Synergistic and Additive Effects of Menadione in Combination with Antibiotics on Multidrug‐Resistant Staphylococcus aureus: Insights from Structure‐Function Analysis of Naphthoquinones

Abstract Antimicrobial resistance (AMR) interferes with the effective treatment of infections and increases the risk of microbial spread and infection‐related illness and death. The synergistic activities of combinations of antimicrobial compounds offer satisfactory approaches to some extent. Structurally diverse naphthoquinones (NQs) including menadione (−CH 3 group at C2) exhibit substantial antimicrobial activities against multidrug‐resistant (MDR) pathogens. We explored the combinations of menadione with antibiotic ciprofloxacin or ampicillin against Staphylococcus aureus and its biofilms. We found an additive (0.5<FICI 1.0) and synergistic (FICI≤0.5) effect of menadione with ciprofloxacin and ampicillin, respectively. High reactive oxygen species (ROS) activity and inhibition of biofilm formation (>90 %) were also observed. However, preformed biofilms were not affected. Dent formation was also evident in S. aureus treated with the test compounds. The structure‐function relationship (SFR) of NQs was used to determine and predict their activity pattern against pathogens. Analysis of 10 structurally distinct NQs revealed that the compounds with −Cl, −Br, −CH 3 , or −OH groups displayed the lowest MICs (32–256 μg/mL). Furthermore, 1,4‐NQs possessing a halogen or −CH 3 moiety showed elevated ROS activity, whereas molecules with an −OH group affected cell integrity. Improved activity of antimicrobial combinations and SFR approaches are significant in antimicrobial therapies.