INTRODUCTION — The genus Yersinia includes 18 species, three of which are important human pathogens: Yersinia pestis, Yersinia enterocolitica, and Yersinia pseudotuberculosis [1,2]. The yersinioses are zoonotic infections of domestic and wild animals; humans are considered incidental hosts that do not contribute to the natural disease cycle.

Y. enterocolitica and Y. pseudotuberculosis cause yersiniosis, a diarrheal illness. The microbiological characteristics, pathogenic determinants and laboratory isolation and characterization of Y. enterocolitica and Y. pseudotuberculosis will be reviewed here. The epidemiology, clinical manifestations and treatment of these infections are discussed separately. (See "Epidemiology of yersiniosis" and "Clinical manifestations and diagnosis of Yersinia infections".)

Y. pestis causes plague and is discussed separately. (See "Epidemiology, microbiology and pathogenesis of plague (Yersinia pestis infection)" and "Clinical manifestations, diagnosis, and treatment of plague (Yersinia pestis infection)".)

MICROBIOLOGY — Members of the genus Yersinia are gram-negative coccobacilli; they are facultative anaerobes in the family Yersiniaceae (formerly part of the family Enterobacteriaceae) [3,4]. Like others in the family, Y. pseudotuberculosis and Y. enterocolitica are bile tolerant and grow on MacConkey agar, they ferment glucose but not lactose, they are oxidase negative, and reduce nitrate to nitrite.

Y. pseudotuberculosis and Y. enterocolitica are readily differentiated by biochemical tests. Y. enterocolitica ferments sucrose and Y. pseudotuberculosis does not, while Y. pseudotuberculosis ferments rhamnose and melibiose and Y. enterocolitica does not. Y. enterocolitica can be further subtyped into biotypes by a combination of phenotypic markers [5]. They can also be serogrouped using antisera produced against cell surface lipopolysaccharide antigens, known classically as the O antigens [5,6]. For Y. enterocolitica, biogroup and serotype are correlated. The most common are serotype O:9 biotype 2, serotype O:3 biotype 4 and serotype O:8 biotype 1B [7]. A similar serotyping system exists for Y. pseudotuberculosis. Both Y. enterocolitica and Y. pseudotuberculosis can be further subtyped using molecular methods such as pulsed field gel electrophoresis or whole genome sequencing.

Y. enterocolitica exhibits three noteworthy microbiological features. It is psychrophilic, which means that on suitable media it is capable of multiplying at refrigerator temperatures (although its optimum growth temperature is 25 to 28°C). Second, most strains lack efficient intrinsic iron uptake mechanisms, and depend on the iron binding strategies of other bacteria to capture the iron that they need [8]. Conditions associated with iron-overload such as chronic liver disease, hemochromatosis, and thalassemias have been associated with an increased risk of invasive yersiniosis [9]. Third, many virulent strains are relatively calcium dependent, and require calcium-supplemented medium to grow at 37°C.

Certain subtypes of Y. enterocolitica are considered virulent for humans [10]. Isolation of non-virulent types from a non-sterile body site is likely to be an incidental finding. Rapid tests can determine whether an isolate is likely to be pathogenic (see below).

Subtyping by serotyping and/or molecular methods is of epidemiological utility. Biotype and serogroup can provide a clue to the environmental source of an infection [11]. Detecting a cluster of infections of the same molecular subtype may indicate that the cases are linked to a common source.

Laboratory isolation — Definitive laboratory identification of virulent strains depends on isolation of the organism via bacteriologic culture. Most laboratories in North America do not routinely screen for Yersinia species, and the organism grows poorly on the Salmonella-Shigella and Campylobacter agars commonly used for the isolation of stool pathogens [6]. Yersinia grows well on MacConkey agar, but at routine incubation temperatures it forms small colorless lactose-negative colonies, which are easily overlooked by the inexperienced eye unless plates are specifically examined for them [6].

