Synergistic staphylocidal interaction of benzoic acid derivatives (benzoic acid, 4-hydroxybenzoic acid and β-resorcylic acid) and capric acid: Mechanism and verification study using artificial skin

Objectives: The present study was designed to investigate a synergistic staphylocidal interaction of antimicrobials. Methods: The widely used preservative benzoic acid (BzA) and its derivatives [4-hydroxybenzoic acid (HA) and β-resorcylic acid (β-RA)] combined with capric acid (CPA) were investigated. Results: β-RA was identified as the most effective antimicrobial exhibiting synergistic action with CPA against both Staphylococcus aureus and MRSA. For example, a complete reduction of bacteria (>7.3 log reduction) was obtained within 5 min after treatment with 5.0 mM β-RA (0.079%) plus 0.20 mM CPA (0.004%), while treatment with each material individually showed low bactericidal effects (<1.5 log reduction). Flow cytometry analysis identified membrane disruption related to the synergistic mechanisms, including the following: (i) membrane disruption by CPA (69.2% of cells were damaged by 0.20 mM CPA treatment); (ii) antimicrobial entry through the damaged membrane; and (iii) cytoplasmic ion imbalance resulting in cell death. We verified that the synergistic combination was also effective against MRSA on artificial skin (99.989% elimination after 5 min). Conclusions: We used only consumer-preferred natural-borne antimicrobials and a very small amount of material was needed based on the synergistic effects. Therefore, these antimicrobials can be widely used as alternative anti-MRSA compounds in healthcare products, cosmetics, pharmaceutical products, foods and for environmental hygiene.