Enzymatic hydrolysis of molecules associated with bacterial quorum sensing using an acyl homoserine lactonase from a novel Bacillus thuringiensis strain

Quorum quenching activity of Bacillus cereus isolate 30b confers antipathogenic effects in Pseudomonas aeruginosa

Background: Quorum quenching, the interference of a Quorum sensing (QS) system that contributes to the pathogenesis through triggering the production of various virulence determinants, is among the newly suggested antivirulence strategies. Purpose: This study aimed at screening of N-Acyl homoserine lactonase activity from local bacterial isolate, and investigating its effect on Pseudomonas aeruginosa (P. aeruginosa) virulence and biofilm formation. Materials and methods: Soil bacteria were screened for aiiA gene coding for lactonase enzyme by Polymerase Chain reaction and sequencing of aiiA gene homologs. Lactonase activity and spectrum were assessed in the cell-free lysate by well diffusion assay using Agrobacterium tumafaciens KYC55. A bacterial isolate showing the highest N-acyl-homoserine lactones degradation percentage was identified by gene amplification and sequencing of the 16S rRNA gene and its aiiA gene homolog. High performance liquid chromatography was used to confirm N-acyl-homoserine lactone degradation. The effect of cell-free lysate on the biofilm formation ability and cytotoxicity of P. aeruginosa PAO1 and P. aeruginosa clinical isolates from different clinical sources were assessed by static microtiter plate and viability assay, respectively Results: Lactonase gene and activity were identified in three Bacillus spp. isolates. They showed broad catalytic activities against tested N-acyl-homoserine lactones. However, The lactonase activity in the cell- free lysate of isolate 30b showed the highest significant degradation percentage on all tested signals; N-butanoyl-L-homoserine lactone (71%), N-hexanoyl-l-homoserine lactone (100%), N-decanoyl-homoserine lactone (100%), N-(3-oxohexanoyl)-L-homoserine lactone (37.5%), N-(oxodecanoyl)-L-homoserine lactone (100%), and N-(3-oxododecanoyl)-L-homoserine lactone (100%). Alignment of the amino acid sequences of AiiA protein of isolate 30b showed 96% identity with Bacillus cereus (B. cereus) homologous lactonases in the GenBank database, and the isolate was designated as B. cereus isolate 30b. Cell-free lysate of B. cereus isolate 30b reduced biofilm formation significantly in 93% of P. aeruginosa isolates. The highest mean percentage of reduction in the biofilm was 86%. Moreover, the viability percentage of human lung carcinoma A549 cells infected by P. aeruginosa and treated with cell-free lysate of B. cereus isolate 30b increased up to 15%. Conclusion: The results of this study highlight the potential of lactonases as a promising strategy to combat Pseudomonas aeruginosa virulence.

Quorum quenching activity of AiiA lactonase KMMI17 from endophytic Bacillus thuringiensis KMCL07 on AHL- mediated pathogenic phenotype in Pseudomonas aeruginosa

Virulence pathways in gram-negative pathogenic bacteria are regulated by quorum sensing mechanisms, through the production and sensing of N-acylhomoserine lactone (AHL) signal molecules. Enzymatic degradation to disrupt quorum-sensing in these bacteria could pave the way for the new development in decreasing resistance strains and are of significant interest for clinical, agricultural, and industrial applications. Isolated endophytic Bacillus thuringiensis strain KMCL07 showing quorum quenching activity on Pseudomonas aeruginosa PAO1 has been studied. AiiA lactonase _KMMI17 identified belongs to metallo- β-lactamase superfamily preserving conserved regions of ¹⁰⁶HXDH-59 amino acids-H¹⁶⁹-21 amino acids-D¹⁹¹ motif, significantly inhibits the biofilm formation and attenuates virulence factor pyocyanin production of PAO1. Insilico molecular docking analysis of lactonase _KMMI17 using alternative catalytic site (PDB entry: 3DHA) with the AHL-based QS system regulators of PAO-1, C4 AHL, C6 AHL and 3-oxo-C12 AHL molecules showed good binding affinity between the protein and ligands, Phe111 and Tyr198 residues plays an important role in binding them. Crude enzyme extract was found to have K_m value for C6-HSL: 134.2702 ± 34.83 μM^-1, C4-HSL: 308.217 ± 139.9 μM^-1 and 3-oxo-C12-HSL: 760.463 ± 251.3 μM^-1. LCMS analysis confirms the degradation activity of lactonase _KMMI17 on AHL molecules and its hydrolytic process, which indicates the potential application of lactonase _KMMI17 as a biocontrol agent or an anti-pathogenic drug.