Selective growth medium is recommended in laboratories not familiar with the appearance of the colonies. The most widely studied is cefsulodin-irgasin-novobiocin (CIN) agar, which inhibits the growth of competing flora and produces a characteristic colony morphology (figure 1) [12]. The optimal growth temperature for Yersinia species is 25 to 28°C, which also inhibits the growth of other organisms, even on non-selective MacConkey agar. The organism can be isolated at this temperature within 24 to 48 hours. If a clinician specifically requests Yersinia culture, the laboratory will usually either use CIN media or reincubate the original MacConkey agar culture. CHROMagar Yersinia is reported to be more selective for pathogenic strains of Y. enterocolitica, while Y. pseudotuberculosis does not grow on it at all [13].

The use of a cold enrichment step at 4°C can facilitate recovery of Y. enterocolitica when the bacterial density is low, such as in the convalescent phase of an infection. However, this procedure is likely to lead to isolation of non-pathogenic strains, and may require incubation for weeks, limiting clinical use. Isolates of Y. enterocolitica identified in this fashion must be further characterized before diagnostic conclusions are drawn.

The use of multi-pathogen syndromic diagnostic panels for fecal specimens has expanded. In 2018, 68 percent of yersiniosis cases reported to the United States Centers for Disease Control and Prevention (CDC) Foodborne Diseases Active Surveillance Network were diagnosed using such culture-independent panels; approximately half of those positive specimens that were cultured yielded a Yersinia [14]. The performance of panels that include a Y. enterocolitica target is difficult to measure because of the rarity of infection [15-17]. Just as with culture of cold-enriched specimens, characterization of the pathogen isolate obtained from reflex culture of the original specimen will still be helpful to determine whether it is pathogenic or not. Reflex culture of specimens that test positive by nucleic acid amplification tests (NAATs) is recommended to help guide treatment and for public health surveillance [18]. Multi-analyte panels typically do not include Y. pseudotuberculosis targets and will not detect that pathogen.

In addition to biotype and serotype determination, several rapid tests have been developed to identify pathogenic strains. They are highly specific and sensitive for fresh isolates, although the virulence plasmid can be lost when strains are stored [19]. The presence of pyrazinamidase also is strongly correlated with virulence, and is part of the Wauters biotype schema [5]. Use of Congo red-magnesium oxalate agar (CR-MOX) facilitates detection of the virulence plasmid capable of binding Congo Red dye, and calcium dependent growth at 37 degrees [20]. Polymerase chain reaction (PCR) assays for the presence of specific chromosomal and plasmid-associated virulence determinants have also been used [21].

Serologic assays have been described and used in some epidemiologic studies, but are of limited clinical utility due to crossreactivity [22].

PATHOGENESIS — Pathogenic Y. enterocolitica pass through the stomach, adhere to gut epithelial cells, invade the gut wall, localize in lymphoid tissue within the gut wall and in regional mesenteric lymph nodes, and evade the host's cell-mediated immune response. A 70 kilodalton virulence plasmid known as pYV has been associated with pathogenic Y. enterocolitica. Variants of this plasmid are also present in Y. pseudotuberculosis and Y. pestis, and the presence of the plasmid is closely correlated with calcium-dependency [23]. The organism’s capacity to produce urease releases ammonia from urea, which buffers local gastric acidity and provides relative acid resistance [24]. Adherence and invasion of the mucosal cells occurs via invasin and other surface proteins [25].

The organism elaborates additional proteins that allow it to evade host defense mechanisms, including phagocytosis and the bactericidal action of serum. Two such proteins, Ail (attachment invasion locus) and YadA, are adhesins that also confer resistance to complement-mediated opsonization. The most complex of these additional proteins are the Yersinia outer membrane proteins (Yops), which are encoded by a 70 kilodalton virulence plasmid [26]. The presence of this plasmid is an important virulence determinant, although some pathogenic strains that lack the virulence plasmid have been described.

