Impact of the structure-activity relationship of AHL analogues on quorum sensing in Gram-negative bacteria

Amide bioisosteric replacement in the design and synthesis of quorum sensing modulators

The prevention or control of bacterial infections requires continuous search for novel approaches among which bacterial quorum sensing inhibition is considered as a complementary antibacterial strategy. Quorum sensing, used by many different bacteria, functions through a cell-to-cell communication mechanism relying on chemical signals, referred to as autoinducers, such as N-acyl homoserine lactones (AHLs) which are the most common chemical signals in this system. Designing analogs of these autoinducers is one of the possible ways to interfere with quorum sensing. Since bioisosteres are powerful tools in medicinal chemistry, targeting analogs of AHLs or other signal molecules and mimics of known QS modulators built on amide bond bioisosteres is a relevant strategy in molecular design and synthetic routes. This review highlights the application of amide bond bioisosteric replacement in the design and synthesis of novel quorum sensing inhibitors.

Roles and regulation of quorum sensing in anaerobic granular sludge: Research status, challenges, and perspectives

Anaerobic granular sludge (AnGS) has a complex and important internal microbial communication system due to its unique microbial layered structure. As a concentration-dependent communication system between bacterial cells through signal molecules, QS (quorum sensing) is widespread in AnGS and exhibits great potential to regulate microbial behaviors. Therefore, the universal functions of QS in AnGS have been systematically summarized in this paper, including the influence on the metabolic activity, physicochemical properties, and microbial community of AnGS. Subsequently, the common QS-based AnGS regulation approaches are reviewed and analyzed comprehensively. The regulation mechanism of QS in AnGS is analyzed from two systems of single bacterium and mixed bacteria. This review can provide a comprehensive understanding of QS functions in AnGS systems, and promote the practical application of QS-based strategies in optimization of AnGS treatment process.

The Autoinducer N-Octanoyl-L-Homoserine Lactone (C8-HSL) as a Potential Adjuvant in Vaccine Formulations

Autoinducers AI-1 and AI-2 play an important role in bacterial quorum sensing (QS), a form of chemical communication between bacteria. The autoinducer N-octanoyl-L-Homoserinehomoserine lactone (C8-HSL) serves as a major inter- and intraspecies communicator or 'signal', mainly for Gram-negative bacteria. C8-HSL is proposed to have immunogenic properties. The aim of this project is to evaluate C8-HSL as a potential vaccine adjuvant. For this purpose, a microparticulate formulation was developed. The C8-HSL microparticles (MPs) were formulated by a water/oil/water (W/O/W) double-emulsion solvent evaporation method using PLGA (poly (lactic-co-glycolic acid)) polymer. We tested C8-HSL MPs with two spray-dried bovine serum albumin (BSA)-encapsulated bacterial antigens: colonization factor antigen I (CFA/I) from Escherichia coli (E. coli.) and the inactive protective antigen (PA) from Bacillus anthracis (B. anthracis). We formulated and tested C8-HSL MP to determine its immunogenicity potential and its ability to serve as an adjuvant with particulate vaccine formulations. An in vitro immunogenicity assessment was performed using Griess's assay, which indirectly measures the nitric oxide radical (NOˑ) released by dendritic cells (DCs). The C8-HSL MP adjuvant was compared with FDA-approved adjuvants to determine its immunogenicity potential. C8-HSL MP was combined with particulate vaccines for measles, Zika and the marketed influenza vaccine. The cytotoxicity study showed that MPs were non-cytotoxic toward DCs. Griess's assay showed a comparable release of NOˑ from DCs when exposed to CFA and PA bacterial antigens. Nitric oxide radical (NOˑ) release was significantly higher when C8-HSL MPs were combined with particulate vaccines for measles and Zika. C8-HSL MPs showed immunostimulatory potential when combined with the influenza vaccine. The results showed that C8-HSL MPs were as immunogenic as FDA-approved adjuvants such as alum, MF59, and CpG. This proof-of-concept study showed that C8-HSL MP displayed adjuvant potential when combined with several particulate vaccines, indicating that C8-HSL MPs can increase the immunogenicity of both bacterial and viral vaccines.

