Viviane Chenal-Francisque,1 Jodie Lopez,1
Thomas Cantinelli, Valerie Caro, Coralie Tran,
Alexandre Leclercq, Marc Lecuit,2
and Sylvain Brisse2
Listeria monocytogenes is a worldwide pathogen,
but the geographic distribution of clones remains
largely unknown. Genotyping of 300 isolates from the 5
continents and diverse sources showed the existence
of few prevalent and globally distributed clones, some
of which include previously described epidemic clones.
Cosmopolitan distribution indicates the need for genotyping
with a particularly high case-fatality rate. Listeriosis is a
major public health concern in all world regions, with an
increasing incidence in Europe, especially among elderly
L. monocytogenes is genetically heterogeneous (3–5).
To help epidemiologic investigation and to defi ne clones,
i.e., groups of genetically similar isolates descending from
a common ancestor, a variety of typing methods have
been used, including pulsed-fi eld gel electrophoresis (5,6),
single nucleotide polymorphism typing (7), and multiple
housekeeping and virulence gene sequencing (8,9). Some
clones implicated in multiple outbreaks have been defi ned
as epidemic clones (EC) (3,5,9–11). ECI and ECIV have
been described in several countries (3,5), but because of the
lack of standardization of genotyping, a defi nition of clones
is not widely accepted, and current knowledge on the global
distribution of L. monocytogenes clones is virtually absent.
Multilocus sequence typing (MLST) is a reference method
for global epidemiology and population biology of bacteria,
isteria monocytogenes is a foodborne pathogen that can
cause listeriosis, a severe invasive infection in humans
and its application to L. monocytogenes (12) effectively
allows isolate comparisons across laboratories (www.
pasteur.fr/mlst). The aim of this study was to investigate
the global distribution of L. monocytogenes MLST-defi ned
Three hundred L. monocytogenes isolates were
collected from different sources from 42 countries on 5
continents (online Appendix Table, www.cdc.gov/EID/
from 1) the collection of the World Health Organization
Collaborating Center for Listeria and 2) the Seeliger Listeria
Culture Collection. When available, up to 10 countries per
continent were included. Only 1 isolate per documented
outbreak was kept, and the isolates from a given country
were selected from various sources, years, and serotypes.
A total of 117 isolates were from humans, 107 from food,
28 from animals, 32 from the environment and vegetation,
and 16 of undocumented origin. The relative proportion
of isolates from distinct sources was similar among world
regions (online Appendix Table), except that no animal
isolate was available from the Western Hemisphere and
that the ratio of human to food isolates was lower from this
Each isolate was hemolytic when streaked for
isolation on blood agar. Genomic DNA was extracted by
using Promega Wizard Genomic DNA purifi cation kit
(Promega, Madison, WI, USA). Serotype information
was confi rmed by PCR serogrouping (13). MLST was
performed as described (12). Alleles and sequence types
(STs) are publicly available at www.pasteur.fr/mlst. Clonal
complexes (CC) were defi ned as groups of STs differing
by only 1 gene from another member of the group (12)
and were considered as clones. The θ estimator of the Fst
statistic, which measures population differentiation, was
determined on the basis of ST frequency by using FSTAT
The 300 isolates represented 111 STs (diversity index
95.4%) grouped into 17 CCs (online Appendix Figure, www.
analysis of the concatenated genes (not shown) indicated
that 199, 98, and 3 isolates belonged to lineages I, II, and
III, respectively (12). In lineage I, 3 CCs were highly
prevalent: CC1 (47 isolates, serotype 4b), CC2 (64 isolates,
4b,) and CC3 (32 isolates, 1/2b). The remaining isolates of
lineage I were of serotype 4b or 1/2b (Table). In lineage II,
CC9 (28, all with serotype 1/2c, except one 1/2a isolate)
was the most frequent, followed by CC7 (15 1/2a isolates).
All other lineage II isolates had serotype 1/2a.
The isolates derived
1110 Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 17, No. 6, June 2011
1These authors contributed equally to this study.
2These authors contributed equally to this study.
Author affi liations: Institut Pasteur, Paris, France (V. Chenal-Fran-
cisque, J. Lopez, T. Cantinelli, V. Caro, A. Leclercq, M. Lecuit, S.
