Submitted to: Applied and Environmental Microbiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/17/2011
Publication Date: 2/20/2012
Citation: Lee, S., Ward, T.J., Siletzky, R.M., Kathariou, S., Graves, L., Sperry, K., Wolf, L. 2012. Atypical Listeria monocytogenes Serotype 4b strains harboring a lineage II-specific gene cassette. Applied and Environmental Microbiology. 78(3):660-667. Interpretive Summary: Listeria monocytogenes is a food-borne bacterium that places a serious burden on human health and the agricultural economy. However, not all L. monocytogenes strains are equally likely to cause disease, and strains designated as serotype 4b appear to pose the greatest risk to human health. As a result, rapid methods to distinguish among different L. monocytogenes and identify serotype 4b strains play an important part in assessing risk and understanding why serotype 4b isolates pose an increased risk to human health. In this study, we determined that one of the most widely used methods to rapidly identify L. monocytogenes serotypes failed to perform as expected for a small group of serotype 4b strains because these strains recently acquired a set of genes from strains of a different serotype. This study provides information to improve methods of strain differentiation, and will be of direct interest to epidemiologists, public health and food inspection agencies, and scientists involved in food safety research.
Technical Abstract: Listeria monocytogenes is the etiological agent of listeriosis, a severe foodborne illness. The population of L. monocytogenes is divided into four lineages (I-IV) and serotype 4b in lineage I has been involved in numerous outbreaks. Several serotype 4b epidemic-associated clonal groups (ECI, II, and Ia) have been identified. In this study, we characterized a panel of strains of serotype 4b that produced atypical results with a serotype-specific multiplex polymerase chain reaction (PCR) and possessed the lmo0734-lmo0739 gene cassette that had been thought to be specific to lineage II. The cassette was harbored in a genomically equivalent locus in these isolates and in lineage II strains. Three distinct clonal groups (Group 1-3) were identified among these isolates based on multilocus genotyping and DNA hybridization data. Groups 1 and 2 had multilocus genotyping (MLGT) haplotypes previously encountered among clinical isolates and were composed of clinical isolates from multiple states in the United States. In contrast, Group 3 consisted of clinical and environmental isolates solely from North Carolina and exhibited a novel haplotype. In addition, all Group 3 isolates had DNA that was resistant to MboI, suggesting methylation of adenines of GATC sites. Sequence analysis of the lmo0734-lmo0739 gene cassette from two strains (Group 1 and Group 3) revealed that the genes were highly conserved (>99% identity). The data suggest relatively recent horizontal gene transfer from lineage II L. monocytogenes into L. monocytogenes serotype 4b, and subsequent dissemination among at least three distinct clonal groups of L. monocytogenes serotype 4b, one of which exhibits restrictions in regional distribution.