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.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/ETD-TAMU-2009-12-7520 |
| Date | 2009 December 1900 |
| Creators | Brandt, Alex Lamar |
| Contributors | Taylor, Thomas M. |
| Source Sets | Texas A and M University |
| Language | en_US |
| Detected Language | English |
| Type | Book, Thesis, Electronic Thesis, text |
| Format | application/pdf |
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