is a Gram-positive bacterium that can cause life-threatening infections for humans and more than 40 animal species. Immunocompromised individuals, pregnant women, the elderly and neonates are at high risk for listeriosis. Outbreaks of listeriosis have been associated with the consumption of ready-to-eat foods, especially meat and dairy products (1
). The disease can result in abortion, stillbirths, septicemia, meningitis, encephalitis and death. The ubiquity of L.monocytogenes
in food processing, distribution and retail environments, coupled with its inherent resistances and ability to grow in many foods, including those stored refrigerated, makes this pathogen particularly difficult to both manage and regulate (1
). In the USA, L.monocytogenes
is responsible for about 2500 cases of listeriosis each year, with a hospitalization rate of 91% and a case fatality rate of 20% (2
). Despite appreciable efforts worldwide by research organizations, regulatory-action agencies and the food industry to reduce the incidence of listeriosis, this pathogen, quite arguably, remains the most critical threat to the safety of our food supply.
There are 13 described serotypes of L.monocytogenes
, with serotypes 1/2a, 1/2b and 4b accounting for 95% of human infections (3
). Among strains recovered from foods or food processing plants, serotype 1/2a strains are over-represented. Serotype 4b strains are, however, over-represented when compared with other serotypes among strains responsible for outbreaks and sporadic cases of listeriosis (4
). The species also exists in two major genomic divisions, with substantial linkage disequilibrium and apparently limited gene flow between the two. Numerous molecular subtyping data indicate that the divisions fall along serotypic cluster lines, division I consisting of serotypes 1/2a, 1/2c, 3a and 3c, and division II of serotypes 1/2b, 4b and 3b (5
). The clonality of the pathogen remains poorly described, and descriptions of diversity at the global genomic level have been lacking.
To date, only L.monocytogenes
strain EGD-e (serotype 1/2a) and Listeria innocua
CLIP 11262 (serotype 6a) have been fully sequenced (8
). Although the initial comparison between these two strains provided considerable insight on the virulence attributes of this pathogen, the sequencing and comparative genomic analysis of additional strains was necessary if a core set of L.monocytogenes
-specific genes was to be defined.
To better understand the molecular mechanisms of L.monocytogenes
virulence in humans and survival of this bacterium in food and in the environment, a genomic survey of three strains of L.monocytogenes
was conducted. These strains were chosen as they are food isolates associated with human listeriosis, and they represent the two main genomic divisions. More specifically, L.monocytogenes
strain F2365 is a serotype 4b (genomic division II) cheese isolate from the Jalisco cheese outbreak of 1985 in California (9
strain F6854 is a serotype 1/2a (genomic division I) turkey frankfurter isolate from a sporadic case in 1988 in Oklahoma (10
), and L.monocytogenes
strain H7858 is a serotype 4b frankfurter isolate from the multistate outbreak of 1998–1999 in the USA (11
). The strains were used in a comparative genomics study that includes a comparison with the two previously published strains: L.monocytogenes
strain EDG-e (serotype 1/2a) and L.innocua
strain CLIP 11262 (8
). The analyses of the newly sequenced L.monocytogenes
genomes have provided novel information that improves on current understanding of this species.