Novel epidemic clones of Listeria monocytogenes, United States, 2011

Abstract

We identified a novel serotype 1/2a outbreak strain and 2 novel epidemic clones of Listeria monocytogenes while investigating a foodborne outbreak of listeriosis associated with consumption of cantaloupe during 2011 in the United States. Comparative analyses of strains worldwide are essential to identification of novel outbreak strains and epidemic clones.

Figure 1

Four AscI/ApaI pulsed-field gel electrophoresis (PFGE) profiles (identified at the time the research was performed) displayed by Listeria monocytogenes clinical isolates (L2624, L2625, L2626, and L2676) and isolates from food or environmental samples (LIS0072, LIS0075, LIS0077, LIS0078, and LIS0087) associated with the 2011 listeriosis outbreak traced to cantaloupe. PFGE profiles 3 and 4 differ by ≈40-kb shift in 1 band in the AscI pattern, likely related to the loss or acquisition of the comK prophage, because the size of this prophage was ≈40 kb as calculated by using the whole genome sequencing data (not shown).

Figure 2

Unrooted neighbor-joining tree computed in MEGA 5.0 (10) for multivirulence locus sequence typing data based on sequencing of 6 virulence genes, prfA, inlB, inlC, dal, clpP, and lisR (3) obtained for the 93 Listeria monocytogenes isolates compared in this study. Nine cantaloupe-associated outbreak isolates were from the Centers for Disease Control and Prevention (CDC) (GenBank accession nos. JQ407055–JQ407078) and the Food and Drug Administration (FDA) (JX141237–JX141275), 23 isolates from Knabel et al., 2012 (4), 29 isolates from Chen et al., 2007 (3), 29 from Berrang et al., 2005 and 2010 (7,8) (JQ946653–JQ946836), and 3 other isolates for which sequence data were available in GenBank. Squares indicate reference strains representing the 5 currently known L. monocytogenes epidemic clones. Underlined text indicates CDC and FDA isolates associated with the isolates from the 2011 cantaloupe-associated outbreak. Circles indicate strains classified as epidemic clone (EC)VI (indicated in blue). Triangles indicate strains classified as ECVII (indicated in red). Diamond indicates the novel outbreak strain (indicated in green).