title: A novel interpretation of the Fractional Inhibitory Concentration Index: The case Origanum vulgare L. and Leptospermum scoparium J. R. et G. Forst essential oils against Staphylococcus aureus strains.
abstract: Origanum vulgare (oregano) and Leptospermum scoparium (manuka) were traditionally employed as natural remedies for infected wounds and skin injuries where Staphylococcus aureus is mainly involved. The first aim of this study was to investigate oregano and manuka essential oils (EOs) chemical compositions and evaluate their antibacterial activity (MIC, Minimum Inhibitory Concentration) against fourteen S. aureus wild strains. The second aim was to evaluate the antibacterial activities of oregano and manuka EOs mixed in different combination (FIC, Fractional Inhibitory Concentration) with an improved chequerboard technique. This allowed to avoid the usual uncertainty in the determination of MIC and FIC values and to obtain a more precise interpretation of FIC indexes (FICIs). Moreover, FICIs were discussed on the basis of a novel interpretation method to evaluate the synergistic/antagonistic effect of EOs mixtures. The most representative compounds in oregano EO were Carvacrol (65.93%), p-Cymene (9.33%) and γ-Terpinene (5.25%), while in manuka EO were Leptospermone (31.65%), cis-Calamenene (15.93%) and Flavesone (6.92%). EOs presented MIC values ranging from 1:2048 to 1:4096 v/v and FIC values ranging from 0.125 to 1. According to our interpretation, a synergistic effect (34.68%), a commutative effect (15.32%) and an indifferent effect (50.00%) and no antagonistic effect were observed. Conversely, according to two previously proposed FICI interpretation models, 1.80% synergistic effect could be observed and, respectively, 98.20% indifferent effect or 48.20% additive effect and 50.00% indifferent effect. As practical results, oregano and manuka EOs may be an effective alternative to chemotherapic drugs in staphylococcal infections and useful tools to enhance food security.
authors.affiliation: Department of Veterinary Sciences, Viale delle Piagge 2, University of Pisa, Italy; Interdepartmental Research Center "Nutraceuticals and Food for Health", Via del Borghetto 80, University of Pisa, Italy. Electronic address: filippo.fratini@unipi.it.
authors.forename: Filippo
authors.initials: F
authors.lastname: Fratini
authors.affiliation: Department of Veterinary Sciences, Viale delle Piagge 2, University of Pisa, Italy.
authors.forename: Simone
authors.initials: S
authors.lastname: Mancini
authors.affiliation: Department of Veterinary Sciences, Viale delle Piagge 2, University of Pisa, Italy.
authors.forename: Barbara
authors.initials: B
authors.lastname: Turchi
authors.affiliation: Department of Veterinary Sciences, Viale delle Piagge 2, University of Pisa, Italy.
authors.forename: Elisabetta
authors.initials: E
authors.lastname: Friscia
authors.affiliation: Interdepartmental Research Center "Nutraceuticals and Food for Health", Via del Borghetto 80, University of Pisa, Italy; Department of Pharmacy, Via Bonanno 6, University of Pisa, Italy.
authors.forename: Luisa
authors.initials: L
authors.lastname: Pistelli
authors.affiliation: Department of Pharmacy, Via Bonanno 6, University of Pisa, Italy.
authors.forename: Giulia
authors.initials: G
authors.lastname: Giusti
authors.affiliation: Department of Veterinary Sciences, Viale delle Piagge 2, University of Pisa, Italy; Interdepartmental Research Center "Nutraceuticals and Food for Health", Via del Borghetto 80, University of Pisa, Italy.
authors.forename: Domenico
authors.initials: D
authors.lastname: Cerri
mesh_terms: D000900:Anti-Bacterial Agents / Q000302:isolation & purification / Q000494:pharmacology*; D004357:Drug Synergism; D031649:Leptospermum / Q000737:chemistry*; D008826:Microbial Sensitivity Tests; D009822:Oils, Volatile / Q000302:isolation & purification / Q000494:pharmacology*; D031341:Origanum / Q000737:chemistry*; D064209:Phytochemicals / Q000302:isolation & purification / Q000494:pharmacology*; D013211:Staphylococcus aureus
chemical_list: D000900:Anti-Bacterial Agents; D009822:Oils, Volatile; D064209:Phytochemicals
keywords: Essential oil; Fractional inhibitory concentration; Manuka; Oregano; Staphylococcus aureus; Synergy