Brisse); World Health Organization Collaborating Center for Liste-
ria, Paris (V. Chenal-Francisque, A. Leclercq, M. Lecuit); Inserm
Avenir U604, Paris (M. Lecuit); and Université Paris Descartes,
Paris (M. Lecuit)
Worldwide Distribution of L. monocytogenes
Comparisons of populations from different sources
(Table) showed a clear partitioning of genotypic diversity
between clinical isolates on the one hand and food or
environmental isolates on the other (θ = 0.033 and
0.050, respectively; p<0.0002). Consistent with common
knowledge (4,5), and even though recent outbreaks in
Canada and Austria/Germany were caused by 1/2a strains,
isolates of serotype 4b were, compared with other serotypes,
relatively more frequent in human cases than in food. This
difference in source distribution was further demonstrated
for individual clones because the human/food ratio of
both CC1 (2.6) and CC2 (2.8) differed signifi cantly from
those of CC3 (0.65) and CC9 (0.5) (χ2 p<0.01 for the 4
A global distribution of L. monocytogenes clones was
evident (Figure). Frequent clones were found in many
countries (up to 30 countries for CC2; online Appendix
Table) and were globally distributed. Remarkably, CC1
and CC2 were predominant in all world regions except
northern Africa for CC1 (Figure). CC3 ranked among the
4 most common clones in all regions, whereas CC9 ranked
third in Europe and the Western Hemisphere. Altogether,
these 4 clones represented 54 (50%) food isolates and 80
(68%) clinical isolates. Our results show that the same
few clones account for a large fraction of nonepidemic L.
monocytogenes isolates in distant world regions. However,
continents and sources were not equally represented in
our sample, and larger studies are needed to confi rm
our hypothesis that the clonal composition is similar
across world regions and countries. Consistent with their
cosmopolitan distribution, 15 of the 17 clones found herein
(except CC199 and CC315, with only 6 and 3 isolates,
respectively) included isolates from our previous analysis
of 360 isolates, mostly from France (12).
This study provides the fi rst global view of L.
monocytogenes clonal diversity. Our results clearly
demonstrate the worldwide distribution and high prevalence
of a few frequent clones in distinct world regions. In the
current debate on the phylogeography of bacterial species
(14), major L. monocytogenes clones clearly fi t in the
“everything is everywhere” group, as do other pathogens
in the environment, e.g., Pseudomonas aeruginosa (15).
Dispersal by human travel, animal or food trade, wild
animal migration, or wind and dust all might contribute to
the global diffusion of L. monocytogenes clones. However,
fi ner phylogenetic resolution will possibly subdivide
Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 17, No. 6, June 2011 1111
Table. Distribution of the major Listeria monocytogenes clonal complexes in lineages I and II among sources
Lineage or clonal
Lineage I, total
Lineage II, total
Other, lineage II (1/2a)
No. (%), by source
Figure. Number of isolates
from 7 world regions where
the most prevalent clones of
Listeria monocytogenes are
widespread MLST-defi ned clones into subclades that might Download full-text
exhibit phylogeographic partitioning and will better clarify
the rate and patterns of strain dispersal.
Remarkably, some ECs correspond with highly
prevalent clones. ECII, described relatively recently (6), and
ECIII, involved in outbreaks from a single plant, correspond
to 2 clones (CC6 and ST11, respectively ), that were rare
herein (5 and 0 isolates, respectively), suggesting that both
clones experienced particular conditions that favored their
diffusion on specifi c occasions. In contrast, the outbreaks
caused by ECI and ECIV, reference strains of which
belong to CC1 and CC2, respectively (12), could have been
favored by their high prevalence in sources. One important
question for future research is whether ECs correspond
entirely to MLST-defi ned clones (i.e., CCs) or whether, on
the contrary, they represent a genotypic subset thereof. The
cosmopolitan distribution of clones, which protects them
against extinction resulting from local disturbances, further
highlights the crucial need to standardize L. monocytogenes
genotyping to improve global epidemiologic knowledge
and monitoring of current emergence trends.
We thank all laboratories that have sent isolates to the World
Health Organization Collaborating Center for Listeria.
This study was supported by Institut Pasteur (Paris, France)
and by the Institut de Veille Sanitaire (Saint-Maurice, France).
Ms Chenal-Francisque is a microbiologist at Institut
Pasteur. Her primary research interests include the molecular
epidemiology of infectious diseases and evolution of bacterial
pathogens, particularly L. monocytogenes.
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Address for correspondence: Sylvain Brisse. Institut Pasteur, Genotyping
of Pathogens and Public Health (PF8), 28 Rue du Dr Roux, F-75724 Paris,
France; email: firstname.lastname@example.org
1112 Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 17, No. 6, June 2011