Plant-derived products as antibiotic enhancers and antibiotic-resistance modifying agents

Combination of conventional antibiotics with natural products represents a promising strategy in overcoming antibiotic resistance. The aim of this work was to assess the interactions between plant-derived products and several antibiotics in order to identify potential synergistic antibacterial combinations. Both extracts and pure phytochemicals were included in this study. The effects of the combinations were evaluated against Gram-positive including methicillin-resistant Staphylococcus aureus (MRSA) and Gram-negative bacteria. Our studies on essential oils showed that coriander essential oil and its major constituent, linalool, acted synergistically with several antibiotics (oxacillin, gentamicin, tetracycline, ciprofloxacin) against both Gram-positive and Gram-negative bacteria with coriander essential oil reversing tetracycline resistance of Staphylococcus epidermitis [2]. Despite its moderate antibacterial activity, white mulberry leaf ethanolic extract was found to reverse oxacillin resistance of MRSA and to act synergistically with gentamicin against MRSA. Additionally, the extract showed synergy with gentamicin and tetracycline against Staphylococcus epidermitis [3]. Two of its constituents, namely morusin and kuwanon G, also reversed oxacillin resistance of MRSA and acted synergistically with gentamicin, tetracycline and ciprofloxacin against MRSA. Morusin also reversed tetracycline resistance of Staphylococcus epidermitis. Both morusin and kuwanon G increased the permeability of the bacterial membrane as shown by fluorescence and differential interference contrast microscopy. Conclusion: Plant-derived products are promising candidates for the development of novel treatment strategies in bacterial infections.