Curvularin Isolated From Phoma macrostoma Is an Antagonist of RhlR Quorum Sensing in Pseudomonas aeruginosa

Abstract

Quorum sensing (QS) is an attractive target for the treatment of multidrug-resistant Pseudomonas aeruginosa, against which new antibiotics are urgently needed. Because LasR is at the top of the QS hierarchy controlling Rhl and PQS systems, most QS inhibitors have been targeted to LasR. However, it has recently been reported that in clinical isolates of P. aeruginosa, LasR is frequently mutated and nonfunctional, and RhlR independently acts to produce virulent factors that maintain toxicity. Thus, for effective treatment of chronic cystic fibrosis infections, RhlR antagonists is needed to prevent the LasR-independent Rhl system, but RhlR antagonists have rarely been reported. In this study, we found that curvularin, an aromatic compound with a cyclized alkyl side chain isolated from Phoma macrostoma, at a low micromolar concentration of 1–30 μM potently and selectively inhibited pyocyanin and rhamnolipid production without affecting the cell viability of P. aeruginosa. Only high concentration (more over 100 μM) curvularin negligibly inhibited biofilm formation and elastase production, suggesting that curvularin at low concentrations selectively inhibits RhlR. The QS antagonism by curvularin was investigated in experiments using QS competition and signaling molecules assays with QS gene expression analysis, and the results showed that, indeed, at low concentrations, curvularin selectively antagonized RhlR; in contrast, it negligibly antagonized LasR only when applied at a high concentration. The exclusive RhlR antagonizing activity of curvularin at low concentrations was confirmed using QS mutants; specifically, curvularin at low concentrations inhibited pyocyanin and rhamnolipid production by selectively antagonizing N-butanoyl homoserine lactone (BHL)-activated RhlR. Moreover, by targeting RhlR, curvularin reduced the in vivo virulence of wild-type P. aeruginosa as well as lasR mutants in Caenorhabditis elegans. Overall, low-concentration curvularin is a pure RhlR antagonist in P. aeruginosa, and to the best of our knowledge, this is the first report describing an RhlR antagonist from natural resources. Hence, curvularin has great potential for the development of chronic P. aeruginosa infection therapeutics and for the study of RhlR function in the complex QS system.

Molecular Mechanisms and Applications of N-Acyl Homoserine Lactone-Mediated Quorum Sensing in Bacteria

“…In another study, lasR , lasI , and rhlI mutant strains diminished pathogenicity in a burn wound mouse model [ 85 ]. LasR mutants also demonstrated lower pathogenicity in the Arabidopsis nematode [ 86 ]. These findings imply that lasR-dependent virulence factor expression is critical for the establishment of P. aeruginosa infection.…”

Research Progress on the Combination of Quorum-Sensing Inhibitors and Antibiotics against Bacterial Resistance

“…The regulation of the QS system, an important intercellular communication system, is performed by the release of autoinducers (AIs). AIs can track changes in bacterial density, control the expression of bacterial-related genes [7,8], and adjust the bacteria to adapt to the environment by controlling bioluminescence, pigment synthesis, biofilm formation, and the secretion of virulence factors [9]. Interestingly, the formation of bacterial biofilms causes drug resistance in bacteria [10].…”