Key role of hydrogen peroxide in antimicrobial activity of spring, Honeydew maquis and chestnut grove Corsican honeys on Pseudomonas aeruginosa DNA.

In honeys, several molecules have been known for their antibacterial or wound healing properties. Corsican honeys just began to be tested for their antimicrobial activity with promising results on Pseudomonas aeruginosa. So, identification of active molecules and their mode of action was determined. Hydrogen peroxide concentrations were evaluated and, in parallel, the minimal inhibitory concentrations (MIC) values were performed with and without catalase. More, the quantity of phenolic compounds and ORAC assay were measured. Observation of antibacterial action was done using scanning electron microscopy (SEM) followed by plasmidic DNA extraction. MIC values of chestnut grove and honeydew maquis honeys vary between 7 and 8%, showing a strong antimicrobial capacity, associated with a plasmidic DNA degradation. When catalase is added, MIC values significatively increase (25%) without damaging DNA, proving the importance of H2 O2 . This hypothesis is confirmed by SEM micrographies which did not show any morphological damages but a depletion in bacterial population. Although, such low concentrations of H2 O2 (between 23 µmol l-1 and 54 µmol l-1 ) cannot explain antimicrobial activity and might be correlated with phenolic compounds concentration. Thus, Corsican honeys seem to induce DNA damage when H2 O2 and phenolic compounds act in synergy by a putative pro-oxidant effect. SIGNIFICANCE AND IMPACT OF THE STUDY: We started to determine the antibacterial efficiency of Corsican chestnut grove and honeydew maquis honeys on Pseudomonas aeruginosa. No morphological alteration of the bacterial surface was observed. Antimicrobial action seems to be related to the synergy between hydrogen peroxide and phenolic compounds. The exerted pro-oxidant activity leads to a degradation of P. aeruginosa plasmidic DNA. This is the first study that investigate the primary antibacterial mechanism of Corsican honeys.

Cellular effects induced by Inula graveolens and Santolina corsica essential oils on Staphylococcus aureus.

This study was conducted to evaluate the antibacterial activity of Inula graveolens and Santolina corsica essential oils on Staphylococcus aureus and investigate their effects at the cellular level. The mode of inhibition of both essential oils against S. aureus ATCC 6538P (CIP 53.156) was assessed by determining the minimum inhibitory concentration (MIC) and the minimum bactericidal concentration (MBC). The effects of time and treatment dose on cell viability were determined by time-kill and bacteriolysis assays. Marked structural changes were observed by transmission electron microscopy (TEM). A bactericidal mode of inhibition was established for both essential oils, which rapidly reduced the cell viability of S. aureus at their MIC (5 mg.ml(-1)). No lysis occurred after treatments with the MIC and eight times the MIC of each essential oil. Invaginations of the plasmic membrane with thickenings of the cell wall as well as an aggregation of the cytoplasmic contents were observed in S. aureus cells treated with the MIC of both essential oils. These results suggest that the cytoplasmic membrane and the cell wall are involved in the toxic action of Inula graveolens and Santolina corsica essential oils.