Novel Epidemic Clones of Listeria monocytogenes, United States, 2011

Sara Lomonaco, Bindhu Verghese, Peter Gerner-Smidt, Cheryl Tarr, Lori Gladney, Lavin Joseph, Lee Katz, Maryann Turnsek, Michael Frace, Yi Chen, Eric Brown, Richard Meinersmann, Mark Berrang, Stephen Knabel

, and Authors

Author affiliations: Author affiliations: Università degli Studi di Torino, Turin, Italy (S. Lomonaco); Accugenix, Newark, Delaware, USA (B. Verghese); The Pennsylvania State University, University Park, Pennsylvania, USA (B. Verghese, S. Knabel); Centers for Disease Control and Prevention, Atlanta, Georgia, USA (P. Gerner-Smidt, C. Tarr, L. Gladney, L. Joseph, L. Katz, M. Turnsek, M. Frace); Food and Drug Administration, College Park, Maryland, USA (Y. Chen, E. Brown); US Department of Agriculture, Athens, Georgia, USA (R. Meinersmann, M. Berrang)

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.

In September 2011, the Centers for Disease Control and Prevention (CDC) in Atlanta, GA, was notified of an increase of listeriosis cases linked to eating cantaloupe (1). The outbreak isolates were categorized into 4 pulsed-field gel electrophoresis (PFGE) profiles and serotypes 1/2a and 1/2b, the latter being seldom associated with large outbreaks (1,2). During August 2012, a fifth outbreak-associated subtype responsible for 1 case was detected, and CDC reported a final total of 147 cases from 28 US states, causing 33 deaths and 1 miscarriage (www.cdc.gov/listeria/outbreaks/cantaloupes-jensen-farms/index.html). The Food and Drug Administration (FDA) inspected the involved farm; outbreak strains matching 3 of the PFGE profiles from clinical samples were isolated from washed cantaloupes and various environmental surfaces within the facility (www.fda.gov/Food/FoodSafety/CORENetwork/ucm272372.htm#report).
Epidemic clones (ECs) of Listeria monocytogenes are defined as isolates of a presumably common ancestor that are genetically related and involved in different temporally and geographically unrelated outbreaks (2). Previously, multivirulence locus sequence typing (MVLST) accurately identified the 5 known ECs of L. monocytogenes, ECI–V (3,4). Also, comK prophage junction fragment (JF) sequences were demonstrated to be unique to EC strains of L. monocytogenes in individual facilities that processed ready-to-eat meat and poultry or in multiple plants manufacturing similar ready-to-eat products (5). The comK prophage may represent a rapid adaptation island that enables L. monocytogenes to rapidly adapt to and form biofilms in specific environmental niches (5).
Nine foodborne outbreak-associated isolates related to cantaloupe, representing the 4 outbreak strains initially identified, were selected for multilocus sequence typing (MLST) (6), MVLST (3), and comK prophage JF sequencing (5) to determine if they represented previously identified outbreak strains or known/novel ECs of L. monocytogenes (2–4). Isolates from cantaloupe samples were also compared with 29 US Department of Agriculture (USDA) isolates of L. monocytogenes retrieved from 2 US chicken processing plants (7,8).