Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/27/2004
Publication Date: 5/1/2004
Citation: Wonderling, L.D., Bayles, D.O. 2004. Survival of listeria monocytogenes strain h7762 and resistance to simulated gastric fluid following exposure to frankfurter exudate. Journal of Food Protection. 67:1170-1176 Interpretive Summary: Listeria monocytogenes is a bacterium responsible for sporadic and outbreak cases of food-borne listeriosis. It is currently unclear if foods alter the potential for L. monocytogenes to survive conditions found in the stomach and cause disease. Since listeriosis outbreaks have been linked to the consumption of contaminated ready-to-eat (RTE) meats such as deli meats sand frankfurters, the present work was conducted to understand if L. monocytogenes can survive conditions found in RTE foods and subsequently become more or less likely to cause disease. L. monocytogenes was exposed at 4C to the fluid obtained from packaged frankfurters (exudate) or laboratory media (BHI) and was subsequently tested to measure how well the bacteria survived in simulated gastric fluid (SGF). This provided a measure of whether L. monocytogenes exposed to exudate was more or less able to survive stomach-like conditions. Results indicated that initial exposure to exudate caused L. monocytogenes to be relatively resistant to SGF. Over a 25-day period of exudate exposure, the cells were generally more resistant to SGF; however, exudate-exposed L. monocytogenes cells were generally more resistant to SGF than controls (BHI-exposed cells), possibly due to bacterial adaptations induced in response to exudate. This study indicates that frankfurter package fluid may prime L. monocytogenes to withstand acidic stomach conditions. Identifying these factors may lead to interventions that allow the virulence potential of L. monocytogenes to be minimized.
Technical Abstract: Listeria monocytogenes was tested to determine if it was able to survive at 4C in frankfurter exudate or brain-heart infusion broth (BHI) and to determine if food exposure affects the acid sensitivity of the organism. For acid sensitivity testing, each culture was sampled at intervals and challenged with simulated gastric fluid (SGF) from the day of inoculation until day 25. SGF challenges performed immediately after inoculation revealed that between 20 to 26% of the cells survived the full 30 minutes of SGF challenge regardless of the media type. The cells in both cultures had decreased SGF-sensitivity after 2 days; however, the cells exposed to exudate were significantly more SGF resistant then cells in BHI (after 15 min SGF treatment, exudate-exposed cells had 33% survivors and BHI-exposed cells had 12% survivors). L. monocytogenes exposed to exudate had greater SGF resistance at all challenge times compared to BHI exposed cells through hday 4. After 5 minutes of SGF challenge, exudate-exposed cells continued to have greater SGF resistance than BHI-exposed cells from days 6 to 15, but were as sensitive as the BHI-exposed cells after 20-30 min of challenge. By day 25, cells exposed to exudate were significantly more sensitive to SGF challenge then BHI-exposed cells. The survivor data generated from SGF challenges was modeled, using a non-linear regression analysis, to calculate the underlying distribution of SGF sensitivity found in challenged populations. These analyses indicated that L. monocytogenes exposed to exudate at 4C had a broader distribution of resistance to SGF compared to cells exposed to BHI at 4C. The mean time of death resulting from SGF treatment was higher after exposure to exudate, indicating that cells exposed to exudate were more resistant to killing by SGF.