Abstract
In the present study, we have evaluated the in vitro antibacterial activity of essential oils from Origanum vulgare, Thymus vulgaris, Lavandula angustifolia, Mentha piperita, and Melaleuca alternifolia against 32 erythromycin-resistant [Mininum Inhibitory Concentration (MIC) ≥1 μg/mL; inducible, constitutive, and efflux-mediated resistance phenotype; erm(TR), erm(B), and mef(A) genes] and cell-invasive Group A streptococci (GAS) isolated from children with pharyngotonsillitis in Italy. Over the past decades erythromycin resistance in GAS has emerged in several countries; strains combining erythromycin resistance and cell invasiveness may escape β-lactams because of intracellular location and macrolides because of resistance, resulting in difficulty of eradication and recurrent pharyngitis. Thyme and origanum essential oils demonstrated the highest antimicrobial activity with MICs ranging from 256 to 512 μg/mL. The phenolic monoterpene carvacrol [2-Methyl-5-(1-methylethyl) phenol] is a major component of the essential oils of Origanum and Thymus plants. MICs of carvacrol ranged from 64 to 256 μg/mL. In the live/dead assay several dead cells were detected as early as 1 h after incubation with carvacrol at the MIC. In single-step resistance selection studies no resistant mutants were obtained. A synergistic action of carvacrol and erythromycin was detected by the checkerboard assay and calculation of the Fractional Inhibitory Concentration (FIC) Index. A 2- to 2048-fold reduction of the erythromycin MIC was documented in checkerboard assays. Synergy (FIC Index ≤0.5) was found in 21/32 strains and was highly significant (p < 0.01) in strains where resistance is expressed only in presence of erythromycin. Synergy was confirmed in 17/23 strains using 24-h time-kill curves in presence of carvacrol and erythromycin. Our findings demonstrated that carvacrol acts either alone or in combination with erythromycin against erythromycin-resistant GAS and could potentially serve as a novel therapeutic tool.
Highlights
The increase in antibiotic-resistant bacteria has revived the interest in plant products as alternative/adjunct antimicrobial agents to control pathogenic micro-organisms (Cowan, 1999; Hemaiswarya et al, 2008; Hyldgaard et al, 2012)
We evaluated the antibacterial activity of different essential oils and of carvacrol, alone and in combination with erythromycin, against erythromycin-resistant Group A streptococci (GAS) isolated from children with pharyngotonsillitis in Italy
One of the alternative strategies to fight antibiotic-resistant bacteria is the use of natural antimicrobial substances such as plant essential oils and their components
Summary
The increase in antibiotic-resistant bacteria has revived the interest in plant products as alternative/adjunct antimicrobial agents to control pathogenic micro-organisms (Cowan, 1999; Hemaiswarya et al, 2008; Hyldgaard et al, 2012). A major group of plant antimicrobial compounds is represented by essential oils, which are complex mixtures of volatile secondary metabolites. They are used in the food industry because of their preservative potency against food-borne pathogens—thanks to their antimicrobial, antibacterial, and antifungal properties. Beside anti-inflammatory, antioxidant, antitumor, analgesic, anti-hepatotoxic, and insecticidal properties, several studies have demonstrated that carvacrol has antimicrobial properties (Hyldgaard et al, 2012). The ability of carvacrol to exert synergistic effects in combination with a number of antibiotics, including macrolides, has been recently reported (Langeveld et al, 2014)
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