Quinuclidin-3-one-based quaternary ammonium compounds as potent antibacterials against <i>Staphylococcus aureus </i>and <i>Listeria monocytogenes</i>

Bacterial resistance and the cytotoxicity of conventional quaternary ammonium compounds (QACs) highlight the necessity for safer and more effective membrane-active antibacterials. This study aimed to synthesize a homologous series of quinuclidin-3-one-based QACs (QO-C12, QO-C14, and QO-C16) and to elucidate their structure-activity relationships through comprehensive antibacterial, antibiofilm, cytotoxicity, embryotoxicity, mechanistic, and physicochemical analyses. The compounds were evaluated against clinically relevant pathogens using broth microdilution, growth and time-kill assays, biofilm inhibition testing, membrane permeabilization analysis, cytotoxicity testing in human retinal pigment epithelial (RPE1) and human embryonic kidney (HEK293) cells, zebrafish embryotoxicity assessments, and critical micelle concentration (CMC) measurements. Antibacterial potency was found to increase with alkyl chain length, with QO-C16 exhibiting the highest activity, including minimum inhibitory concentrations (MICs) of 8 µM against Staphylococcus aureus (including methicillin-resistant strains) and 4 µM against Listeria monocytogenes. In Dulbecco's modified Eagle medium (DMEM), MIC values were up to 32-fold lower than those observed in broth media, indicating a significant medium-dependent effect. QO-C14 and QO-C16 demonstrated robust antibiofilm activity, while QO-C16 produced the fastest bactericidal and membrane-disruptive effects. The critical micelle concentration decreased with increasing chain length, aligning with enhanced hydrophobicity and membrane affinity. Compared to commercial QACs, the novel compounds exhibited lower cytotoxicity in human cell lines; however, zebrafish embryotoxicity did increase with chain length. Overall, quinuclidin-3-one represents a promising scaffold for the development of QACs, with QO-C16 identified as the lead compound for further optimization toward selective antibacterial agents.