Discovery of novel β-turn mimetic-based peptides as novel quorum sensing inhibitors of gram-negative bacteria

Identification of novel phenylalanine derivatives bearing a hydroxamic acid moiety as potent quorum sensing inhibitors

Phenylalanine derivatives are a well-known small moiety responsible for controlling the virulence factors of several bacteria. Herein, for the first time, we report novel structures of phenylalanine derivatives bearing a hydroxamic acid moiety which were designed, synthesized, and evaluated for use as quorum sensing inhibitors. Biological results reveal that six compounds showed good quorum sensing inhibitors properties with an IC50 ranging from 7.12 ± 2.11 μM-92.34 ± 2.09 μM (4NPO, a reference compound, IC50 = 29.13 ± 0.88 μM). In addition, three out of the six compounds (4a, 4c, 4h) showed strong anti-biofilm formation and CviR inhibitory activity when compared to that of 4NPO. These biological data were also confirmed by computational studies. In this series of compounds, 4h is the most promising compound for future drug development targeting quorum sensing. Our results concluded that the fragment-based drug design is a good approach for the discovery of novel quorum-sensing inhibitors in the future.

Bacterial virulence regulation through soluble peptidoglycan fragments sensing and response: knowledge gaps and therapeutic potential

Given the growing clinical-epidemiological threat posed by the phenomenon of antibiotic resistance, new therapeutic options are urgently needed, especially against top nosocomial pathogens such as those within the ESKAPE group. In this scenario, research is pushed to explore therapeutic alternatives and, among these, those oriented toward reducing bacterial pathogenic power could pose encouraging options. However, the first step in developing these antivirulence weapons is to find weak points in the bacterial biology to be attacked with the goal of dampening pathogenesis. In this regard, during the last decades some studies have directly/indirectly suggested that certain soluble peptidoglycan-derived fragments display virulence-regulatory capacities, likely through similar mechanisms to those followed to regulate the production of several β-lactamases: binding to specific transcriptional regulators and/or sensing/activation of two-component systems. These data suggest the existence of intra- and also intercellular peptidoglycan-derived signaling capable of impacting bacterial behavior, and hence likely exploitable from the therapeutic perspective. Using the well-known phenomenon of peptidoglycan metabolism-linked β-lactamase regulation as a starting point, we gather and integrate the studies connecting soluble peptidoglycan sensing with fitness/virulence regulation in Gram-negatives, dissecting the gaps in current knowledge that need filling to enable potential therapeutic strategy development, a topic which is also finally discussed.

Evolution of two-component quorum sensing systems

Quorum sensing (QS) is a cell-to-cell communication system that enables bacteria to coordinate their gene expression depending on their population density, via the detection of small molecules called autoinducers. In this way bacteria can act collectively to initiate processes like bioluminescence, virulence and biofilm formation. Autoinducers are detected by receptors, some of which are part of two-component signal transduction systems (TCS), which comprise of a (usually membrane-bound) sensor histidine kinase (HK) and a cognate response regulator (RR). Different QS systems are used by different bacterial taxa, and their relative evolutionary relationships have not been extensively studied. To address this, we used the Kyoto Encyclopedia of Genes and Genomes (KEGG) database to identify all the QS HKs and RRs that are part of TCSs and examined their conservation across microbial taxa. We compared the combinations of the highly conserved domains in the different families of receptors and response regulators using the Simple Modular Architecture Research Tool (SMART) and KEGG databases, and we also carried out phylogenetic analyses for each family, and all families together. The distribution of the different QS systems across taxa, indicates flexibility in HK–RR pairing and highlights the need for further study of the most abundant systems. For both the QS receptors and the response regulators, our results indicate close evolutionary relationships between certain families, highlighting a common evolutionary history which can inform future applications, such as the design of novel inhibitors for pathogenic QS systems.

