Tarek Msadek, Elisabeth Carniel, Annie Guiyoule, Carmen Buchrieser, Michael B. Prentice, Sandrine Bach, Bactériologie Moléculaire et Médicale, Institut Pasteur [Paris] (IP), Bart's and The London School of Medicine and Dentistry, Biochimie Microbienne, Institut Pasteur [Paris] (IP)-Centre National de la Recherche Scientifique (CNRS), S. Bach received a grant from the Ministère de l’Education Nationale, de la Recherche et de la Technologie. M. Prentice acknowledges support from the Trustees of the Joint Research Board of St. Bartholomew’s Hospital. C. Buchreiser received a grant from the Austrian Program for Advanced Research and Technology., Institut Pasteur [Paris], and Institut Pasteur [Paris]-Centre National de la Recherche Scientifique (CNRS)
Yersinia spp. are gram-negative bacteria belonging to the family Enterobacteriaceae (3). The genus is composed of 11 species which can be divided into three pathogenicity groups. Strains nonpathogenic for humans are found mainly in the environment and sometimes also as transient intestinal saprophytes. These include Yersinia intermedia, Y. frederiksenii, Y. kristensenii, Y. mollaretii, Y. bercovieri, Y. aldovae, Y. rohdei, Y. ruckeri, and Y. enterocolitica biotype 1A (38). Strains with a moderate level of pathogenicity are widespread in cold and temperate countries and are responsible for mild enteric infections in humans (38). They are not lethal for mice at low doses and correspond to Y. enterocolitica strains of biotypes 2 to 5 (14). The group of high-pathogenicity strains is composed of Y. pestis, Y. pseudotuberculosis, and Y. enterocolitica biotype 1B (46), which are all lethal for mice at low doses and cause disseminated infections in humans. This high level of pathogenicity correlates with the presence of a pathogenicity island (PAI) called the high-pathogenicity island (HPI) in Yersinia, as it differentiates low- and high-pathogenicity strains (10). PAIs are large pieces of chromosomal DNA that carry virulence genes, possibly acquired by phage-mediated horizontal transfer (24). The HPI of Y. enterocolitica 1B (10) is a chromosomal fragment of 45 kb that carries virulence genes known as the yersiniabactin locus that are involved in iron uptake (10, 39). Yersiniabactin is a siderophore (26) that endows the bacterium with the ability to acquire the iron molecules necessary for its in vivo growth and dissemination. Most of the length of the HPI is occupied by the yersiniabactin locus (Fig. (Fig.1),1), which comprises the genes irp1 to irp5, involved in nonribosomal biosynthesis of the siderophore (22, 39), and the outer membrane siderophore receptor gene fyuA (27, 42). The G+C content of these genes is higher than the chromosomal background. The yersiniabactin locus is highly conserved among Y. enterocolitica 1B strains (10) and in the two other high-pathogenicity species, Y. pseudotuberculosis (7) and Y. pestis (2, 8, 20). The region located on the right side of this locus, between irp2 and the right-hand boundary of the Y. enterocolitica HPI (Fig. (Fig.1),1), has not yet been sequenced. However, based on hybridization experiments with Y. pestis probes (8), it is most likely that this region carries genes recently described as composing another part of the yersiniabactin locus in Y. pestis (17). The right-hand border of the Y. enterocolitica HPI is defined by an asn-tRNA gene (10). In contrast, the ≈15-kb region of the Y. enterocolitica HPI extending on the left side of the yersiniabactin locus (Fig. (Fig.1)1) is not well conserved among various isolates of this species (10) and is different from the regions found on the HPIs of the two other high-pathogenicity species (7, 8). The only genes identified until now on this 15-kb region correspond to a cluster of three repeated sequences (IS1400 [10], IS1328 [41], and RS3 [10]) (Fig. (Fig.1).1). The left-hand boundary of the island has not been precisely identified. FIG. 1 Genetic organization of the HPI of Y. enterocolitica Ye8081. The HPI is represented by the gray line, and the identified genes are represented by arrows. Dashed lines below the diagram represent probes, named on the basis of their size in kilobases and ... The HPI of Y. enterocolitica 1B thus shows nearly all of the criteria of a typical PAI as defined by Hacker and colleagues (24): (i) it is a large chromosomal DNA fragment, (ii) it carries virulence genes essential for the expression of the high-virulence phenotype, (iii) it harbors several repeated sequences, (iv) it is bordered on one side by a tRNA gene, and (v) its G+C content is different from that of the rest of the chromosome. However, while PAIs are often described as mobile elements, and while the HPIs of Y. pestis (18, 19) and Y. pseudotuberculosis (7) are able to undergo spontaneous deletion from the host chromosome, the Y. enterocolitica HPI seems to be stable. Our previous attempts to identify spontaneous HPI deletion mutants among natural isolates of Y. enterocolitica 1B or following repeated subcultures of high-pathogenicity strain Ye8081 were unsuccessful (16). To date, no precise excision of this island has been reported. The aim of this study was to confirm whether the HPI of Y. enterocolitica was a stable feature of this organism’s genome and, if it was, determine the species-specific mechanism of stabilization.