Targeting the G-quadruplex as a novel strategy for developing antibiotics against hypervirulent drug-resistant Staphylococcus aureus.

BACKGROUND: The rapid emergence of multiple drug-resistant (MDR) bacterial pathogens and the lack of a novel antibiotic pipeline pose a serious threat to global healthcare. The limited number of established targets further restricts the identification of novel antibiotics to treat life-threatening MDR infections caused by Staphylococcus aureus strains. Therefore, novel targets for developing antibiotics are urgently required. In this study, we hypothesized that the G-quadruplex (G4)-binding ligands can be used as novel antibiotics as their binding can possibly downregulate/block the expression of vital genes.

METHODS: To test this, first we screened the antibiotic properties of representative G4-binding ligands against hypervirulent and MDR S. aureus USA300 and determined the in vitro and in vivo antibacterial activity; and proposed the mechanism of action by applying various microbiological, infection, microscopic, and biophysicochemical techniques.

RESULTS: Herein, among screened G4-binding ligands, N-methyl mesoporphyrin IX (NMM) showed the highest antibacterial activity against S. aureus USA300. NMM exhibited a minimum inhibitory concentration (MIC) of 5 μM against S. aureus USA300, impacting cell division and the cell wall by repressing the expressions of genes in the division cell wall (dcw) gene cluster. Genome-wide bioinformatics analysis of G4 motifs and their mapping on S. aureus genome, identified the presence of G4-motif in the promoter of mraZ, a conserved master regulator of the dcw cluster regulating the coordinated cell division and cell wall synthesis. Physicochemical assessments using UV-visible, circular dichroism, and nuclear magnetic resonance spectroscopy confirmed that the G4-motif present in the mraZ promoter formed an intramolecular parallel G4 structure, interacting with NMM. In vivo reporter followed by coupled in vitro transcription/translation (IVT) assays confirmed the role of mraZ G4 as a target interacting NMM to impose extreme antibacterial activity against both the gram-positive and -negative bacteria. In-cell and in vivo validation of NMM using RAW264.7 cells and Galleria mellonella; respectively, demonstrated that NMM exhibited superior antibiotic activity compared to well-established antibiotics, with no observed cytotoxicity.

CONCLUSIONS: In summary, the current study identified NMM as a broad-spectrum potent antibacterial agent and elucidated its plausible mechanism of action primarily by targeting G4-motif in the mraZ promoter of the dcw gene cluster.