Microbial Genomics: Innovative Targets and Mechanisms

Multidrug resistance (MDR) has become an increasing threat to global health because bacteria can develop resistance to antibiotics over time. Scientists worldwide are searching for new approaches that go beyond traditional antibiotic discovery and development pipelines. Advances in genomics, however, opened up an unexplored therapeutic opportunity for the discovery of new antibacterial agents. Genomic approaches have been used to discover several novel antibiotics that target critical processes for bacterial growth and survival, including histidine kinases (HKs), LpxC, FabI, peptide deformylase (PDF), and aminoacyl-tRNA synthetases (AaRS). In this review, we will discuss the use of microbial genomics in the search for innovative and promising drug targets as well as the mechanisms of action for novel antimicrobial agents. We will also discuss future directions on how the utilization of the microbial genomics approach could improve the odds of antibiotic development having a more successful outcome.

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

Microbial biodiversity includes biotic and abiotic components that support all life forms by adapting to environmental conditions. Climate change, pollution, human activity, and natural calamities affect microbial biodiversity. Microbes have diverse growth conditions, physiology, and metabolism. Bacteria use signaling systems such as quorum sensing (QS) to regulate cellular interactions via small chemical signaling molecules which also help with adaptation under undesirable survival conditions. Proteobacteria use acyl-homoserine lactone (AHL) molecules as autoinducers to sense population density and modulate gene expression. The LuxI-type enzymes synthesize AHL molecules, while the LuxR-type proteins (AHL transcriptional regulators) bind to AHLs to regulate QS-dependent gene expression. Diverse AHLs have been identified, and the diversity extends to AHL synthases and AHL receptors. This review comprehensively explains the molecular diversity of AHL signaling components of Pseudomonas aeruginosa, Chromobacterium violaceum, Agrobacterium tumefaciens, and Escherichia coli. The regulatory mechanism of AHL signaling is also highlighted in this review, which adds to the current understanding of AHL signaling in Gram-negative bacteria. We summarize molecular diversity among well-studied QS systems and recent advances in the role of QS proteins in bacterial cellular signaling pathways. This review describes AHL-dependent QS details in bacteria that can be employed to understand their features, improve environmental adaptation, and develop broad biomolecule-based biotechnological applications.

Effect of Autoinducer-2 Quorum Sensing Inhibitor on Interspecies Quorum Sensing

Bacterial drug resistance caused by overuse and misuse of antibiotics is common, especially in clinical multispecies infections. It is of great significance to discover novel agents to treat clinical bacterial infections. Studies have demonstrated that autoinducer-2 (AI-2), a signal molecule in quorum sensing (QS), plays an important role in communication among multiple bacterial species and bacterial drug-resistance. Previously, 14 AI-2 inhibited compounds were selected through virtual screening by using the AI-2 receptor protein LuxP as a target. Here, we used Vibrio harveyi BB170 as a reporter strain for the preliminary screening of 14 inhibitors and compound Str7410 had higher AI-2 QS inhibition activity (IC₅₀ = 0.3724 ± 0.1091 μM). Then, co-culture of Pseudomonas aeruginosa PAO1 with Staphylococcus aureus ATCC 25923 was used to evaluate the inhibitory effects of Str7410 on multispecies infection in vitro and in vivo. In vitro, Str7410 significantly inhibited the formation of mixed bacterial biofilms. Meanwhile, the combination of Str7410 with meropenem trihydrate (MEPM) significantly improved the susceptibility of mixed-species-biofilm cells to the antibiotic. In vivo, Str7410 significantly increased the survival rate of wild-type Caenorhabditis elegans N2 co-infected by P. aeruginosa PAO1 and S. aureus ATCC 25923. Real-time quantitative PCR analysis showed that Str7410 reduced virulence factor (pyocyanin and elastase) production and swarming motility of P. aeruginosa PAO1 by downregulating the expression of QS-related genes in strain PAO1 in co-culture with S. aureus ATCC 25923. Compound Str7410 is a candidate agent for treating drug-resistant multispecies infections. The work described here provides a strategy for discovering novel antibacterial drugs.

