Enhanced inhibition of Listeria monocytogenes by a combination of cold pressed terpeneless Valencia orange oil and antibiotics.

This study was designed to evaluate the ability of cold pressed terpeneless Valencia orange oil (CPTVO) to enhance the effectiveness of antibiotics against 10 strains of Listeria monocytogenes. Disc diffusion assays were performed to determine the effects of CPTVO and two antibiotics with different mechanisms of action (i.e., penicillin and chloramphenicol) individually and in combination with CPTVO. CPTVO alone produced zones ranging from 16.5 to 19.9 mm. Penicillin at 2 or 10 units produced zones ranging from <6 to 13.4 mm, and from 16 to 19.5 mm, respectively. Chloramphenicol at 5 or 30 µg had zones ranging from <6 to 6.9 mm, and from 10.8 to 15.9 mm, respectively. Penicillin (2 and 10 units) plus CPTVO produced zones ranging from 20.2 to 25.3 mm, and from 21.9 to 28 mm, respectively. Chloramphenicol (5 or 30 µg) plus CPTVO produced zones of from 20.1 to 26.6 mm, and from 19.5 to 23.9 mm, respectively. In conclusion, the combination of antibiotics with CPTVO increases their ability to inhibit L. monocytogenes.