Yersinia

Q: What antigens are part of the antigenic structure of Yersinia pestis?

The antigenic structure of Yersinia pestis includes a complex set of antigens, featuring thermostable somatic (O-antigens) and thermolabile capsular antigens. Species-specific antigens of the plague bacillus include: FI — capsular antigen (possesses protective activity);FII — murine toxin;PI — pesticin;Fb — fibrinolysin;Pq — plasmocoagulase. Additionally, the pathogen shares cross-reactive antigens with human blood group 0(I) erythrocytes, contributing to antigenic mimicry.

Q: What animal species serve as the main natural reservoirs of the plague pathogen?

The primary natural reservoirs of Yersinia pestis are wild, synanthropic, and domestic animals. The most significant are: Rodents — marmots, ground squirrels, gerbils, voles, rats, mice;Lagomorphs — hares, pikas. In secondary anthroprourgic foci, sources include house rats and mice; among domestic animals, camels and cats can become infected via rodent contact. In hibernating rodents, the infection may enter a chronic latent form, maintaining the pathogen between epizootic periods.

For medical students2 min readUpdated 2026-10-10

Yersinia is a genus of Gram-negative rod-shaped bacteria that includes causative agents of severe human infections. The most clinically significant species are Yersinia pestis (the causative agent of plague), Yersinia pseudotuberculosis, and Yersinia enterocolitica (causative agents of yersiniosis).

Pathogenic speciesY. pestis, Y. pseudotuberculosis, Y. enterocolitica

StainingGram-negative ovoid rods

RespirationFacultative anaerobes

Defense mechanismInhibit phagocytosis (Type III secretion system)

General Characteristics of the Genus

The genus Yersinia is named after Alexandre Yersin, who discovered the plague pathogen in 1894. It includes 11 species, classified based on biochemical properties and motility. Morphologically, they are Gram-negative, frequently ovoid rods. Y. pestis is characterized by bipolar staining (ends stain more intensely) and a delicate capsule. All Yersinia species are facultative anaerobes capable of growing on standard nutrient media.

An interesting feature is psychrophilia (cold-tolerance) in many representatives, especially Y. pseudotuberculosis and Y. enterocolitica. They can multiply even at refrigerator temperatures (4 °C) and survive for prolonged periods in a frozen state.

Plague (Yersinia pestis)

Plague is a highly dangerous, quarantinable zoonotic infection maintained in natural foci. The pathogen is extremely aggressive, causing severe intoxication, fever, lymph node enlargement (buboes), and pulmonary involvement, frequently leading to sepsis and high mortality.

Characteristics of the plague pathogen:

Non-motile.
Optimum growth temperature: 28 °C.
On solid media, forms colonies resembling "cracked glass" (early growth) and "lace handkerchiefs" (virulent R-forms).
In liquid media, grows as a surface pellicle with "stalactites".
Possesses a complex antigenic structure, including thermostable O-antigens and the capsular F1 antigen (protection against phagocytosis).

Transmission mechanisms:

Vector-borne — bite of an infected flea (specific vector).
Contact — handling carcasses of infected animals (rodents).
Alimentary — contaminated food or water.
Aerogenic (droplet) — from patients with pneumonic plague.

Enteropathogenic Yersinia

This group includes the agents of pseudotuberculosis (Y. pseudotuberculosis) and intestinal yersiniosis (Y. enterocolitica). These are typical sapronoses — their natural habitat is the external environment (water, soil).

They can accumulate on vegetables in cold storage at low temperatures (cold enrichment). Upon entering a warm-blooded host (at 37 °C), the bacteria activate pathogenicity factors. They penetrate through intestinal M cells, causing mesenteric lymphadenitis and an abdominal syndrome that frequently mimics acute appendicitis. Interaction with macrophages leads to incomplete phagocytosis.

Pathogenicity Genetics

Pathogenicity in Yersinia is controlled by the chromosome and specific plasmids. The transition from a "saprophytic" to a "parasitic" lifestyle depends on temperature.

