Chemical composition, antimicrobial and antibiofilm efficacy of C. limon and L. angustifolia EOs and of their mixtures against Staphylococcus epidermidis clinical strains

Staphylococcus epidermidis is a Gram-positive resident of the human skin, which in certain conditions, can turn into an opportunistic pathogen, being one of the most frequent agents of nosocomial infections, particularly of those associated with indwelling medical devices. The aim of this study was to evaluate the antimicrobial and antibiofilm activity of two essential oils (EOs) to be used alone or in combination as natural alternatives to currently available antibiotics, for treating S. epidermidis infections or preventing the contamination and further development of S. epidermidis biofilm on viable tissues or medical devices. For this purpose C. limon and L. angustifolia EOs were obtained both by hydrodistillation and microwave-assisted extraction and analyzed by GC–MS. The compositions of EOs as a function of the extraction procedure were compared and discussed. Each of the two extraction techniques was more effective in the extraction of different components of the EOs. In the case of C. limon EOs, terpinen-4-ol, α-terpineol and linalool, not found in the microwave assisted extract, are obtained by classical hydrodistillation (being compounds of high polarity) while less polar compounds (α-pinene, β-pinene, limonene, γ-terpinene) are drawn into higher percentage through the microwave-assisted procedure. In the case of L. angustifolia EOs, a higher percentage of compounds with high polarity (such as linalool) is extracted by classical hydrodistillation while the compounds with lower polarity, such as acetates (linalyl acetate and neryl acetate) are extracted in higher proportion by the microwave-assisted extraction method. The antibacterial and antibiofilm effects of the EOs and of their mixtures were tested towards five S. epidermidis clinical strains and their biological activity was correlated with the oils composition. The most active composition was represented by the binary mixture (1:1) of the EOs obtained by hydrodistillation, containing terpene alcohols (such linalool) and d-limonene, which exhibited the lowest minimum inhibitory concentration (3.9 μg/mL) and minimum biofilm eradicating concentration (0.97–7.81 μg/mL), against all tested S. epidermidis strains. Other minor constituents, such as terpinen-4-ol, α-terpineol, linalyl acetate, neryl acetate, geranyl acetate and α-pinene could be potentiated in presence of limonene and could be responsible for the high antimicrobial activity of these EOs mixture.