Involvement of Closely Related Strains of a New Clonal Group of Listeria monocytogenes in the 1998–99 and 2002 Multistate Outbreaks of Foodborne Listeriosis in the United States

In 1998–99, a multistate outbreak of listeriosis in the United States was associated with contaminated hot dogs and was caused by a strain of Listeria monocytogenes serotype 4b that had been only rarely encountered before in the national PulseNet database. Upon further characterization, the strains from this outbreak were designated as Epidemic Clone II (ECII). ECII isolates exhibited diversification in a genomic region ("region 18") that was otherwise conserved among L. monocytogenes of serotype 4b. Additional unique genetic markers were identified through genome sequencing of one of the isolates from the 1998–99 outbreak. In 2002, another multistate outbreak of listeriosis also involved bacteria of serotype 4b and was attributed to contaminated turkey deli meats. Molecular subtyping data revealed that the macrorestriction patterns of the isolates from the 1998–99 and 2002 outbreaks were closely related. In addition, the 2002 outbreak isolates harbored chromosomal genetic markers found to be unique to, and typical of, the 1998–99 outbreak isolates, including diversification in genomic region 18. Macroarray- based subtyping using chromosomal sequences confirmed the close genetic relatedness between the isolates from the two outbreaks. Genomic content was highly conserved among isolates from each outbreak, with differences detected only in prophage and internalin-like gene sequences. However, since these differences were observed among isolates from each of the outbreaks, they did not differentiate the 1998–99 isolates as a group from those of the 2002 outbreak. Two of 15 randomly chosen serotype 4b clinical isolates from a non-outbreak period(calendar year 2003) appeared to be closely related to the 1998–99 and 2002 outbreak isolates. These findings suggest that both multistate outbreaks of listeriosis in the United States involved closely related members of a single clonal group (ECII) that had not been identified in outbreaks prior to 1998. Since the outbreaks involved different food vehicles and processing plants, the findings suggest establishment of ECII in a still unidentified reservoir in the United States, from which the organisms were introduced to different processing plants.