Introduction

Staphylococcus aureus is a Gram-positive bacterium that asymptomatically colonizes approximately 20 to 30 % of the global population [1]. Under conditions of dysbiosis or impaired host immunity, S. aureus can shift from a commensal to an opportunistic pathogen, causing a broad spectrum of infections ranging from mild skin and soft tissue lesions to severe, life-threatening systemic diseases [2]. Treatment of S. aureus infection has become increasingly challenging due to the rising prevalence of antimicrobial resistance [3], which has significantly reduced the effectiveness of available therapeutic options. This resistance is commonly driven by mutations at antimicrobial target sites [4] and is further compounded by S. aureus's ability to form highly structured, resilient biofilms [1], which enhance tolerance to antimicrobial agents and host immune defences.

Natural products have long been recognized as prolific sources of novel therapeutic agents, playing a central role in the discovery and development of treatments for infectious diseases. Their remarkable chemical diversity and evolutionary optimization generate unique molecular scaffolds capable of targeting microbial pathogens through mechanisms often distinct from those of conventional drugs [5,6]. In plants, these bioactive metabolites serve as natural defence compounds, giving rise to a vast, chemically rich reservoir of molecules with antimicrobial potential [7,8]. In this context, Hypericum brasiliense, a species of the Hypericaceae family, has a well-documented history of medicinal use, attributed to its astringent, aromatic, excitant, vulnerary, antispasmodic and antiophidic properties. The apolar extract of H. brasiliense leaves and flowers contains a diverse array of bioactive metabolites, including phloroglucinols such as japonicin A, uliginosin A, uliginosin B, isouliginosin B and various hyperbrasilol derivatives [9-11]. Previous studies from our research group have demonstrated that the hexane extract, as well as the isolated compounds uliginosin B, isouliginosin B and japonicin A, display antibacterial activity against Gram-positive bacterial pathogens, including Staphylococcus epidermidis and S. aureus [12,13]. These findings underscore H. brasiliense as a promising source of natural antimicrobial agents. Nevertheless, the mechanisms underlying its antibacterial effects, particularly against drug-resistant S. aureus clinical isolates, remain insufficiently explored.

The present study aimed to advance current knowledge on the anti-staphylococcal activity of the hexane extract of H. brasiliense and its major phloroglucinol derivatives, including uliginosin B, isouliginosin B and japonicin A, against both reference strains and drug-resistant clinical isolates of S. aureus. Building upon previous findings, we comprehensively investigated their antimicrobial potential, including their ability to inhibit biofilm formation and disrupt established biofilms through assessments of metabolic activity, total biomass and extracellular matrix production. In addition, we explored potential mechanisms of action using transmission and scanning electron microscopies to visualize ultrastructural alterations in S. aureus. Complementary assays were performed to evaluate plasma membrane integrity, oxidative stress induction and changes in microbial metabolic activity. Finally, in silico molecular docking and pharmacokinetic analyses were conducted to test the hypothesis that uliginosin B interacts with and potentially disrupts components of the S. aureus respiratory chain, providing mechanistic insights into its antibacterial activity.