Yersinia pestis: Introduction and morphology


  • Formerly known as Pasteurella pestis.
  • Falls in family enterobacteriaceae.
  • Discovered in Honkong in 1894 AD by Yersin and Kitasato.
  • Important medical species are Y.pestis, Y. pseudotuberculosis and Y. enterocolitica.
  • Y. pestis responsible for causing plague whereas Y. pseudotuberculosis is a primary pathogen of rodents.
  • Similarly, Y. enterocolitica causes enteric and systemic diseases.
  • These diseases are common in animals and humans.

Morphology & Culture Characteristics of Yersinia pestis

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  • Gram-negative, short, ovoid, plump, slow growing bacillus.
  • Size is about 5 × 0.7 µm.
  • Arranged singly in small groups or short chains.
  • With rounded ends and convex sides.
  • Generally stained with Giemsa or methylene blue.
  • Safety pin like appearance.
  • Bipolar staining in smears shows two ends densely stained where central area appears clear.
  • Shows pleomorphism.
  • Different involution forms are seen like coccoid, club-shaped.
  • Sometimes, filamentous ad giant forms are also noticed.
  • These forms are observed mostly in old cultures.
  • Media with 3 % NaCl enhances pleomorphism more comparatively.
  • Envelope or capsule surrounds the bacillus.
  • Also these surrounding structures can be called slime layer.
  • Non-motile, non-sporing and non-acid fast in nature.

Yersinia pestis -

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Cultural characteristics

  • Aerobic and facultatively anaerobic.
  • Can grow over wide range of pH, i.e. pH 5-9.6.
  • Also, growth occurs over wide range of temperature (2-45 °C).
  • Optimum pH for growth is 7.2 and temperature is 27 °C.
  • Development of envelope occurs best at 37 °C.
  • Pinpoint colonies after 24 hour.
  • 1-2 mm size colonies are formed.
  • Growth can be observed on basal media.
  • No specific nutritional requirements.

Plague bacteria Yersinia pestis, 3D illustration. Gram-negative bacteria with bipolar staining which cause plague infection Image - Stock by Pixlr

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On nutrient agar

  1. Colonies appear small.
  2. Seem delicate, transparent discs.
  3. Become opaque if incubation is continued further.

On blood agar

  1. Colonies are generally dark brown.
  2. This is due to absorption of hemin pigment.

On MacConkey agar

  1. Colonies are found colorless.

In broth

  1. Flocculent growth occurs at the bottom.
  2. Little or no turbidity noticed.
  3. Formation of delicate pellicle may occur later.
  4. Shows characteristics growth i.e., stalactites growth in ghee broth.
  5. Colonies are found hanging down into the broth from the surface.

Biochemical reactions

  • Glucose, maltose, mannitol: fermentation +ve.
  • Lactose, sucrose and rhamnose: fermentation  -ve
  • During fermentation, production of acid: +ve, Gas -ve.
  • Hydrogen sulphide (H2S): negative
  • Indole: negative
  • Methyl Red (MR): positive.
  • VP and citrate: negative
  • Catalase: positive
  • Esculin: positive
  • Oxidase and urease: negative
  • Gelatin liquefaction: negative
  • Growth in KCN: negative
  • Hemolysis: negative
  • Nitrate reduction: positive
  • Salicin: variable
  • Sorbitol: variable
  • Raffinose: negative

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  • Easily destroyed by activity of heat, sunlight.
  • Also, drying and chemical disinfectants kill them.
  • Heating at 55 °C for 15 minutes is lethal to bacteria.
  • 5% phenol treatment for 15 minutes kills them.
  • Remains viable in cold, moist environments for long time.
  • Soil of rodent burrows is the most suitable environment.
  • Can survive for several months and also multiply in that soil.
  • Easily lysed by anti-plague bacteriophage at 22°C.

Antigens, toxins and virulence factors

  • Serotypes do not exist so homogeneous.
  • Complex antigenic structure noticed.
  • At least 20 different antigens detected.
  • Detection done by biochemical analysis and gel diffusion.
  • Many of them act or function as virulence factors.
  • Some of them have been mentioned below:

i) Fraction I or F-I antigen

  • Is a heat labile protein envelope antigen.
  • Formation is best observed in culture at 37°C.
  • Related with phagocytosis inhibition.
  • Virulent strains contain them generally.
  • Are plasmid encoded antigens thus, acts as virulence determinants.
  • Sometimes, fatal human cases have been found from the strains lacking this antigen.
  • Antibody formed to this antigen seems protective in mice.

ii) V (LcrV) and W antigens

  • Production always takes place together.
  • Also considered as one of the important virulence factors.
  • Inhibition of phagocytosis takes place.
  • Thus, intracellular killing of bacillus does not occur.
  • Production of these antigens is plasmid mediated.

iii) Bacteriocin (Pesticin I, coagulase and fibrinolysin)

  • