Gram-negative bacteria cause infections that are difficult to treat due to the emergence of multidrug resistance. This review summarizes the current status of the studies investigating the capacity of essential oils and their components to modulate antibiotic activity against Gram-negative bacteria. Synergistic interactions are particularly discussed with reference to possible mechanisms by which essential oil constituents interact with antibiotics. Special emphasis is given to essential oils and volatile compounds that inhibit efflux pumps, thus reversing drug resistance in Gram-negative bacteria. In addition, indifference and antagonism between essential oils/volatile compounds and conventional antibiotics have also been reported. Overall, this literature review reveals that essential oils and their purified components enhance the efficacy of antibiotics against Gram-negative bacteria, being promising candidates for the development of new effective formulations against Gram-negative bacteria.

The aim of this study was to evaluate the antibacterial activity of coriander essential oil and its major constituent, linalool, in combination with antibiotics against Gram‐positive (methicillin‐susceptible and methicillin‐resistant Staphylococcus aureus, S. epidermidis) and Gram‐negative bacteria (Pseudomonas aeruginosa, Escherichia coli). The chemical composition of coriander essential oil was analyzed by gas chromatography with flame ionization and mass spectrometry detection. The antibacterial activity of coriander essential oil, linalool and their combinations with antibiotics were assessed by the broth microdilution and checkerboard assays respectively. Thirty‐four compounds were identified in coriander essential oil, linalool (70·11%) being predominant. Coriander essential oil and linalool showed synergistic interactions with antibiotics (oxacillin, amoxicillin, gentamicin, ciprofloxacin, tetracycline) against both Gram‐positive and Gram‐negative bacteria. In these synergistic combinations, minimum inhibitory concentrations of antibiotics were markedly reduced; even antibiotic resistance reversal activity was recorded. These findings are very promising for the development of new therapeutic options for bacterial infections. Significance and Impact of the Study Methicillin‐resistant Staphylococcus aureus (MRSA) and Gram‐negative bacteria are still major threats to human health and, therefore, identification of new antibacterial agents or combinations with high potency is needed. Our study found synergistic interactions between coriander essential oil/linalool and antibiotics against MRSA and other Gram‐positive bacteria (methicillin‐susceptible S. aureus, S. epidermidis), but also Gram‐negative bacteria (Pseudomonas aeruginosa, Escherichia coli). Increase in antibiotic susceptibility and reversal of antibiotic resistance were also demonstrated. Combinations of coriander essential oil/linalool and antibiotics are thus very promising for the development of novel antibacterials.

No abstract

Combination of antibiotics with natural products is a promising strategy for potentiating antibiotic activity and overcoming antibiotic resistance. The purpose of the present study was to investigate whether morusin and kuwanon G, prenylated phenolics in Morus species, have the ability to enhance antibiotic activity and reverse antibiotic resistance in Staphylococcus aureus and Staphylococcus epidermidis . Commonly used antibiotics (oxacillin, erythromycin, gentamicin, ciprofloxacin, tetracycline, clindamycin) were selected for the combination studies. Checkerboard and time-kill assays were used to investigate potential bacteriostatic and bactericidal synergistic interactions, respectively between morusin or kuwanon G and antibiotics. According to both fractional inhibitory concentration index and response surface models, twenty combinations (14 morusin-antibiotic combinations, six kuwanon G-antibiotic combinations) displaying bacteriostatic synergy were identified, with 4–512-fold reduction in the minimum inhibitory concentration values of antibiotics in combination. Both morusin and kuwanon G reversed oxacillin resistance of methicillin-resistant Staphylococcus aureus. In addition, morusin reversed tetracycline resistance of Staphylococcus epidermidis . At half of the minimum inhibitory concentrations, combinations of morusin with oxacillin or gentamicin showed bactericidal synergy against methicillin-resistant Staphylococcus aureus. Fluorescence and differential interference contrast microscopy and scanning electron microscopy showed an increase in the membrane permeability and massive leakage of cellular content in methicillin-resistant Staphylococcus aureus exposed to morusin or kuwanon G . Overall, our findings strongly indicate that both prenylated compounds are good candidates for the development of novel antibacterial combination therapies.

No abstract