4. P ATHOGENIC Y ERSINIAE
4.1. General Taxonomy and Phylogeny of Yersiniae
Bacteria of the genus Yersinia are facultative anaerobic psychrotrophic bacilli belonging to the family Enterobacteriaceae. Seventeen Yersinia species, discriminated from each other by significant DNA-based criteria and biochemical properties, have been identified so far (Savin et al., 2014). Among them, three are pathogenic to humans: Yersinia pestis, Yersinia pseudotuberculosis and Yersinia
enterocolitica. The two latter ones are qualified as enteropathogenic since they cause enteric disorders and cause infection through an oro- fecal mode of transmission (Carniel et al., 2002). Y. pestis, transmitted by rodent flea bites, is the etiological agent of plague (Perry and Fetherston, 1997; Stenseth et al., 2008). Y. pestis causes primarily the so- called bubonic form of plague after migrating from the insect bite to the proximal lymph node, resulting in the formation of a bubo. If not treated, the disease evolves to a systemic disease which is lethal in >90% of cases. Occasionally, the disease may evolve to a pneumonic stage resulting in massive invasion of the lungs by the plague bacilli.
Pneumonic plague can spread rapidly due to the highly contagious aerosols expectorated by infected individuals (Perry and Fetherston, 1997). Yersinia ruckeri is a fish pathogen causing the enteric redmouth disease in salmonid fish like the rainbow trout (Ewing et al., 1978).
Yersinia similis, termed that way for its close genetic proximity with Y.
pseudotuberculosis, has been isolated from rabbit and surface water but has never been isolated from diseased or dead mammals (Sprague et al., 2008). Another Yersinia was identified as a member of the Y.
pseudotuberculosis group. This species was isolated in Korea and has been differentiated from Y. pseudotuberculosis thanks to Multiple Locus Sequence Typing (MLST), a molecular typing technique based on the nucleotide sequencing of selected house-keeping genes (Laukkanen- Ninios et al., 2011a), and was named Yersinia wautersii. Conversely to
Y. similis, Y. wautersii can cause human and animal gastro-enteric infection (Savin et al., 2014). Comparatively, other Yersinia species are closely related to Y. enterocolitica: Yersinia krirtensenii, Yersinia
intermedia, Yersinia mollaretii, Yersinia frederiksenii and Yersinia
bercovieri. No human or animal pathogenic potential has been reported for these Y. enterocolitica-like species. Other Yersiniae diverged into non-pathogenic species: Yersinia aldovae, Y. aleksiciae, Y. rohdei and Y.
massiliensis (Sulakvelidze 2000; Sprague and Neubauer 2005; Merhej et al. 2008). In 2011, a novel species of Yersinia was isolated from a diseased larva of a Scarabaeidae from New Zealand. This Yersinia species was shown to be entomopathogenic towards a broad range of insects and was thus termed Yersinia entomophaga (Hurst et al. 2011; Hurst et al. 2011). Even if Y. wautersii, Y. ruckeri and Y. entomophaga are animal pathogens, in this work we will gather under the name “pathogenic Yersiniae” the three following species: Y. enterocolitica, Y.
Although they cause the same disease and have the same transmission mode, Y. enterocolitica and Y. pseudotuberculosis diverged from a distant common ancestor (41 – 186 million years ago). Despite its totally different etiology and mode of transmission, Y. pestis evolved from Y. pseudotuberculosis very recently (no more than 20,000 years ago). Indeed, while Y. pseudotuberculosis and Y. enterocolitica are food- and water-borne pathogens usually transmitted by the faecal-oral route, Y. pestis uses fleas as vector to infect new hosts (mainly rodents) (Achtman et al., 1999; Wren, 2003). The main difference between Y.
