Allicin Inhibits The Production Of Pseudomonas Aeruginosa Extracellular Secretory Virulence Factors By Suppressing The Rhl And Pqs Quorum-sensing Systems

Pseudomonas aeruginosa(P.aeruginosa),is a most infectious non-fermenting Gram-negative bacillus that secretes a variety of virulence factors such as exotoxin A,pyocyanin,protease,elastase,and forming biofilms to help them adapt to the new environment and invade the host.PA exhibits its high infectivity and toxic effects through the virulence phenotype controlled by the quorum sensing(QS)system.QS system is a cell density-dependent regulation network of bacterial virulence.It contains three circuits:two N-acylhomoserine lactone(AHL)regulatory circuits,known as las and rhl QS systems,and one quinolone signaling circuit,termed pqs system.In las and rhl systems,the LasI and RhlI synthetases catalyze the synthesis of signal molecules N-(3-oxododecanoyl)-l-homoserine lactone(3-oxo-C12-HSL)and N-butanoyl-l-homoserine lactone(C4-HSL),respectively.In quinolone(pqs)system,the pqsABCD gene products catalyze the synthesis of2-Heptyl-4-quinolone(HHQ),a precursor of 2-heptyl-3-hydroxy-4-quinolone(PQS).Then,HHQ is converted to PQS by monooxygenase enzyme PqsH.When the 3-oxo-C12-HSLs,C4-HSLs and PQS signal molecules interact with their cognate transcriptional regulators LasR,RhlR and PqsR,respectively,the complex modulates its down-stream genes related to the synthesis of bacterial virulence,such as elastase,pyocyanin,and pyoverdine et al.Although the quorum sensing systems which consist of las,rhl and pqs system feedback loop regulate the extracellular secretory virulence factors,the diverse systems regulate altered virulence factors respectively.Both las and rhl systems control the expression of elastase and alkaline protease,while the rhl sysyem modulates the expression of haemolysin,pyocyanin,and the pqs system modulates the secretion of siderophore.In view of the critical role of QS system in P.aeruginosa virulence regulation,many studies have focused on developing the effective methods to prevent P.aeruginosa infectionsby targeting QS system,such as the chemical QS inhibitor G1 and the quorum quenching enzyme,AHL-lactonase(AiiA).In addition,researchers have also shown that the compounds extracted from the plant or fruit exert anti-P.aeruginosa activity by disrupting the QS circuits.Allicin,derived from garlic,has been shown to inhibit virulence factor production and biofilm formation in P.aeruginosa.Nevertheless,the underlying mechanisms regarding allicin-mediated regulation of P.aeruginosa virulence remain unclear.In this study,we investigated the possible mechanisms of allicin-mediated virulence regulation in P.aeruginosa.Our results show that allicin attenuates the production of P.aeruginosa virulence-associated factors,such as elastase,pyocyanin,pyoverdine and rhamnolipids,through inhibiting rhl and pqs quorum-sensing systems.Further analysis reveales that rhl and pqs systems play different roles during the allicin-mediated regulation process.The pqs system regulates pyoverdine directly,while rhl system mainly regulates elastase,pyocyanin and rhamnolipids.Pqs system also can regulate elastase,pyocyanin and rhamnolipids through modulating rhl system mediately.Taken together,these results provide further evidence regarding the potential role and mechanism of allicin as a therapeutic agent in controlling P.aeruginosa infection.