Synergistic bactericidal activity between hyperosmotic stress and membrane-disrupting nanoemulsions

There is a clear clinical need for alternative types of non-antibiotic biocides due to the rising global health concern of microbial drug resistance. In this work, a novel antibacterial concept was delineated that utilized hyperosmotic stress (H) in concert with membrane-disrupting nanoemulsions (NEs). The antibacterial effects of either H or a NE, as well as in combination (H+NE), were assessed in vitro using an Escherichia coli model. It was found that exposure to H or NE alone produced dose-dependent bacteriostatic and bactericidal effects, respectively. However, the bactericidal action of NE was significantly amplified in the presence of H. Outcomes following H+NE exposure included rapid efflux of K⁺ and nucleic acids, increased membrane permeability and a reduction in both intracellular ATP and cell viability. Further inspection of morphology by electron microscopy highlighted cell shrinkage, membrane dissolution and bacteriolysis. Pathogen inactivation occurred immediately upon contact with H+NE. The effects of H, NE and H+NE against Enterococcus faecalis, Staphylococcus aureus and meticillin-resistant S. aureus isolates were also examined. Similar to the Escherichia coli model, H+NE showed antibacterial synergism in these organisms when classified by the Chou–Talalay combination index for two-agent interactions. This synergistic interaction suggests that the H+NE platform may potentially serve as a new paradigm in disinfectants, antiseptics and antibacterial wound dressings. The H+NE mechanism of action was termed osmopermeation, as a descriptor for the underlying inactivation process.