General characteristics of the genus
The genus Yersinia is named after Alexandre Yersin, who discovered the plague pathogen in 1894. It includes 11 species, whose classification is based on biochemical properties and motility. Morphologically, they are gram-negative bacilli, often ovoid in shape. Y. pestis is characterized by bipolar staining (the ends stain more intensively) and the presence of a delicate capsule. All yersiniae are facultative anaerobes capable of growing on simple nutrient media.
An interesting feature is the psychrophilia (cold-loving nature) of many representatives, especially Y. pseudotuberculosis and Y. enterocolitica. They can multiply even at refrigerator temperatures (4 °C) and survive for a long time in a frozen state.
Plague (Yersinia pestis)
Plague is a particularly dangerous, quarantinable natural focal infection. The pathogen possesses tremendous aggressiveness, causing severe intoxication, fever, lymph node involvement (buboes), and pulmonary damage, frequently resulting in sepsis and high mortality.
Features of the plague pathogen:
- Non-motile.
- Optimum growth temperature: 28 °C.
- On solid media, it forms "cracked glass" colonies (at the beginning of growth) and "lace handkerchief" colonies (virulent R-forms).
- In liquid media, it grows as a surface pellicle with "stalactites".
- Contains a complex antigenic complex, including heat-stable O-antigens and capsule F1-antigen (protection against phagocytosis).
Modes of transmission:
- Transmissive — bite of an infected flea (specific vector).
- Contact — when dressing carcasses of infected animals (rodents).
- Alimentary — via contaminated food products.
- Aerogenic — from patients with the pneumonic form.
Enteropathogenic yersiniae
These include the causative agents of pseudotuberculosis (Y. pseudotuberculosis) and intestinal yersiniosis (Y. enterocolitica). These are typical sapronoses—their natural habitat is the external environment (water, soil).
They are able to accumulate on vegetables in vegetable stores at low temperatures (cold enrichment). Upon entering the warm-blooded organism (at 37 °C), the bacteria activate pathogenicity factors. They penetrate through intestinal M cells, causing mesenteric lymphadenitis and abdominal syndrome, often mimicking acute appendicitis. Interaction with macrophages leads to incomplete phagocytosis.
Genetics of pathogenicity
The pathogenicity of yersiniae is controlled by the chromosome and specific plasmids. The transition from "saprophytic" to "parasitic" existence depends on temperature.
- Chromosomal genes (inv gene): activated at 37 °C, ensure penetration (transcytosis) through the intestinal barrier.
- pYV / pCad plasmid: encodes the type III secretion system (molecular "syringe") and Yops effector proteins, which block phagocytosis, allowing bacteria to survive inside macrophages.
- The plague pathogen additionally possesses pPst plasmids (invasion enzymes) and pFra plasmids (capsular F1-antigen and "murine toxin").
Briefly: answers to topical questions
What antigens are included in the antigenic structure of Yersinia pestis?
The antigenic structure of Yersinia pestis includes a complex of antigens, comprising heat-stable somatic (O-antigens) and heat-labile capsular antigens. The species-specific antigens of the plague microbe are:
- FI — capsular antigen (possesses protective activity);
- FII — murine toxin;
- PI — pesticin;
- Fb — fibrinolysin;
- Pq — plasmocoagulase.
The pathogen also shares common antigens with human 0(I) blood group erythrocytes, which causes antigenic mimicry.
What pathogenicity factors of yersiniae are encoded by chromosomal genes?
Chromosomal genes of yersiniae encode factors that ensure the initial stage of infection, dissemination, and protection against phagocytosis. These include:
- Outer membrane protein (encoded by the inv gene) — ensures interaction with M cells and transcytosis across the intestinal barrier;
- Yersiniabactin (siderophore) — an iron uptake system necessary for utilizing host iron and rapid dissemination;
- pH6 antigen — fimbria-like structures that play the role of Fc receptors (antigenic mimicry), exhibiting cytotoxicity to macrophages and inhibiting their digestive capacity.
What biochemical tests are used for intra-genus differentiation of yersiniae?
For intra-genus differentiation of yersiniae (separation of species within the genus), motility and biochemical properties are assessed. As differential tests, the ability of bacteria to do the following is determined:
- liquefy gelatin;
- hydrolyze urea;
- ferment carbohydrates (rhamnose, sucrose, dextrin).
For intra-species classification (isolation of chemo- and biovars), tests for the ability to utilize glycerol and reduce nitrates are additionally used.
Which animal species are the main natural reservoirs of the plague pathogen?
The main natural reservoirs of the plague pathogen (Yersinia pestis) are wild, synanthropic, and domestic animals. Of greatest importance are:
- rodents — marmots, susliks, gerbils, voles, rats, hares;
- lagomorphs — hares, pikas.
In secondary anthropurgic foci, house rat and mouse species serve as sources; among domestic animals, camels and cats are involved, contracting the infection from rodents.
In hibernating rodents, the infection can transition into a chronic latent form, which preserves the pathogen during the inter-epizootic period.
What are the main clinical forms of plague classified according to the localization of the pathological process?
Depending on the localization and spread of the pathological process (according to G.P. Rudnev's classification), the following main clinical forms of plague are distinguished:
- Predominantly localized forms — cutaneous, bubonic, and cutaneobubonic;
- Internally disseminated (generalized) forms — primary septic and secondary septic;
- Externally disseminated forms — primary pneumonic, secondary pneumonic, and intestinal.
Additionally, according to ICD-10, plague meningitis and other forms (abortive, asymptomatic, minor) are distinguished.
Class and exam questions
Why is the plague pathogen difficult to eradicate in nature?
In natural foci, the infection is maintained among rodents. In hibernating animals, the infection transitions into a latent form, preserving the bacteria until spring. In addition, they are resistant to low temperatures.
What are the diagnostic features of pseudotuberculosis?
The "cold enrichment" method is used: the culture is incubated at 4 °C for a prolonged period, as these bacteria are psychrophilic and multiply in the cold, outpacing other flora.
How does the antiphagocytic defense of Yersinia work?
Using a type III secretion system, they inject special Yops proteins into phagocytes. This leads to incomplete phagocytosis—the bacteria are engulfed but not digested, surviving inside macrophages.
What else to study on this topic
- Clinical forms of plague and their pathogenesis
- Microbiological diagnosis of plague in specialized laboratories
- Vaccine prevention of plague (EV strain)
- Far Eastern scarlatiniform fever
- Biochemical classification of Y. enterocolitica
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