The influence of the presence of synthetic structurally related N-oxide and corresponding cationic surfactants with different chain lengths on the properties of 1,2-dimyristoyl-sn-glycero-3-phosphocholine liposomes was investigated. Their potentiality as delivery systems of (+)-usnic acid was also evaluated by studying its entrapment efficiency and its antioxidant activity. In fact, (+)-usnic acid has many pharmacological properties that, as many natural substances, are often strictly linked to their antioxidant power. N-oxide surfactants can enhance this property and improve the efficacy of the system. Based on this premise, we verified how and to what extent the molecular structure of liposomes components affects this effect: the best antioxidant effectiveness was observed when (+)-usnic acid was included in liposomes containing the N-oxide surfactant with C14 alkyl chain. Our results underline the importance of the hydrophilic/hydrophobic balance of the monomer in determining the properties of the aggregates in which it is included.

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The dramatic intensification of antimicrobial resistance occurrence in pathogenic bacteria concerns the global community. The revitalisation of inactive antibiotics is, at present, the only way to go through this health system crisis and the use of antimicrobial adjuvants is turning out the most promising approach. Due to their low toxicity, eco-friendly characteristics and antimicrobial activity, amphoteric surfactants are good candidates. This study investigated the adjuvant potentialities of commercial acyclic and newly cyclic N-oxide surfactants combined with therapeutically available antibiotics against MDR methicillin-resistant Staphylococcus aureus (MRSA). The safety profile of the new cyclic compounds, compared to commercial surfactants, was preliminarily assessed, evaluating the cytotoxicity on human peripheral mononuclear blood cells and the haemolysis in human red blood cells. The compounds show an efficacious antimicrobial activity strongly related to the length of the carbon atom chain. In drug–drug interaction assays, all surfactants act synergistically, restoring sensitivity to oxacillin in MRSA, with dodecyl acyclic and cyclic derivatives being the most effective. After evaluating the cytotoxicity and considering the antimicrobial action, the most promising compound is the L-prolinol amine-oxide C12NOX. These findings suggest that the combination of antibiotics with amphoteric surfactants is a valuable therapeutic option for topical infections sustained by multidrug-resistant S. aureus.

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