2-Difluoromethylpyridine as a bioisosteric replacement of pyridine-N-oxide: the case of quorum sensing inhibitors

Herein, we demonstrate that 2-difluoromethylpyridine is a bioisosteric replacement of pyridine-N-oxide. Using the quorum sensing inhibitor 4NPO as a model compound, a library of 2-difluoromethylpyridine derivatives was designed, synthesized, and evaluated toward quorum sensing activity, biofilm formation, anti-violacein activity, and protease activity. As a result, compounds 1 (IC50 of 35 ± 1.12 μM), 5 (IC50 of 19 ± 1.01 μM), and 6 (IC50 of 27 ± 0.67 μM) showed a similar or better activity in comparison to 4NPO (IC50 of 33 ± 1.12 μM) in a quorum sensing system of Pseudomonas aeruginosa. In addition, compounds 1, 5, 6, and 4NPO showed good antibiofilm biomass of Pseudomonas aeruginosa and reduced violacein production in Chromobacterium violaceum. In terms of protease activity, compounds 1, 5, and 6 showed significant activity compared to 4NPO. Overall, the replacement of pyridine-N-oxide by 2-difluoromethylpyridine enhances the activity of the model compound, which could open a new path for bioisosteric replacement in drug discovery and development.

Fluorinated quorum sensing inhibitors: enhancement of potency through conformational control

The bacterial quorum sensing (QS) system is a target for non-lethal antibacterial agents that do not encourage the development of resistance. QS inhibitors commonly contain a polar "head" moiety and a lipidic "tail" moiety. In this work, we synthesised novel QS inhibitor candidates in which the lipidic "tail" is decorated with stereospecifically positioned fluorine atoms. The presence of fluorine is shown to bias the molecules into distinctive conformations that are pre-organised for binding to the QS receptor. This translates into significant increases in QS inhibitory potency.

A novel bis(pyrazolyl)methane compound as a potential agent against Gram-positive bacteria

This study was designed to propose alternative therapeutic compounds to fight against bacterial pathogens. Thus, a library of nitrogen-based compounds bis(triazolyl)methane (1T–7T) and bis(pyrazolyl)methane (1P–11P) was synthesised following previously reported methodologies and their antibacterial activity was tested using the collection strains of Staphylococcus aureus, Enterococcus faecalis, Escherichia coli, and Pseudomonas aeruginosa. Moreover, the novel compound 2P was fully characterized by IR, UV–Vis and NMR spectroscopy. To evaluate antibacterial activity, minimal inhibitory concentrations (MICs), minimal bactericidal concentrations (MBCs), minimum biofilm inhibitory concentrations (MBICs), and minimum biofilm eradication concentrations (MBECs) assays were carried out at different concentrations (2–2000 µg/mL). The MTT assay and Resazurin viability assays were performed in both human liver carcinoma HepG2 and human colorectal adenocarcinoma Caco-2 cell lines at 48 h. Of all the synthesised compounds, 2P had an inhibitory effect on Gram-positive strains, especially against S. aureus. The MIC and MBC of 2P were 62.5 and 2000 µg/mL against S. aureus, and 250 and 2000 µg/mL against E. faecalis, respectively. However, these values were > 2000 µg/mL against E. coli and P. aeruginosa. In addition, the MBICs and MBECs of 2P against S. aureus were 125 and > 2000 µg/mL, respectively, whereas these values were > 2000 µg/mL against E. faecalis, E. coli, and P. aeruginosa. On the other hand, concentrations up to 250 µg/mL of 2P were non-toxic doses for eukaryotic cell cultures. Thus, according to the obtained results, the 2P nitrogen-based compound showed a promising anti-Gram-positive effect (especially against S. aureus) both on planktonic state and biofilm, at non-toxic concentrations.

“Left-hand strategy” for the design, synthesis and discovery of novel triazole–mercaptobenzothiazole hybrid compounds as potent quorum sensing inhibitors and anti-biofilm formation of Pseudomonas aeruginosa

Triazole–benzothiazole hybrids as potent quorum sensing inhibitors and antibiofilm formation of Gram-negative bacteria.