Antibacterial activity of propylene glycol against Staphylococcus aureus and Staphylococcus epidermidis in neutral and mild acidic conditions

BACKGROUND: Staphylococcus aureus and Staphylococcus epidermidis normally coexist on the skin of healthy individuals. Their ratio and pathogenicity, however, change with different skin diseases and thus, the ability to control bacterial growth on the skin is important. Propylene glycol (PG), a widely used component in topical formulations, has been proved to have antimicrobial activity. OBJECTIVES: To investigate the concentration-dependent antimicrobial properties of PG against S. aureus and S. epidermidis skin isolates in mono- and co-culture at neutral (pH 7.4) and mildly acidic (pH 5) conditions. RESULTS AND DISCUSSION: The minimum inhibitory concentration of PG was 12.5% at both pH 5 and pH 7.4 for the selected bacterial strains. The viability of S. aureus exposed to 12.5% PG was lower at pH 5 than at pH 7.4, and post-treatment regrowth of S. aureus occurred slowly at acidic pH. When both bacterial strains were incubated in media containing 12.5% PG at pH 5 for 48 hours, S. epidermidis retained significantly higher viability, while at pH 7.4, the results were opposite. CONCLUSIONS: Under the acidic conditions of healthy skin, pathogenic S. aureus is more suppressed by 12.5% PG than commensal S. epidermidis.