The Yops are not simple outer membrane proteins, but an extraordinary array of effectors that are injected into host cells via a type III secretion system [27]. The injector apparatus is assembled at 37 degrees, and injection occurs on contact with the target cell. The injected Yops rapidly paralyze phagocytes, block secretion of recruitment molecules such as TNF-alpha and IL-8, and appear to inhibit activation of macrophages [27,28]. The cumulative effect is suppression of inflammation and evasion of phagocytosis.

Some Y. enterocolitica strains have genes for an iron-binding siderophore known as yersiniabactin, which can efficiently bind iron in iron deprived sites, permitting continued rapid growth of Y. enterocolitica biotype 1B [29]. Similar virulence mechanisms have been described in Y. pseudotuberculosis.

The expression of many of these virulence characteristics is temperature dependent [11]. At 25°C, the organism is motile and expresses urease and inv, while at 37°C the organism is non-motile but other virulence factors are expressed [30]. This temperature dependence may be important for survival of the organism outside of the host and for pathogenesis in the host.

A specific Yersinia heat-stable enterotoxin Yst has been described which is similar to the heat stable enterotoxin of enterotoxigenic E. coli [31,32]. Yst is secreted under temperature, pH, and osmolality conditions similar to that of the mammalian ileum [31,32].

A distinct strain of Y. pseudotuberculosis associated with the Far East Asia scarlet-like fever syndrome produces a superantigen, Y. pseudotuberculosis-derived mitogen A, that likely plays a role in the pathogenesis [33].

Y. enterocolitica biotype 1A has generally been regarded as avirulent, although some strains have been described that are clearly enteropathogenic even though they lack pYV and other known virulence determinants [10,34]. Therefore, it is likely that more virulence determinants remain to be identified.

Immunologic sequelae — The pathogenesis of reactive arthritis caused by Yersinia infection is likely to be related to an immune response to Yersinia antigens that cross-react with host antigens in susceptible individuals. The putative inciting Yersinia antigen has not been determined. Host tissue type appears to be the predominant co-factor; HLA-B27 is typically present [35]. T-cells derived from the joint fluid of patients with reactive arthritis have been reported to selectively kill HLA-B27–bearing cells that are also infected with Y. enterocolitica [36]. Although biotype 1A is usually regarded as avirulent, it may lead to reactive arthritis [37].

Erythema nodosum has also been reported in association with yersiniosis; it does not appear to be associated with HLA-B27 [38].

SUMMARY

●Yersinia enterocolitica and Yersinia pseudotuberculosis cause yersiniosis, a diarrheal illness. Members of the genus Yersinia are gram-negative coccobacilli; they are facultative anaerobes in the family Enterobacteriaceae. (See 'Introduction' above.)

●Y. enterocolitica exhibits three noteworthy microbiological features. It is capable of multiplying at refrigerator temperatures (although its optimum growth temperature is 25 to 28°C), many strains lack efficient intrinsic iron uptake mechanisms, and virulent strains require calcium-supplemented medium to grow at 37°C. (See 'Microbiology' above.)

●Yersinia grows well on MacConkey agar, but at routine incubation temperatures it forms small colorless lactose-negative colonies, which are easily overlooked unless plates are specifically examined for them. Selective growth medium is recommended in laboratories not familiar with the appearance of the colonies. The most widely studied is cefsulodin-irgasin-novobiocin (CIN) agar, which inhibits the growth of competing flora and produces a characteristic colony morphology (figure 1). After initial isolation, additional characterization may be helpful as some Y. enterocolitica are likely to be non-pathogenic. (See 'Laboratory isolation' above.)

●Rapid diagnosis of Y. enterocolitica infections is available on commercial multiplex polymerase chain reaction platforms, but these can detect avirulent as well as virulent strains. Culture and further characterization of isolates can assess virulence. (See 'Laboratory isolation' above.)

●Pathogenic Y. enterocolitica pass through the stomach, adhere to gut epithelial cells, invade the gut wall, localize in lymphoid tissue within the gut wall and in regional mesenteric lymph nodes, and evade the host's cell-mediated immune response. They can evoke reactive immune phenomena, including reactive arthritis and erythema nodosum. (See 'Pathogenesis' above.)