Chromosomal genes (inv gene): activated at 37 °C, mediating penetration (transcytosis) across the intestinal barrier.
pYV / pCad plasmid: encodes the Type III secretion system (a molecular "syringe") and Yop effector proteins, which block phagocytosis and allow bacteria to survive inside macrophages.
The plague pathogen additionally harbors the pPst plasmid (invasion enzymes) and pFra plasmid (capsular F1 antigen and "murine toxin").

Frequently asked questions

What antigens are part of the antigenic structure of Yersinia pestis?

The antigenic structure of Yersinia pestis includes a complex set of antigens, featuring thermostable somatic (O-antigens) and thermolabile capsular antigens. Species-specific antigens of the plague bacillus include:

FI — capsular antigen (possesses protective activity);
FII — murine toxin;
PI — pesticin;
Fb — fibrinolysin;
Pq — plasmocoagulase.

Additionally, the pathogen shares cross-reactive antigens with human blood group 0(I) erythrocytes, contributing to antigenic mimicry.

Which pathogenicity factors of Yersinia are encoded by chromosomal genes?

Chromosomal genes encode factors essential for the initial stages of infection, dissemination, and defense against phagocytosis. These include:

Outer membrane protein (encoded by the inv gene) — mediates binding to M cells and transcytosis across the intestinal barrier;
Yersiniabactin (siderophore) — an iron-scavenging system required to utilize host iron and facilitate rapid dissemination;
pH6 antigen — fimbriae-like structures acting as Fc-receptors (antigenic mimicry), exhibiting cytotoxicity toward macrophages and suppressing their microbicidal activity.

What biochemical tests are used for intra-genus differentiation of Yersinia?

Intra-genus differentiation (distinguishing species within the genus) relies on motility and biochemical properties. Differential tests determine the ability of bacteria to:

liquefy gelatin;
hydrolyze urea;
ferment carbohydrates (rhamnose, sucrose, dextrin).

For intraspecies classification (biotyping and chemotyping), additional tests involving glycerol utilization and nitrate reduction are used.

What animal species serve as the main natural reservoirs of the plague pathogen?

The primary natural reservoirs of Yersinia pestis are wild, synanthropic, and domestic animals. The most significant are:

Rodents — marmots, ground squirrels, gerbils, voles, rats, mice;
Lagomorphs — hares, pikas.

In secondary anthroprourgic foci, sources include house rats and mice; among domestic animals, camels and cats can become infected via rodent contact.

In hibernating rodents, the infection may enter a chronic latent form, maintaining the pathogen between epizootic periods.

What are the primary clinical forms of plague classified by localization?

Depending on the localization and dissemination of the pathological process (according to the Rudnev classification), the main clinical forms of plague include:

Localized forms — cutaneous, bubonic, and cutaneobubonic;
Disseminated (generalized) forms — primary septic and secondary septic;
Externally disseminated forms — primary pneumonic, secondary pneumonic, and intestinal.

Additionally, ICD-10 classifies plague meningitis and other forms (abortive, asymptomatic, mild).

Why is the plague pathogen difficult to eradicate in nature?

In natural foci, the infection is maintained among wild rodent populations. In hibernating animals, the infection can enter a latent form, preserving the bacteria until spring. Additionally, they are highly resilient to low temperatures.

What is the key diagnostic feature for pseudotuberculosis?

The "cold enrichment" method is used: clinical samples are incubated at 4 °C for a prolonged period, as these bacteria are psychrophilic and multiply in the cold while outgrowing other flora.

How does the anti-phagocytic defense of Yersinia work?

Using the Type III secretion system, they inject specific Yop proteins into phagocytes. This results in incomplete phagocytosis — the bacteria are engulfed but not degraded, allowing them to survive inside macrophages.

Go deeper

Clinical forms of plague and their pathogenesis
Microbiological diagnosis of plague in specialized laboratories
Plague vaccine prophylaxis (EV strain)
Far Eastern scarlet-like fever
Biochemical classification of Y. enterocolitica