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. 2004 Oct;70(10):5833-41.
doi: 10.1128/AEM.70.10.5833-5841.2004.

Listeria Monocytogenes Isolates From Foods and Humans Form Distinct but Overlapping Populations

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Free PMC article

Listeria Monocytogenes Isolates From Foods and Humans Form Distinct but Overlapping Populations

Michael J Gray et al. Appl Environ Microbiol. .
Free PMC article

Abstract

A total of 502 Listeria monocytogenes isolates from food and 492 from humans were subtyped by EcoRI ribotyping and PCR-restriction fragment length polymorphism analysis of the virulence gene hly. Isolates were further classified into genetic lineages based on subtyping results. Food isolates were obtained through a survey of selected ready-to-eat food products in Maryland and California in 2000 and 2001. Human isolates comprised 42 isolates from invasive listeriosis cases reported in Maryland and California during 2000 and 2001 as well as an additional 450 isolates from cases that had occurred throughout the United States, predominantly from 1997 to 2001. Assignment of isolates to lineages and to the majority of L. monocytogenes subtypes was significantly associated with the isolate source (food or human), although most subtypes and lineages included both human and food isolates. Some subtypes were also significantly associated with isolation from specific food types. Tissue culture plaque assay characterization of the 42 human isolates from Maryland and California and of 91 representative food isolates revealed significantly higher average infectivity and cell-to-cell spread for the human isolates, further supporting the hypothesis that food and human isolates form distinct populations. Combined analysis of subtype and cytopathogenicity data showed that strains classified into specific ribotypes previously linked to multiple human listeriosis outbreaks, as well as those classified into lineage I, are more common among human cases and generate larger plaques than other subtypes, suggesting that these subtypes may represent particularly virulent clonal groups. These data will provide a framework for prediction of the public health risk associated with specific L. monocytogenes subtypes.

Figures

FIG. 1.
FIG. 1.
Comparison of plaque area (A) and infectivity (B) for 41 human isolates and 88 food isolates of L. monocytogenes tested in a mouse L cell plaque assay. Data for four isolates that formed no plaques were not included in this figure. Plaque area and infectivity (expressed as log-transformed CFU per PFU) are given relative to those of the reference strain, 10403S. Grey and black bars represent the numbers of food and human isolates, respectively. Grey and black lines represent the normalized probability distributions of results from food and human isolates, respectively.
FIG. 2.
FIG. 2.
Association between two measures of in vitro cytopathogenicity (CP) for the four most common L. monocytogenes ribotypes isolated from foods and humans. Low plaque area and high ln(CFU/PFU) indicate low cytopathogenicity. Plaque area and CFU per PFU are expressed as percentages of those of a reference strain (10403S) assigned a plaque area of 1 and a ln(CFU/PFU) of 0. Open symbols represent food isolates, and closed symbols represent human isolates. The ribotype and the R2 of the association between plaque area and ln(CFU/PFU) are given in each panel.

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