Enhanced bactericidal effect of enterocin A in combination with thyme essential oils against L. monocytogenes and E. coli O157:H7

The combined effects of enterocin A with Thymus vulgaris essential oils (EOs) against Listeria monocytogenes and Escherichia coli O157:H7 were investigated in vitro by enumeration of surviving populations of testing pathogens and minimal inhibitory concentration (MIC) determination. Enterocin A was purified to homogeneity by RP-HPLC from the culture fluid of Enterococcus strain and thyme EOs were extracted from local Thymus vulgaris plants. The major constituent of thyme EOs oils determined by GC-MS was thymol (78.4 %). Combination of enterocin A with thyme EOs showed an enhanced bactericidal effect against Listeria monocytogenes. Checkerboard assay and isobologram construction displayed a synergistic interaction between these compounds against Listeria (FIC index <0.5). Moreover, the MIC value of enterocin A has fallen fivefold (from 4.57 to 0.9 mu g/ml), while the MIC of thyme EOs decreased threefold (from 3.6 to 1.2 mu g/ml). Treatments with enterocin A alone did not affect the growth of the enteric pathogen E. coli O157:H7. However, the addition of thyme EOs and enterocin A yielded a synergistic antimicrobial effect against E. coli (MIC thyme EOs decrease from 2.2 to 0.71 mu g/ml). This is the first report on the combined effect of enterocin A and thyme EOs against food pathogen bacteria. This combination could be useful in food bio-preservation.