Growth of Listeria monocytogenes and spoilage microflora in ready -to -eat ham
Abstract
According to the 2003 USDA-FDA Risk Assessment for Listeria monocytogenes (L. monocytogenes) in ready-to-eat (RTE) foods, RTE deli meats had the highest risk for listeriosis on a per annum and per serving basis. Control measures for L. monocytogenes in RTE meat products have not prevented an influx of recent outbreaks and recalls associated with post-processing contamination of RTE deli meats. In order to determine the overall risk posed by L. monocytogenes in RTE deli meats and develop effective control strategies, more information is needed. In the L. monocytogenes risk assessment, the lack of knowledge on the interactions of spoilage microflora with this pathogen in RTE deli meats was identified as a future need. Therefore, the growth of L. monocytogenes and spoilage microflora in RTE deli ham was investigated at refrigerated storage temperatures of 4 and 10°C. The main objectives were to determine the types of spoilage microflora associated with RTE deli ham, to determine if these spoilage microflora prevent the growth of L. monocytogenes, and to examine the interactions of the spoilage microflora with L. monocytogenes at 4 and 10°C in RTE deli ham at low inoculum levels of 10 to 100 cells. Lactic acid bacteria (LAB) and Bacillus spp. were isolated from RTE deli ham and were screened for their ability to grow in similar products at 4 and 10°C and for their antilisterial activity by an agar spot assay. Isolates with the highest antilisterial activity and broadest spectrum of activity were inoculated with outbreak-associated L. monocytogenes strains onto RTE deli ham, which was stored at 4 and 10°C, and were examined for their interactions at low levels. One LAB and one Bacillus isolate demonstrated bacteriocin production. Antilisterial activity of one LAB isolate was observed to be concentration independent; however, Bacillus isolates were not as effective at lower inoculum levels as were the LAB, as evidenced by the agar spot assay results. Three LAB and one Bacillus isolate inhibited all eleven L. monocytogenes strains, and another Bacillus isolate inhibited 7 of the 11 indicator strains. Numbers of L. monocytogenes inoculated with spoilage isolates onto RTE deli ham that were incubated at 4 and 10°C were significantly lower (P<0.05) than the positive control samples. The mechanisms responsible for the observed inhibition were thought to be competition for nutrients, decreased pH, and the production of one or more antimicrobially active metabolites such as lactic acid and bacteriocins. Data obtained from this study could add to future revisions of the L. monocytogenes risk assessment and aid in the development of novel control strategies for L. monocytogenes in RTE deli meat products. The spoilage isolates observed in this study hold promise for use as bio-protective cultures and the bacteriocins they produce may have application as antilisterial agents for foods.
Degree
Ph.D.
Advisors
Cousin, Purdue University.
Subject Area
Microbiology
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