pseudotuberculosis and Y. pestis is the acquisition by the latter of extra virulence plasmids (Table 2). The three pathogenic Yersiniae do possess one large highly-conserved plasmid (about 70kb) called pYV (for
Yersinia Virulence) or pCD (for Calcium Dependence) which encodes virulence factors such as the Yersinia adhesine (YadA) or Outer membrane Proteins (Yops), which are virulent effectors secreted by a Type Three Secretion System composed of the Ysc proteins (Yop Secretion) and encoded by the same plasmid. In vitro, the presence of this plasmid confers a so-called low-calcium response (Lcr) phenotype rendering bacteria unable to grow at 37°C in the absence of Ca2+. This is an ecological regulation mechanism avoiding virulence factors production outside a susceptible mammal host (Straley and Bowmer 1986; Cornelis et al. 1998; Zadernowska et al. 2013). In addition to this common plasmid, Y. pestis acquired two other plasmids termed pFra (or pMT for murine toxin) and pPla. The former encodes a phospholipase D, described as a murine toxin (Ymt) required for the colonization of the plague flea vector (Hinnebusch et al., 2002). The pPla (for Plasminogen activator) plasmid encodes a protease termed Pla that activates plasminogen in order to facilitate circulation of the plague bacillus in the infected blood (Lathem et al., 2007). Together
with a high pathogenicity island (HPI) and a number of minor genetic mutations in the Y. pestis genome as compared to Y. pseudotuberculosis, these plasmids cause the deadly famous disease Y. pestis is sadly known for.
Table 2: Virulence plasmids repartition through human pathogenic
Yersiniae
Plasmid name Host species Main virulence factors encoded
pYV (or pCD) Y. enterocolitica Y. pseudotuberculosis Y. pestis Adhesines Yops T3SS
pMT (or pFra) Y. pestis Murine toxin (phospholipase D)
pPla Y. pestis Plasminogen activator protease
There is a huge diversity among enteropathogenic Yersiniae while Y. pestis is a monomorphic clone of its parental species, Y.
pseudotuberculosis. Y. enterocolitica is subdivided into 6 biotypes based on their biochemical characteristics and pathogenicity (1A, 1B, 2, 3, 4 and 5). Biotype 1B is thought to be the most pathogenic to human (Zadernowska et al. 2013). Y. enterocolitica can also be subdivided into 76 distinct serotypes based of the variable structure of the O-specific polysaccharide chain of the LPS. Only a few serotypes are pathogenic. O:3 is found primarily in Europe, Canada, China and Australia, O:8 is mainly found in the USA and Japan and O:9 causes gastro-intestinal infections in Scandinavia, the Netherlands and China (Sabina et al., 2011). There are 15 distinct major O-serotypes among Y.
to their virulence factors (plasmid and HPI) content (Laukkanen- Ninios et al., 2011b). Like for Y. enterocolitica, the prevalence of Y.
pseudotuberculosis serotypes varies geographically. In Europe, the most encountered serotypes are O:1, O:2 and O:3 while in Japan isolates most often belong to serotypes O:4b, O:3, O:5a, and O:5b. Y. pestis cannot be divided into serotypes because it does not express the LPS O-specific polysaccharide chain (Castro et al., 2009). Based on some minor biochemical properties, Y. pestis has been subdivided into 4 biovars: Antiqua (from the Justinian plague around the Mediterranean basin during Antiquity period), Medievalis (from the Black Death and subsequent epidemics in Europe between 1346 and the early 19th
century), Orientalis (from China ad spread globally via marine shipping from 1860s till now) and Pestoides (from Russian and Mongolian enzootic isolates) (Anisimov et al., 2004; Morelli et al., 2010; Perry and Fetherston, 1997). If variations in biochemical activities exist between these 4 biovars, it is unlikely that each of them are specifically related to the time of the plague epidemic waves their names refer to. This was demonstrated after the finding of Y. pestis Orientalis DNA from European burial places aged from the 12th – 15th centuries
(Haensch et al., 2010; Tran et al., 2011; Welford and Bossak, 2010).