Berberine Synergistically Enhances the Antibacterial Activity of Ampicillin against Ampicillin-Resistant Staphylococcus epidermidis

Abstract

Although S. epidermidis is a skin commensal bacterium, it is thought to be a causative pathogen of skin infections. Prolonged and extensive use of antibiotics for skin infections, including ampicillin (AMP), has contributed to the increasing emergence of antibiotic-resistant bacteria and the shaping of skin commensal S. epidermidis into opportunistic pathogens. This highlights the urgent necessity for alternative therapeutic strategies to enhance antimicrobial efficiency and minimize toxic effects on host cells. The present study investigated the potential of berberine (BBR), with specific attention to AMP resistance and BBR as an antibiotic adjunct to conventional antibiotics against S. epidermidis. Minimum inhibitory concentration (MIC) was used to determine the antibacterial susceptibility profiles. BBR reduced the MIC of AMP against AMP-resistant S. epidermidis from 16 to 64 μg/mL to 2–16 μg/mL, and the fractional inhibitory concentration index (FICI) values of 0.3–0.5 confirmed the synergistic interactions of BBR and AMP in combination. A time-kill analysis of the BBR+AMP combination suggested bacteriostatic activity and synergistic interaction. An antibacterial resistance induction study of BBR and AMP, either alone or in combination, demonstrated no substantial change in MIC in BBR alone and BBR+AMP over 15 consecutive passages. S. epidermidis isolates harboring resistant genes determined by the multiplex polymerase chain reaction (PCR) revealed multiple resistance genes (mecA, blaZ, aacA, and tetK). The effects of the BBR+AMP combination were visualized using scanning electron microscopy (SEM) and exhibited abnormal septation and cell surface damage. Propidium iodide (PI) and SYTO-9 demonstrated that the BBR+AMP combination disrupted membrane integrity. The cytotoxicity effect of the combination of BBR+AMP on human keratinocyte skin cells (HaCaT) was determined by the MTT assay, which demonstrated no significant toxicity, and a therapeutic index (TI) analysis suggested an acceptable safety margin. These findings support combinations of BBR and AMP as alternative therapeutics for the treatment of skin infections caused by AMP-resistant S. epidermidis.