Discovering, Characterizing, and Applying Acyl Homoserine Lactone-Quenching Enzymes to Mitigate Microbe-Associated Problems Under Saline Conditions

Quorum quenching (QQ) is proposed as a new strategy for mitigating microbe-associated problems (e.g., fouling, biocorrosion). However, most QQ agents reported to date have not been evaluated for their quenching efficacies under conditions representative of seawater desalination plants, cooling towers or marine aquaculture. In this study, bacterial strains were isolated from Saudi Arabian coastal environments and screened for acyl homoserine lactone (AHL)-quenching activities. Five AHL quenching bacterial isolates from the genera Pseudoalteromonas, Pontibacillus, and Altererythrobacter exhibited high AHL-quenching activity at a salinity level of 58 g/L and a pH of 7.8 at 50°C. This result demonstrates the potential use of these QQ bacteria in mitigating microbe-associated problems under saline and alkaline conditions at high (>37°C) temperatures. Further characterizations of the QQ efficacies revealed two bacterial isolates, namely, Pseudoalteromonas sp. L11 and Altererythrobacter sp. S1-5, which could possess enzymatic QQ activity. The genome sequences of L11 and S1-5 with a homologous screening against reported AHL quenching genes suggest the existence of four possible QQ coding genes in each strain. Specifically, two novel AHL enzymes, AiiA_S1-5 and Est_S1-5 from Altererythrobacter sp. S1-5, both contain signal peptides and exhibit QQ activity over a broad range of pH, salinity, and temperature values. In particular, AiiA_S1-5 demonstrated activity against a wide spectrum of AHL molecules. When tested against three bacterial species, namely, Aeromonas hydrophila, Pseudomonas aeruginosa, and Vibrio alginolyticus, AiiA_S1-5 was able to inhibit the motility of all three species under saline conditions. The biofilm formation associated with P. aeruginosa was also significantly inhibited by AiiA_S1-5. AiiA_S1-5 also reduced the quorum sensing-mediated virulence traits of A. hydrophila, P. aeruginosa, and V. alginolyticus during the mid and late exponential phases of cell growth. The enzyme did not impose any detrimental effects on cell growth, suggesting a lower potential for the target bacterium to develop resistance over long-term exposure. Overall, this study suggested that some QQ enzymes obtained from the bacteria that inhabit saline environments under high temperatures have potential applications in the mitigation of microbe-associated problems.

Quorum sensing is a language of chemical signals and plays an ecological role in algal-bacterial interactions

Algae are ubiquitous in the marine environment, and the ways in which they interact with bacteria are of particular interest in marine ecology field. The interactions between primary producers and bacteria impact the physiology of both partners, alter the chemistry of their environment, and shape microbial diversity. Although algal-bacterial interactions are well known and studied, information regarding the chemical-ecological role of this relationship remains limited, particularly with respect to quorum sensing (QS), which is a system of stimuli and response correlated to population density. In the microbial biosphere, QS is pivotal in driving community structure and regulating behavioral ecology, including biofilm formation, virulence, antibiotic resistance, swarming motility, and secondary metabolite production. Many marine habitats, such as the phycosphere, harbour diverse populations of microorganisms and various signal languages (such as QS-based autoinducers). QS-mediated interactions widely influence algal-bacterial symbiotic relationships, which in turn determine community organization, population structure, and ecosystem functioning. Understanding infochemicals-mediated ecological processes may shed light on the symbiotic interactions between algae host and associated microbes. In this review, we summarize current achievements about how QS modulates microbial behavior, affects symbiotic relationships, and regulates phytoplankton chemical ecological processes. Additionally, we present an overview of QS-modulated co-evolutionary relationships between algae and bacterioplankton, and consider the potential applications and future perspectives of QS.