>Listeria</i> infection and its persistence in various environments. Herein, we report synthetic peptide tools that will enable the study of QS in <i>Listeria</i>. We identified a 6-mer AIP signal in <i>L. monocytogenes</i> supernatants and selected it as a scaffold around which a collection of non-native AIP mimics was designed and synthesized. These peptides were evaluated in cell-based <i>agr</i> reporter assays to generate structure-activity relationships for AIP-based agonism and antagonism in <i>L. monocytogenes</i>. We discovered synthetic agonists with increased potency relative to native AIP and a synthetic antagonist capable of reducing <i>agr</i> activity to basal levels. Notably, the latter peptide was able to reduce biofilm formation by over 90%, a first for a synthetic QS modulator in wild-type <i>L. monocytogenes</i>. The lead <i>agr</i> agonist and antagonist in <i>L. monocytogenes</i> were also capable of antagonizing <i>agr</i> signaling in the related pathogen <i>Staphylococcus aureus</i>, further extending their utility and suggesting different mechanisms of <i>agr</i> activation in these two pathogens. This study represents an important first step in the application of chemical methods to modulate QS and concomitant virulence outcomes in <i>L. monocytogenes</i>.