Chemical Interrogation of LuxR-type Quorum Sensing Receptors Reveals New Insights into Receptor Selectivity and the Potential for Interspecies Bacterial Signaling.

Cell-cell signaling between bacteria, including quorum-sensing (QS) communication systems, may play a role in the establishment and maintenance of polymicrobial communities. To better understand and model these interactions, we must uncover the degree to which neighboring species recognize each another's signals. In the current study, we tested the likelihood of whether the QS systems of two opportunistic pathogens (Acinetobacter baumannii and Pseudomonas aeruginosa) that frequently arise in polymicrobial infections would be affected by the QS signals of neighboring species. Through the synthesis and screening of a library of native and non-native N-acyl l-homoserine lactones (AHLs), we found that the AbaR LuxR-type receptor protein of A. baumannii is highly selective for its native AHL signal. However, a homologous LuxR-type receptor in P. aeruginosa, LasR, is far more promiscuously activated by AHLs relative to AbaR, suggesting that LasR-regulated QS could be more susceptible to activation by neighboring species. To explain the observed difference in signal selectivity between AbaR and LasR, we developed a model based on (i) the activity profiles of these proteins and (ii) previously reported structural data and activity profiles for related LuxR-type receptors. This model may facilitate the study of signal selectivities for hundreds of LuxR-type QS receptors from bacteria, many of which grow in polymicrobial communities and may sense each other's signals. In addition, we discovered a set of AHLs that could be used to selectively activate LasR and selectively inhibit AbaR in polymicrobial experiments.

Synthesis and biological evaluation of triazole-containing N-acyl homoserine lactones as quorum sensing modulators.

Many bacterial species are capable of assessing their local population densities through a cell-cell signaling mechanism termed quorum sensing (QS). This intercellular communication process is mediated by small molecule or peptide ligands and their cognate protein receptors. Numerous pathogens use QS to initiate virulence once they achieve a threshold cell number on a host. Consequently, approaches to intercept QS have attracted considerable attention as potential anti-infective therapies. Our interest in the development of small molecule tools to modulate QS pathways motivated us to evaluate triazole-containing analogs of natural N-acyl L-homoserine lactone (AHL) signals as non-native QS agonists and antagonists in Gram-negative bacteria. We synthesized 72 triazole derivatives of five broad structure types in high yields and purities using efficient Cu(I)-catalyzed azide-alkyne couplings. These compounds were evaluated for their ability to activate or inhibit two QS receptors from two prevalent pathogens - LasR from Pseudomonas aeruginosa and AbaR from Acinetobacter baumannii- using bacterial reporter strains. Several triazole derivatives were identified that were capable of strongly modulating the activity of LasR and AbaR. These compounds represent a new and synthetically accessible class of AHL analogs, and could find utility as chemical tools to study QS and its role in bacterial virulence.