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. 2006 Jan;72(1):776-83.
doi: 10.1128/AEM.72.1.776-783.2006.

Competitive fitness of Listeria monocytogenes serotype 1/2a and 4b strains in mixed cultures with and without food in the U.S. Food and Drug Administration enrichment protocol

Lisa Gorski  1 , Denise Flaherty, Robert E Mandrell
Affiliations

Competitive fitness of Listeria monocytogenes serotype 1/2a and 4b strains in mixed cultures with and without food in the U.S. Food and Drug Administration enrichment protocol

Lisa Gorski et al. Appl Environ Microbiol. 2006 Jan.

Abstract

Thirteen different serotypes of the food-borne pathogen Listeria monocytogenes have been described. Serotype 4b strains are most often associated with illness, and serotype 1/2a strains are most often isolated from foods and processing plants. Different abilities to respond to stresses have been described for serotype 4b and 1/2a strains. One of the common enrichment protocols used to test foods for the presence L. monocytogenes is described in the U.S. Food and Drug Administration (FDA) Bacterial Analytical Manual (BAM). We compared three strains of L. monocytogenes serotype 4b and five strains of serotype 1/2a in direct competition with each other in two-strain mixed cultures by using the FDA BAM enrichment protocol, which includes both enrichment broth and selective agar, with and without added food to mimic the conditions that occur during attempts to isolate Listeria species from contaminated foods. Using a colony immunoblot procedure and analyzing over 112,000 colonies, we observed differences in strain fitness, but these differences were not attributable to serotype or genetic lineage.

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Figures

FIG. 1.
FIG. 1.
Growth of cultures in BLEB before and after addition of BAM supplements. The time of addition of the supplements is indicated by an arrow. See text for details.
FIG. 2.
FIG. 2.
Typical immunoblot with mixed serotype colonies of L. monocytogenes. The pink colonies are RM2387 colonies (serotype 4b), and the blue colonies are RM3364 colonies (serotype 1/2a).
FIG. 3.
FIG. 3.
Percentages of serotype 4b and 1/2a strains resulting from mixed enrichments in BLEB without added food. Shaded bars, serotype 4b strains; open bars, serotype 1/2a strains. The serotype 1/2a strains are indicated on the x axes. (A) RM2387 versus serotype 1/2a strains; (B) RM2199 versus serotype 1/2a strains; (C) RM3176 versus serotype 1/2a strains.
FIG. 4.
FIG. 4.
Percentages of serotype 4b and 1/2a strains resulting from mixed enrichments in BLEB with cheese. Shaded bars, serotype 4b strains; open bars, serotype 1/2a strains. The serotype 1/2a strains are indicated on the x axes. (A) RM2387 versus serotype 1/2a strains; (B) RM2199 versus 1/2a strains; (C) RM3176 versus 1/2a strains.
FIG. 5.
FIG. 5.
Percentages of serotype 4b and 1/2a strains resulting from mixed enrichments in BLEB with lettuce. Shaded bars, serotype 4b strains; open bars, serotype 1/2a strains. The serotype 1/2a strains are indicated on the x axes. (A) RM2387 versus serotype 1/2a strains; (B) RM2199 versus serotype 1/2a strains; (C) RM3176 versus serotype 1/2a strains.
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