Altered Antimicrobial and Anti-biofilm Forming Effect of Thyme Essential Oil due to Changes in Composition

The antimicrobial and anti-biofilm forming effect of thyme (Thymus vulgaris) essential oil (TEO) with different compositions was evaluated. Normally the main component in this TEO is thymol, but in 2014 we found that the proportions of y-terpinene and p-cymene (the precursors in thymol biosynthesis) increased and that of thymol decreased. This altered composition led to changes in the antimicrobial and anti-biofilm forming capacity of the essential oil depending also on the type of microorganisms. In the case of bacteria, minimal inhibitory (MIC) and minimal bactericidal (MBC) concentrations either decreased or increased. In the case of yeasts, minimal fungicidal concentrations (MFC) increased 4- and 8-fold for TEO containing p-cymene as the main component. On the contrary, MIC values decreased for all the tested moulds. Anti-biofilm forming activity of TEO containing p-cymene as its main component decreased in almost all cases and P. fluorescens biofilm forming capacity was even enhanced.