Determination Of Effective Antimicrobial Treatments To Control Listeria Monocytogenes On Cold-Smoked Salmon And Elucidation Of Food Antimicrobial Resistance Mechanisms

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Contamination of ready-to-eat (RTE) foods by a foodborne pathogen L. monocytogenes is a serious public health concern, particularly for RTE foods that support growth of this pathogen to hazardous levels. Due to the high mortality rate associated with this pathogen among susceptible hosts, continued efforts to identify effective control strategies for L. monocytogenes on RTE foods are imperative to mitigate the risk of a future recall or an outbreak. In these studies, we combined both applied and basic research approaches to identify strategies that may improve the safety of smoked seafood. Specifically, the main objectives of the research were to (i) determine the effect of curing method and storage condition on subsequent growth of L. monocytogenes on cold-smoked salmon, (ii) evaluate combinations of bactericidal and bacteriostatic antimicrobials as a cost-effective treatment option to control L. monocytogenes on cold-smoked salmon, and (iii) elucidate L. monocytogenes resistance mechanisms against commercially-available food antimicrobials. Our results indicated that L. monocytogenes grew more extensively on salmon that has undergone a freeze-thaw cycle compared to refrigerated storage. On salmon that has been stored frozen (prior to inoculation), L. monocytogenes also initiated growth more rapidly on wet-cured salmon during refrigerated storage (7oC), with the lag phase of less than 24h whereas the lag phase on dry-cured salmon ranged from 4 to 11 days. Strain variation was observed with regard to growth potential on salmon, highlighting the importance of assessing multiple strains with different genetic backgrounds. The evaluation of antimicrobial combinations indicated that nisin is most effective at inactivating L. monocytogenes on salmon and when combined with the blend of potassium lactate and sodium diacetate, the combined treatment can deliver a considerable initial lethality (2 log CFU/g) as well as prevent growth of L. monocytogenes during expected shelf-life of salmon. Further, a L. monocytogenes twocomponent system response regulator, VirR, was found to be the key resistance factor against a range of cell-envelope targeting food antimicrobials. Specifically, VirR-mediated regulation of dltABCD appears to be the major resistance determinant. Thus, novel agents that interfere with these regulatory pathways have the potential to further improve the control of this burdensome pathogen in a variety of RTE foods.
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Listeria monocytogenes; Antimicrobials; cold-smoked salmon
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Food Science and Technology
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Ph. D., Food Science and Technology
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Doctor of Philosophy
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dissertation or thesis
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