Antimicrobial action of selected plant-derived compounds against Listeria monocytogenes
Listeria monocytogenes causes listeriosis, a disease that can be fatal to immunocompromised individuals. Due to the number of outbreaks, recalls, and deaths linked to the consumption of contaminated ready-to-eat (RTE) foods, the United States government issued a directive for the control of L. monocytogenes in the production of RTE meats including the use of a post-lethality treatment and/or the addition of a growth inhibitor. Several methods to inhibit pathogenic bacteria in RTE foods are currently utilized. One method actively being studied involves the use of natural plant products as food antimicrobials. The present study evaluated the effectiveness of plant-derived compounds in controlling the growth of L. monocytogenes . Four antimicrobials (cranberry-CB, grape seed-GS, oregano-OR, and green tea-GT) were evaluated in culture media and two (CB and GS in combination with sodium lauryl sulfate-SLS) as dips for frankfurters (formulated with or without sodium lactate-SL). In culture media, GS had the highest inhibitory activity at both storage temperatures (4 and 10°C) and at all concentrations tested. In frankfurters without SL, 4 and 6 log reductions in the initial numbers of the pathogen were obtained with CB (+SLS) and GS (+SLS), respectively, and growth of survivors was inhibited up to day 14. In frankfurters with SL, a similar bacteriocidal effect was obtained with both CB (+SLS) and GS (+SLS) but growth of survivors was prevented for 90 days. The survival of acid adapted L. monocytogenes in culture media containing CB and GS was also assessed. Acid adapted cells were less affected by CB than the non-acid adapted at weeks 1 and 2; no differences in sensitivity to GS was observed between acid adapted and non-acid adapted cells. Lastly, preliminary evidence on the mode of action of GS on L. monocytogenes cells using transmission electron microscopy indicated possible damage to the cytoplasmic membrane.