Population Structure of Listeria monocytogenes Serotype 4b Isolates from Sporadic Human Listeriosis in the United States, 2003-2008.

Lee S1, Ward TJ, Graves LM, Tarr CL, Siletzky RM, Kathariou S.

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Abstract

Listeria monocytogenes can cause severe foodborne disease (listeriosis). Numerous outbreaks have involved three serotype 4b epidemic clones (ECs): ECI, ECII and ECIa. However, little is known about the population structure of L. monocytogenes serotype 4b from sporadic listeriosis in the United States, even though most cases of human listeriosis are in fact sporadic. Here we analyzed 136 serotype 4b isolates from sporadic cases in the United States, 2003-2008, utilizing multiple tools including multilocus genotyping, pulsed-field gel electrophoresis and sequence analysis of the inlAB locus. ECI, ECII and ECIa were frequently encountered (32, 17 and 7%, respectively). However, annually 30-68% of isolates were outside of these ECs and several novel clonal groups were identified. An estimated 33 and 17% of the isolates, mostly among the ECs, were resistant to cadmium and arsenic, respectively, but resistance to benzalkonium chloride was uncommon (3%) among the sporadic isolates. Frequency of clonal groups fluctuated within the six-year study period, without consistent trends. However, on several occasions, temporal clusters of isolates with indistinguishable genotypes were detected, suggesting the possibility of hidden multistate outbreaks. Our analysis suggests a complex population structure of serotype 4b L. monocytogenes from sporadic disease, with important contributions by ECs and several novel clonal groups. Continuous monitoring will be needed to assess long term trends in clonality patterns and population structure of L. monocytogenes from sporadic listeriosis.