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Applied and Environmental Microbiology, April 2006, p. 2439-2448, Vol. 72, No. 4
0099-2240/06/$08.00+0     doi:10.1128/AEM.72.4.2439-2448.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Discovery of Natural Atypical Nonhemolytic Listeria seeligeri Isolates

Center for Biologics Evaluation and Research, Food and Drug Administration, Kensington, Maryland 20895,¹ Center for Food Safety and Applied Nutrition, Food and Drug Administration, College Park, Maryland 20740-3835²

Received 4 October 2005/ Accepted 20 January 2006

We found seven Listeria isolates, initially identified as isolates with the Xyl⁺ Rha^– biotype of Listeria welshimeri by phenotypic tests, which exhibited discrepant genotypic properties in a well-validated Listeria species identification oligonucleotide microarray. The microarray gives results of these seven isolates being atypical hly-negative L. seeligeri isolates, not L. welshimeri isolates. The aberrant L. seeligeri isolates were D-xylose fermentation positive, L-rhamnose fermentation negative (Xyl⁺ Rha^–), and nonhemolytic on blood agar and in the CAMP test with both Staphylococcus aureus (S^– reaction) and Rhodococcus equi (R^– reaction). All genes of the prfA cluster of L. seeligeri, located in the prs-ldh region, including the orfA2, orfD, prfA, orfE, plcA, hly, orfK, mpl, actA, dplcB, plcB, orfH, orfX, orfI, orfP, orfB, and orfA genes, were checked by PCR and direct sequencing for evidence of their presence in the atypical isolates. The prs-prfA cluster-ldh region of the L. seeligeri isolates was approximately threefold shorter due to the loss of orfD, prfA, orfE, plcA, hly, orfK, mpl, actA, dplcB, plcB, orfH, orfX, and orfI. The genetic map order of the cluster genes of all the atypical L. seeligeri isolates was prs-orfA2-orfP-orfB-orfA-ldh, which was comparable to the similar region in L. welshimeri, with the exception of the presence of orfA2. DNA sequencing and phylogenetic analysis of 17 housekeeping genes indicated an L. seeligeri genomic background in all seven of the atypical hly-negative L. seeligeri isolates. Thus, the novel biotype of Xyl⁺ Rha^– Hly^– L. seeligeri strains can only be distinguished from Xyl⁺ Rha^– L. welshimeri strains genotypically, not phenotypically. In contrast, the Rha⁺ Xyl⁺ biotype of L. welshimeri would not present an identification issue.

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