Effectiveness of Essential Oils as Inhibitors of Quorum Sensing Activity

Abstract

Quorum sensing (QS) represents a specific way of bacterial intercellular communication, which is enabled owing to their ability to detect and respond to cell population density by gene regulation. QS mechanisms are responsible for controlling the pathogenesis, virulence luminescence, motility, sporulation, and biofilm formation of many organisms by regulating gene expression. Therefore, research in this field is an attractive target for developing new natural antibacterial agents. Considering the importance of QS during bacterial pathogenesis, this research has been focused on the evaluation of the anti-QS properties of four essential oils (EOs) Origanum heracleoticum, Origanum vulgare, Thymus vulgare, and Thymus serpyllum, using biomonitor strain Chromobacterium violaceum CV026. Tests were conducted on Luria Bertani agar supplemented with N hexanol DL homoserine lactone (HHL) 10µl/50ml of agar. The anti-QS potential of the EOs was assayed in a range of 200 – 0.39 µl/ml using the disc diffusion method. EOs of T. vulgaris and T. serpyllum exhibited anti-QS activity indicated by a non-pigmented ring in a dilution-dependent manner. The lowest dilution of T. vulgaris and T. serpyllum in which they exhibited visually detectable inhibition of violacein synthesis was 6.25 µl/ml for both tested EOs. O. heracleoticum and O. vulgare displayed different active principles, i.e. antimicrobial activity indicated by the inner clear ring and anti-QS activity indicated by the outer non-pigmented ring, in a concentration-dependent manner. The lowest dilution of O. heracleoticum and O. vulgare which exhibited visually detectable inhibition of violacein synthesis was 1.56 and 3.25 µl/ml, respectively. The main constituents of the tested EOs are monoterpenes (carvacrol, thymol, γ-terpinene, and p-cymene) and anti-QS properties of tested EOs can be mainly attributed to their activity. In particular, from the scientific literature, carvacrol and thymol show a sub-inhibitory effect against foodborne pathogens. Previous studies indicated that sub-lethal concentrations of carvacrol reduced the mobility of bacteria due to the ability of interference using the QS mechanism between the bacterial cells, thereby reducing the ability of biofilm formation. The precise mechanism by which carvacrol inhibits biofilm formation is still not fully understood. The results imply that EOs represent a promising alternative for effective control of the emergence and spread of resistant pathogens.