The antimicrobial activity of a commercial Lippia citriodora extract (LCE) at 2500 mµ mL-1 (maximum sensorial aceptable level) and vanillin (MIC and 2 MIC) alone and in combination were analyzed in four strains of Escherichia coli (two non-verotoxigenic and two verotoxigenic ones) in cation adjusted Mueller Hinton medium (CA-MHB) and in “Piel de Sapo” melon juice (MJ) stored under refrigeration (4 ºC) for 7 days. The bacterial counts of the four strains together in untreated samples were higher (≈ 4 log CFU/ml) in CA-MHB than in stored MJ. LCE showed higher antimicrobial activity in MJ than in standard culture broth, but vanillin showed a higher effect in broth. The verotoxigenic strain E. coli O146:H stx2 was the most sensitive to LCE in refrigerated MJ. Combinations of vanillin (at MIC and 2 MIC) with LCE were very effective in reducing E. coli counts either in broth or in refrigerated MJ to undetectable levels. Bactericidal effects were observed for the combinations in all strains in broth and in MJ. Also, these combinations showed an antimicrobial synergistic effect after day 3 of storage in MJ in three of the bacterial strains tested. These results indicate that the combination of LCE (at maximum sensorial acceptable levels) and vanillin (at low concentrations) could be considered as a promising natural antimicrobial agent to inhibit verotoxigenic E. coli growth in refrigerated “Piel de Sapo” MJ and improve its quality.

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