Atypical Non-hemolytic Listeria seeligeri Isolated in the USA
D. V. Volokhov1 , V. E. Chizhikov1 , R. E. Duvall2 , A. D. Hitchins2 , 1CBER, FDA, Rockville, MD, 2CFSAN, FDA, College Park, MD
Background: Regulation of food pathogens, like Listeria monocytogenes, requires accurate identification of them and their related species. This may be confounded when aberrant variants are isolated as is exemplified by this study of non-hemolytic L. seeligeri isolates presenting as L. welshimeri.
Methods: Identities of 7extant isolates of the Rha- biotype of L. welshimeri were re-determined phenotypically. Their hybridization patterns with general iap- and hly-specific probes were determined with a validated species identification oligonucleotide microarray. Their reactivity with L. seeligeri hly-specific oligonucleotide probes was determined. The hly region of the isolates' genome was compared with that of L. seeligeri, ATCC 35967, the type strain, using PCR and nucleotide sequencing. Housekeeping genes were sequenced and phylogenetically analyzed.
Results: The 7 isolates were confirmed as the Rha- biotype of L. welshimeri, not its Rha+ biotype, by standard tests. In contrast, they identified as L. seeligeri based on their hybridization patterns in the species identification oligonucleotide microarray. The isolates did not hybridize with special L. seeligeri hly-specific probes. PCR and sequencing showed that the PrfA regulated virulence-gene cluster was altered in the variants by the loss of 13 genes: orfD-prfA-orfE-plcA-hly-orfK-mpl-actA-dplcB-plcB-orfH-orfX-orfI. Thus the isolates' cluster is: prs-orfA2-[13-gene deletion]-orfP-orfB-orfA-ldh. The isolates' housekeeping genes (iap, 16S rRNA gene, the inter-16S-23S rRNA gene region, prs, comK, groEL, cat, sigB, recA, hsp60, and ldh) were L. seeligeri specific.
Conclusions: Atypical variants of Listeria species, like Hly- L. seeligeri, can sometimes only be recognized genotypically. Phenotypic tests may be insufficient.