Plant compounds and nonsteroidal anti-inflammatory drugs interfere with quorum sensing in Chromobacterium violaceum

Chromobacterium violaceum is a Gram-negative, saprophytic bacterium that can infect humans and its virulence may be regulated by quorum sensing via N-acyl homoserine lactones. A virtual screening study with plant compounds and nonsteroidal anti-inflammatory drugs for inhibition of C. violaceum quorum sensing system has been performed. In vitro evaluation was done to validate the in silico results. Molecular docking showed that phytol, margaric acid, palmitic acid, dipyrone, ketoprofen, and phenylbutazone bound to structures of CviR proteins of different C. violaceum strains. Phytol presented higher binding affinities than AHLs and furanones, recognized inducers, and inhibitors of quorum sensing, respectively. When tested in vitro, phytol at a non-inhibitory concentration was the most efficient tested compound to reduce phenotypes regulated by quorum sensing. The results indicate that in silico compound prospection to inhibit quorum sensing may be a good tool for finding alternative lead molecules.

Heterocyclic Chemistry Applied to the Design of N-Acyl Homoserine Lactone Analogues as Bacterial Quorum Sensing Signals Mimics

N-acyl homoserine lactones (AHLs) are small signaling molecules used by many Gram-negative bacteria for coordinating their behavior as a function of their population density. This process, based on the biosynthesis and the sensing of such molecular signals, and referred to as Quorum Sensing (QS), regulates various gene expressions, including growth, virulence, biofilms formation, and toxin production. Considering the role of QS in bacterial pathogenicity, its modulation appears as a possible complementary approach in antibacterial strategies. Analogues and mimics of AHLs are therefore biologically relevant targets, including several families in which heterocyclic chemistry provides a strategic contribution in the molecular design and the synthetic approach. AHLs consist of three main sections, the homoserine lactone ring, the central amide group, and the side chain, which can vary in length and level of oxygenation. The purpose of this review is to summarize the contribution of heterocyclic chemistry in the design of AHLs analogues, insisting on the way heterocyclic building blocks can serve as replacements of the lactone moiety, as a bioisostere for the amide group, or as an additional pattern appended to the side chain. A few non-AHL-related heterocyclic compounds with AHL-like QS activity are also mentioned.

Microbial Genomics as a Catalyst for Targeted Antivirulence Therapeutics

Virulence arresting drugs (VAD) are an expanding class of antimicrobial treatment that act to “disarm” rather than kill bacteria. Despite an increasing number of VAD being registered for clinical use, uptake is hampered by the lack of methods that can identify patients who are most likely to benefit from these new agents. The application of pathogen genomics can facilitate the rational utilization of advanced therapeutics for infectious diseases. The development of genomic assessment of VAD targets is essential to support the early stages of VAD diffusion into infectious disease management. Genomic identification and characterization of VAD targets in clinical isolates can augment antimicrobial stewardship and pharmacovigilance. Personalized genomics guided use of VAD will provide crucial policy guidance to regulating agencies, assist hospitals to optimize the use of these expensive medicines and create market opportunities for biotech companies and diagnostic laboratories.

Targeted and untargeted quantification of quorum sensing signalling molecules in bacterial cultures and biological samples via HPLC-TQ MS techniques

Quorum sensing (QS) is the ability of some bacteria to detect and to respond to population density through signalling molecules. QS molecules are involved in motility and cell aggregation mechanisms in diseases such as sepsis. Few biomarkers are currently available to diagnose sepsis, especially in high-risk conditions. The aim of this study was the development of new analytical methods based on liquid chromatography-mass spectrometry for the detection and quantification of QS signalling molecules, including N-acyl homoserine lactones (AHL) and hydroxyquinolones (HQ), in biofluids. Biological samples used in the study were Pseudomonas aeruginosa bacterial cultures and plasma from patients with sepsis. We developed two MS analytical methods, based on neutral loss (NL) and product ion (PI) experiments, to identify and characterize unknown AHL and HQ molecules. We then established a multiple-reaction-monitoring (MRM) method to quantify specific QS compounds. We validated the HPLC-MS-based approaches (MRM-NL-PI), and data were in accord with the validation guidelines. With the NL and PI MS-based methods, we identified and characterized 3 and 13 unknown AHL and HQ compounds, respectively, in biological samples. One of the newly found AHL molecules was C12-AHL, first quantified in Pseudomonas aeruginosa bacterial cultures. The MRM quantitation of analytes in plasma from patients with sepsis confirmed the analytical ability of MRM for the quantification of virulence factors during sepsis.