Antibacterial and Resistance-Modifying Activities of Carvacrol and p-Cymene Against Listeria monocytogenes

Introduction:  Listeria monocytogenes, a Gram-positive, facultative intracellular bacterium, is capable of causing serious human infections and is, generally, susceptible to a wide range of antibiotics. However, during the last few years, increasing numbers of strains have been reported as resistant to one or more antibiotics. Carvacrol and p-cymene are two monoterpenes that are constituents of many essential oils, including oregano and thyme. They have been proven as efficient antimicrobials.

Purpose: The aim of this study was to investigate the effect of carvacrol and p-cymene on L. monocytogenes by studying their antimicrobial and resistance-modifying activity.

Methods:   In this study the MICs of carvacrol, p-cymene and ampicillin (Amp) were determined in the absence and presence of sub-inhibitory concentration of carvacrol and p-cymene (1/2 MIC) on 10 L. monocytogenes strains. In addition, ethidium bromide accumulation assays were carried out to determine the influence of carvacrol and p-cymene (1/2 and 1/4 MICs) on the cell efflux activity, where reserpine (chemical efflux pump inhibitor) was used as control. The LIVE/DEAD BacLight cell viability assay was carried out for determining cells membrane integrity in the absence and presence of carvacrol and p-cymene (1/2 and 1/4 MICs).

Results: Carvacrol and p-cymene inhibited the growth of all L. monocytogenes strains. Carvacrol, in combination with Amp, increased the susceptibility of L. monocytogenes; a reduction in the MIC was observed (up to 16-fold). No effect was observed with p-cymene in combination with Amp. The ethidium bromide accumulation increased in the presence of carvacrol or p-cymene and was comparable to reserpine. Membrane integrity disintegrated.

Significance: These data warrant further studies on the use of carvacrol and p-cymene in the control of antibiotic resistance in L. monocytogenes.