INTRODUCTION. The growing threat of antimicrobial drug resistance has stimulated the search for new therapeutic alternatives, including essential oils (EO) that are now recognized for their potential antimicrobial role against microorganisms. Mentha x piperita L. Huds has been used in folk medicine since antiquity and recent studies documented the antimicrobial efficacy of its EO. In Piedmont (Pancalieri,TO) it has spread the EO production of a cultivar of M. x piperita. Since clinical experience showed that the efficacy of antimicrobial drugs depends both on their direct effect on microorganisms and on the activity of the host immune system, we evaluated the influence of this oil on intracellular killing by PMNs against Candida krusei in comparison with anidulafungin (AND), as reference drug. MATERIALS AND METHODS.A clinical C.krusei strain was used. The EO was purchased from Erbe Aromatiche Pancalieri,Turin and analysed by GC (Drug Science and Technology Dept.). AND (Pfizer) was provided by Prof. Milici (University of Palermo). Susceptibility testing was based on the CLSI M27-A3 method, with some modifications. Intracellular killing was investigated by incubating yeasts (106cfu/mL) and PMNs (106cells/mL) at 37°C for 30,60,90 min in presence of 1/4xMIC or 1/8×MIC of EO and 1/2MIC of AND. Killing values were expressed as Survival Index. To differentiate between any separated effect of EO/AND on yeasts/PMNs, the experiments were conducted after exposure of each of them to EO or AND for 1h. EO/AND-free controls were included. Statistical evaluation of the differences between test and control results was performed by Tukey’s test. RESULTS.EO at 1/4 and 1/8xMIC produced a significantly further decrease in the survival of yeasts, with killing values ranging from 56 to 61% and 53 to 67%, respectively during the 90min period, in comparison with controls (22-33%) (P<0.01). In the presence of ½×MIC AND, intracellular yeasts were killed in similar values to the control (27 to 33%). The data showed that pre-treatment with EO acts both on the yeasts and on phagocytic cells, suggesting a positive interaction between EO and PMNs. In fact, pre-treatment of PMNs with 1/8xMIC EO resulted in a significant enhancement of intracellular killing, as compared with 1/2xMIC AND and controls. Pre-treated yeasts with ¼ - 1/8xMIC EO were killed more efficiently by PMNs than AND pre-treated yeasts: 46-44-57% and 50-64-60% vs 17-18-13% (1/2xMIC AND) and 22-29-33% (yeasts not treated) (P<0.01). CONCLUSIONS.We observed that EO was more efficacy at 1/8xMIC than at 1/4xMIC (eagle effect), indicating that decreasing concentrations did not cause lower candidacidal activity. These data show a promising potential application of this EO against C.krusei, often resistant to conventional drugs.
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