title: Determination of Evolutionary Relationships of Outbreak-Associated Listeria monocytogenes Strains of Serotypes 1/2a and 1/2b by Whole-Genome Sequencing.
abstract: We used whole-genome sequencing to determine evolutionary relationships among 20 outbreak-associated clinical isolates of Listeria monocytogenes serotypes 1/2a and 1/2b. Isolates from 6 of 11 outbreaks fell outside the clonal groups or "epidemic clones" that have been previously associated with outbreaks, suggesting that epidemic potential may be widespread in L. monocytogenes and is not limited to the recognized epidemic clones. Pairwise comparisons between epidemiologically related isolates within clonal complexes showed that genome-level variation differed by 2 orders of magnitude between different comparisons, and the distribution of point mutations (core versus accessory genome) also varied. In addition, genetic divergence between one closely related pair of isolates from a single outbreak was driven primarily by changes in phage regions. The evolutionary analysis showed that the changes could be attributed to horizontal gene transfer; members of the diverse bacterial community found in the production facility could have served as the source of novel genetic material at some point in the production chain. The results raise the question of how to best utilize information contained within the accessory genome in outbreak investigations. The full magnitude and complexity of genetic changes revealed by genome sequencing could not be discerned from traditional subtyping methods, and the results demonstrate the challenges of interpreting genetic variation among isolates recovered from a single outbreak. Epidemiological information remains critical for proper interpretation of nucleotide and structural diversity among isolates recovered during outbreaks and will remain so until we understand more about how various population histories influence genetic variation.
authors.affiliation: North Dakota State University, Fargo, North Dakota, USA.
authors.forename: Teresa M
authors.initials: TM
authors.lastname: Bergholz
authors.affiliation: Cornell University, Ithaca, New York, USA.
authors.forename: Henk C
authors.initials: HC
authors.lastname: den Bakker
authors.affiliation: Centers for Disease Control and Prevention, Atlanta, Georgia, USA.
authors.forename: Lee S
authors.initials: LS
authors.lastname: Katz
authors.affiliation: Centers for Disease Control and Prevention, Atlanta, Georgia, USA.
authors.forename: Benjamin J
authors.initials: BJ
authors.lastname: Silk
authors.affiliation: Centers for Disease Control and Prevention, Atlanta, Georgia, USA.
authors.forename: Kelly A
authors.initials: KA
authors.lastname: Jackson
authors.affiliation: Centers for Disease Control and Prevention, Atlanta, Georgia, USA.
authors.forename: Zuzana
authors.initials: Z
authors.lastname: Kucerova
authors.affiliation: Centers for Disease Control and Prevention, Atlanta, Georgia, USA.
authors.forename: Lavin A
authors.initials: LA
authors.lastname: Joseph
authors.affiliation: Centers for Disease Control and Prevention, Atlanta, Georgia, USA.
authors.forename: Maryann
authors.initials: M
authors.lastname: Turnsek
authors.affiliation: Centers for Disease Control and Prevention, Atlanta, Georgia, USA.
authors.forename: Lori M
authors.initials: LM
authors.lastname: Gladney
authors.affiliation: Centers for Disease Control and Prevention, Atlanta, Georgia, USA.
authors.forename: Jessica L
authors.initials: JL
authors.lastname: Halpin
authors.affiliation: Colorado Department of Public Health and Environment, Denver, Colorado, USA.
authors.forename: Karen
authors.initials: K
authors.lastname: Xavier
authors.affiliation: Colorado Department of Public Health and Environment, Denver, Colorado, USA.
authors.forename: Joseph
authors.initials: J
authors.lastname: Gossack
authors.affiliation: U.S. Department of Agriculture, Agricultural Research Service, Peoria, Illinois, USA.
authors.forename: Todd J
authors.initials: TJ
authors.lastname: Ward
authors.affiliation: Centers for Disease Control and Prevention, Atlanta, Georgia, USA.
authors.forename: Michael
authors.initials: M
authors.lastname: Frace
authors.affiliation: Centers for Disease Control and Prevention, Atlanta, Georgia, USA ctarr@cdc.gov.
authors.forename: Cheryl L
authors.initials: CL
authors.lastname: Tarr
mesh_terms: D004196:Disease Outbreaks; D019143:Evolution, Molecular; D022761:Gene Transfer, Horizontal; D014644:Genetic Variation; D016680:Genome, Bacterial; D006801:Humans; D008089:Listeria monocytogenes / Q000235:genetics* / Q000302:isolation & purification; D008088:Listeriosis / Q000453:epidemiology* / Q000382:microbiology*; D010802:Phylogeny; D017354:Point Mutation; D017422:Sequence Analysis, DNA; D065288:Serogroup; D012703:Serotyping; D014481:United States / Q000453:epidemiology