Vannina Lorenzi

Geraniol Restores Antibiotic Activities against Multidrug-Resistant Isolates from Gram-Negative Species

ABSTRACT The essential oil of Helichrysum italicum significantly reduces the multidrug resistance of Enterobacter aerogenes, Escherichia coli, Pseudomonas aeruginosa, and Acinetobacter baumannii. Combinations of the two most active fractions of the essential oil with each other or with phenylalanine arginine β-naphthylamide yield synergistic activity. Geraniol, a component of one fraction, significantly increased the efficacy of β-lactams, quinolones, and chloramphenicol.

A lipoxygenase with dual positional specificity is expressed in olives (Olea europaea L.) during ripening.

Plant lipoxygenases (LOXs) are a class of widespread dioxygenases catalysing the hydroperoxidation of polyunsaturated fatty acids. Although multiple isoforms of LOX have been detected in a wide range of plants, their physiological roles remain to be clarified. With the aim to clarify the occurrence of LOXs in olives and their contribution to the elaboration of the olive oil aroma, we cloned and characterized the first cDNA of the LOX isoform which is expressed during olive development. The open reading frame encodes a polypeptide of 864 amino acids. This olive LOX is a type-1 LOX which shows a high degree of identity at the peptide level towards hazelnut (77.3%), tobacco (76.3%) and almond (75.5%) LOXs. The recombinant enzyme shows a dual positional specificity, as it forms both 9- and 13-hydroperoxide of linoleic acid in a 2:1 ratio, and would be defined as 9/13-LOX. Although a LOX activity was detected throughout the olive development, the 9/13-LOX is mainly expressed at late developmental stages. Our data suggest that there are at least two Lox genes expressed in black olives, and that the 9/13-LOX is associated with the ripening and senescence processes. However, due to its dual positional specificity and its expression pattern, its contribution to the elaboration of the olive oil aroma might be considered.

Investigating the antibacterial action of Corsican honeys on nosocomial and foodborne pathogens

Honey is known for its medicinal uses, with compounds such as hydrogen peroxide or phenolic compounds. For the first time, the antibacterial activity of protected designation of origin (PDO) “Miel de Corse – Mele di Corsica” commercial Corsican honeys was investigated. According to the PDO recommendation, melissopalynological analysis were used to classify 11 Corsican honeys. They have been tested for antimicrobial action on seven foodborne and nosocomial bacterial strains by Minimum Inhibitory Concentration (MIC90) then statistical analyses helped to correlate their action on the different strains. The varietal range of honey samples were obtained by melissopalynological analysis leading to 2: “Spring”, 3 “Chestnut grove”, 1 “Summer maquis”, 2 “Honeydew maquis” and 1 “Autumn maquis” and 2 samples classified as “generic” Corsican honeys. 29 MIC values determined were under 10%. Only Enterococcus faecalis didn’t seem sensitive to the action of the honey. However, “Honeydew maquis”, “generic” Corsican honeys and 2 “Chestnut grove” honeys appeared as the most effective (MIC values ranging from under 5 to 9% on Pseudomonas aeruginosa and Staphylococcus aureus). The antimicrobial activity of PDO “Miel de Corse – Mele di Corsica” honeys demonstrate bacterial growth inhibition on 6 of the 7 bacterial strains. Now, an in-depth study must be performed to identify the mode of action of the active honeys.

Anti-Quorum Sensing Activity of 12 Essential Oils on chromobacterium violaceum and Specific Action of cis-cis-p-Menthenolide from Corsican Mentha suaveolens ssp. Insularis

Quorum sensing (QS) is a bacterial communication mechanism used to express various survival or virulence traits leading to enhanced resistance. Chromobacterium violaceum is a commonly used strain that highlights anti-QS action of bioactive substances. Here, we wanted to see if 12 selected essential oils (EO) could exert anti-QS activity. We measured the sublethal minimal QS inhibitory concentration (MQSIC) by assessing violacein production of C. violaceum along with bacterial growth. To confirm the QS disruption, we also proceed to surface bacterial observations using scanning electron microscopy (SEM). We showed that cis-cis-p-menthenolide extracted and isolated from a plant endemic to occidental Mediterranean Sea islands, Mentha suaveolens ssp. insularis, acts as an inhibitor of violacein production and biofilm formation. Measured MQSIC was much lower than the minimal inhibitory concentration (MIC): 0.10 mg·mL−1 vs. 3.00 mg·mL−1. Moreover, disturbance of QS-related traits was confirmed by the degradation of C. violaceum biofilm matrix. There is a clear structure–activity relationship between cis-cis-p-menthenolide and anti-QS activity. Indeed, its isomer molecule (mintlactone) exerts a poor anti-QS action. These results indicate that inhibition of violacein production and biofilm formation by cis-cis-p-menthenolide might be related to a disruption in the QS mechanism.

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 H2O2. 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 H2O2 (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 H2O2 and phenolic compounds act in synergy by a putative pro‐oxidant effect.