N-Butanoyl-L-homoserine lactone

Acylated homoserine lactones (AHLs) are self-generated diffusible signal molecules that mediate population density dependent gene expression (quorum sensing) in a variety of Gram-negative bacteria, and several virulence genes of human pathogens are known to be controlled by AHLs. In this study, strains of Pseudomonas aeruginosa, Acinetobacter baumannii, Escherichia coli and Klebsiella pneumoniae, isolated from intensive care patients, were screened for AHL production by using AHL responsive indicator strains of Chromobacterium violaceum CV026 and Agrobacterium tumefaciens NT1. Positive reactions were recorded for all 50 isolates of P. aeruginosa and 10 isolates of Acinetobacter baumannii with Agrobacterium tumefaciens NT1. Surprisingly, most P. aeruginosa isolates gave negative results with C. violaceum CV026 in contrast to previous reports. This suggests that the new isolates of P. aeruginosa either failed to make short chain AHLs or the level of the signal molecule is very low.

Pseudomonas aeruginosa is an opportunistic pathogen that coordinates the production of many virulence phenotypes at high population density via quorum sensing (QS). The LuxR-type receptor RhlR plays an important role in the P. aeruginosa QS process, and there is considerable interest in the development of chemical approaches to modulate the activity of this protein. RhlR is activated by a simple, low molecular weight N-acyl l-homoserine lactone signal, N-butanoyll-homoserine lactone (BHL). Despite the emerging prominence of RhlR in QS pathways, there has been limited exploration of the chemical features of the BHL scaffold that are critical to its function. In the current study, we sought to systematically delineate the structure-activity relationships (SARs) driving BHL activity for the first time. A focused library of BHL analogues was designed, synthesized, and evaluated in cell-based reporter gene assays for RhlR agonism and antagonism. These investigations allowed us to define a series of SARs for BHL-type ligands and identify structural motifs critical for both activation and inhibition of the RhlR receptor. Notably, we identified agonists that have ~10-fold higher potencies in RhlR relative to BHL, are highly selective for RhlR agonism over LasR, and are active in the P. aeruginosa background. These compounds and the SARs reported herein should pave a route toward new chemical strategies to study RhlR in P. aeruginosa.

The discovery of quorum sensing (QS) communication systems regulating bacterial virulence has afforded a novel opportunity for controlling infectious bacteria by interfering with QS. Pseudomonas aeruginosa is an example of an opportunistic human pathogen for which N-acyl homoserine lactone (AHL)-related compounds have been described as potent inhibitors of biofilm formation and virulence factors, given their similarity to the natural QS autoinducers (AHLs). Our purpose was to design potent analogs of N-butanoyl-L-homoserine lactone (C4-HSL) and to screen them for biological activity. Eleven original compounds characterized by the modification of the lactone moiety were screened for their ability to impair biofilm formation. Among them, compound 11 was able to modify the growth kinetics and to restrict the number of adherent cells when added from the early stages of biofilm formation (i.e., adhesion and microcolony formation) in a dose-dependent manner. To demonstrate antagonism with C4-HSL, we showed that the inhibition of biofilm formation by compound 11 was impaired when C4-HSL was added. Structure-activity relationships are discussed with respect to the results obtained.