Refinement of Listeria monocytogenes (L. monocytogenes) Low Dose Data from Pregnant Guinea Pigs for Human Risk Assessment

This research used animal models to develop dose response information for human listeriosis risk assessment modeling. It gathered information on the low dose region of the dose response curve and attempt to identify mechanisms by which L. monocytogenes causes illness and death.

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Objectives

L. monocytogenes is a foodborne bacterium that has been found on a variety of foods including deli meats, and its occurrence has resulted in many recalls.  To investigate its pathogenicity, the investigators used pregnant guinea pigs as an animal model for human listeriosis (Williams et al, 2007) demonstrating that maternal exposure to high concentrations of L. monocytogenes resulted inthe pathogen crossing the intestinal barrier, colonizing maternal and fetal tissues and eventually resulting in stillbirths, the same as seen in humans.  To be useful in assessing human risk from exposure to L. monocytogenes, data were needed for exposure to low concentrations.  The overall objective of this project was to conduct studies investigating exposures to low doses of L. monocytogenes, to identify biomarkers of L. monocytogenes infection and to use that information to refine the dose response curve, and ultimately the human health risk assessment for exposure to L. monocytogenes.  The specific objectives were:  (1) to use L. monocytogenes labeled with green fluorescent protein (gfp) to examine the dose-related invasion of maternal liver, maternal spleen, placenta, fetal liver and fetal brain in guinea pigs, (2) to identify sub-lethal biomarkers for L. monocytogenes infection at low doses, and (3) focusing on the low dose area of the dose response curve, to develop a dose response curve for the sub-lethal endpoints and ultimately compare these dose response curves to those from primates and the FDA/USDA/CDC human risk assessment.

Conclusions

Using the pregnant guinea pig model, investigators showed that following ingestion of gfp-labeled L. monocytogenes, the pathogen was isolated from maternal liver and spleen, placenta, fetal liver and brain.  Using fluorescent microscopy, they also demonstrated that the gfp maintained its fluorescence throughout the experimental procedure including and in culture without antibiotic pressure.  Biomarkers of L. monocytogenes infection were identified including hepatic lesions, placental apoptosis, and changes in cytokine levels.

L. monocytogenes cells invaded maternal tissue in pregnant guinea pigs treated with 10² CFU L. monocytogenes; yet there was no fetal invasion.  Exposure to L. monocytogenes at an early gestation did not affect invasion of maternal or fetal tissues. Using data from two previously established animal models, the guinea pig and nonhuman primate, investigators calculated LD₅₀ valuesof 10⁷ CFU, which is similar to the estimate from the FAO/WHO (1.9 x 10⁶ CFU) but unlike the 10¹³ CFU estimation by the FDA/USDA/CDC.

Deliverable

The current study further characterized the guinea pig as a surrogate for human listeriosis.  Deliverables from our specific aims are:  1) gfp-L. monocytogenes was visible using fluorescent microscopy but was not significantly better than traditional culture methods at low doses, 2) sublethal biomarkers of listeriosis were hepatic lesions, placental apoptosis, and changes in maternal cytokine levels, and 3) at 10² CFU L. monocytogenes invaded maternal liver and spleen but not fetal tissues.  These studies resulted in four publications (plus one in preparation), and three awards for best presentations at national meetings.