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The quorum sensing (QS) system of bacteria helps them to communicate with each other in a density-dependent manner and regulates pathogenicity. The concentrations of autoinducers, peptides, and signaling factors are required for determining the expression of virulence factors in many pathogens. The QS signals of the pathogen are regulated by the signal transduction pathway. The binding of signal molecules to its cognate receptor brings changes in the structure of the receptor, makes it more accessible to the DNA, and thus regulates diverse expression patterns, including virulence factors. Degrading the autoinducer molecules or disturbing the quorum sensing network could be exploited to control the virulence of the pathogen while avoiding multidrug-resistant phenotypes. The rhizosphere is a tremendous source of beneficial microbes that has not yet been explored properly for its anti-quorum sensing potential. Lelliottia amnigena causes soft rot diseases in onion, potato, and other species. The present investigation was carried out with the aim of isolating the anti-quorum sensing metabolites and elucidating their role in controlling the virulence factors of the pathogen by performing a maceration assay. The ethyl acetate extracts of various bacteria are promising for violacein inhibition assay using Chromobacterium violaceum MTCC2656 and pyocyanin inhibition of Pseudomonas aeruginosa MTCC2297. Therefore, the extract was used to deduce its role in attenuation of soft rot in potato, carrot, and cucumber. The maximum reduction of macerated tissue in carrot, potato, and cucumber was given by Bacillus cereus RC1 at 91.22, 97.59, and 88.78%, respectively. The concentration-dependent inhibition of virulence traits was observed during the entire experiment. The quorum quenching potential of the bacterial extract was used to understand the regulatory metabolites. The data of the diffusible zone and gas chromatography–mass spectrometry (GC–MS) analysis showed that diketopiperazines, viz . Cyclo( d -phenylalanyl- l -prolyl), Cyclo Phe-Val, Cyclo(Pro-Ala), Cyclo( l -prolyl- l -valine), Cyclo (Leu-Leu), and Cyclo(-Leu-Pro), are prominent metabolites that could modulate the pathogenicity in L. amnigena RCE. The interaction of bacterial extracts regulates various metabolites of the pathogens during their growth in liquid culture compared to their control counterparts. This study might help in exploiting the metabolites from bacteria to control the pathogens, with concurrent reduction in the pathogenicity of the pathogens without developing antibiotic resistance.

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The quorum-sensing (QS) cascade is responsible for the colonization and phenotypic behavior of the pathogenic organism and the regulation of diverse signal molecules. The disruption of the quorum-sensing system is an effective strategy to overcome the possibility of antibiotic resistance development in the pathogen. The quorum quenching does not kill the microbes. Instead, it hinders the expression of pathogenic traits. In the present experiment, Pseudomonas aeruginosa RKC1 was used to extract the metabolites responsible for quorum-sensing inhibition in soft rot pathogen Lelliottia amnigena RCE. During the initial screening, P. aeruginosa RKC1 was found to be most promising and inhibits violacein of Chromobacterium violaceum MTCC2656 pyocyanin, swarming-swimming motility of P. aeruginosa MTCC2297. The characterization of metabolites produced by the microbes which are responsible for quorum-sensing inhibition through GC-MS is very scarce in scientific literature. The ethyl acetate extract of P. aeruginosa RKC1 inhibits biofilm formation of L. amnigena RCE while inhibiting growth at higher concentrations. The GC-MS analysis suggested that Cyclic dipeptides (CDPs) such as Cyclo (L-prolyl-L-valine), Cyclo (Pro-Leu), and Cyclo(D-phenylalanyl-L-prolyl) were predominantly found in the ethyl acetate extract of the P. aeruginosa RKC1 (93.72%). This diketopiperazine (DKPs) exhibited quorum-sensing inhibition against the pathogen in liquid media during the active growth phase and regulated diverse metabolites of the pathogen. Moreover, the metabolites data from the clear zone around wells showed a higher concentration of DKSs (9.66%) compared to other metabolites. So far, very few reports indicate the role of DKPs or CDPs in inhibiting the quorum-sensing system in plant pathogenic bacteria. This is one such report that exploits metabolites of P. aeruginosa RKC1. The present investigation provided evidence to use quorum-sensing inhibitor metabolites, to suppress microbes' pathogenesis and thus develop an innovative strategy to overcome antibiotic resistance.

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