Antimicrobial efficacy of Lippia citriodora natural extract against Escherichia coli and Enterococcus faecalis in “Piel de Sapo” melon juice

BACKGROUND: The minimal inhibitory concentration (MIC) of an aqueous extract of Lippia citriodora with reported functional properties (PLX(®)) was determined on two strains of Escherichia coli (E. coli) belonging to serogroups commonly associated with foodborne illnesses (E. coli O157:H7 ATCC 700728 and E. coli O111 isolate 172) in vegetable products and two control strains for antimicrobial tests assays (E. coli ATCC 25922 and Enterococcus–En. faecalis ATCC 29212). RESULTS: Mean MIC values at standard pH (7.4) in broth for the E. coli strains tested ranged from 4,444 µg/ml (35ºC) to 1,250 µg/ml (10ºC) and to 182 µg/ml (4ºC). At pH 5.5, conditions resembling those of melon juice, MIC was about 2 times higher at 35 and 10ºC compared with 4ºC. The MIC of En. faecalis was similar or slightly lower than those of E. coli at the conditions tested. In melon juice fortified with PLX(®) (2,500 µg/ml, maximum sensorial acceptable limit), the three strains of E. coli maintained their viability although none showed growth potential after 4 days at 4ºC. CONCLUSIONS: PLX(®) could be added to melon juice to control E. coli O157:H7 and E. coli O111 during refrigerated storage, reducing the risk of microbiological contamination in this food.