Brandt, Alex Lamar (2009-12). In Vitro Inhibition of Listeria Monocytogenes by Novel Combinations of Food Antimicrobials. Master's Thesis. Thesis uri icon

abstract

  • Listeria monocytogenes is a foodborne pathogenic bacterium responsible for ~500 deaths and a financial burden of ~$2.3 billion each year in the United States. Though a zero tolerance policy is enforced with regard to its detection in cooked ready-to-eat foods, additional preemptive control alternatives are required for certain products. Among these alternatives are strategies permitting the usage of food antimicrobial combinations to control the pathogen. Research on antimicrobial combinations can provide insight into more efficient control of the pathogen, but is currently lacking. The purpose of this study was to evaluate the in vitro inhibition of L. monocytogenes exposed to the antimicrobials e-Poly-L-Lysine (EPL), lauric arginate ester (LAE), and sodium lactate (SL) at pH 7.3, octanoic acid (OCT) at pH 5.0, and nisin (NIS) and acidic calcium sulfate (ACS) at both pH 5.0 and 7.3. A broth dilution assay was used to determine single antimicrobial minimum inhibitory and bactericidal concentrations for L. monocytogenes Scott A, 310, NADC 2783, and NADC 2045. Optical density differences (delta<0.05 at 630 nm) were used to denote inhibition. Concentrations producing population decreases of greater than or equal to 3.0 log10 CFU/ml after incubation were considered bactericidal. Inhibition resulting from combinations of antimicrobials (NIS+ACS, EPL+ACS, SL+ACS, NIS+LAE, OCT+ACS, and OCT+NIS) was assessed using a checkerboard assay, and fractional inhibitory concentrations (FIC) were determined. FIC values were plotted on isobolograms and were used to create FIC indices (FICI). Isobologram curvature was used to classify combinations as synergistic, additive, or antagonistic. From FIC indices, interactions were defined as antagonistic (FICI >1.0), additive (FICI =1.0), or synergistic (FICI <1.0). Strain-dependent factors had a bearing on MIC and MBC values for NIS and EPL. At pH 7.3, NIS+ACS displayed synergistic inhibition, NIS+LAE and EPL+ACS demonstrated additive-type interactions, and the SL+ACS pairing was unable to be defined. At pH 5.0, interpretation of the OCT+NIS interaction also presented challenges, while the OCT+ACS combination resulted in synergistic behavior. Additional studies are needed to validate in vitro findings on surfaces of ready-to-eat meats. Future in vivo studies should investigate the ability of synergistic combinations (NIS+ACS and OCT+ACS) to control the pathogen. Better characterizations of inhibitory mechanisms should also be performed.
  • Listeria monocytogenes is a foodborne pathogenic bacterium responsible for ~500

    deaths and a financial burden of ~$2.3 billion each year in the United States. Though a zero

    tolerance policy is enforced with regard to its detection in cooked ready-to-eat foods, additional

    preemptive control alternatives are required for certain products. Among these alternatives are

    strategies permitting the usage of food antimicrobial combinations to control the pathogen.

    Research on antimicrobial combinations can provide insight into more efficient control of the

    pathogen, but is currently lacking.

    The purpose of this study was to evaluate the in vitro inhibition of L. monocytogenes

    exposed to the antimicrobials e-Poly-L-Lysine (EPL), lauric arginate ester (LAE), and sodium

    lactate (SL) at pH 7.3, octanoic acid (OCT) at pH 5.0, and nisin (NIS) and acidic calcium sulfate

    (ACS) at both pH 5.0 and 7.3. A broth dilution assay was used to determine single antimicrobial

    minimum inhibitory and bactericidal concentrations for L. monocytogenes Scott A, 310, NADC

    2783, and NADC 2045. Optical density differences (delta<0.05 at 630 nm) were used to denote

    inhibition. Concentrations producing population decreases of greater than or equal to 3.0 log10 CFU/ml after incubation

    were considered bactericidal.

    Inhibition resulting from combinations of antimicrobials (NIS+ACS, EPL+ACS,

    SL+ACS, NIS+LAE, OCT+ACS, and OCT+NIS) was assessed using a checkerboard assay, and fractional inhibitory concentrations (FIC) were determined. FIC values were plotted on

    isobolograms and were used to create FIC indices (FICI). Isobologram curvature was used to

    classify combinations as synergistic, additive, or antagonistic. From FIC indices, interactions

    were defined as antagonistic (FICI >1.0), additive (FICI =1.0), or synergistic (FICI <1.0).

    Strain-dependent factors had a bearing on MIC and MBC values for NIS and EPL. At

    pH 7.3, NIS+ACS displayed synergistic inhibition, NIS+LAE and EPL+ACS demonstrated

    additive-type interactions, and the SL+ACS pairing was unable to be defined. At pH 5.0,

    interpretation of the OCT+NIS interaction also presented challenges, while the OCT+ACS

    combination resulted in synergistic behavior.

    Additional studies are needed to validate in vitro findings on surfaces of ready-to-eat

    meats. Future in vivo studies should investigate the ability of synergistic combinations

    (NIS+ACS and OCT+ACS) to control the pathogen. Better characterizations of inhibitory

    mechanisms should also be performed.

publication date

  • December 2009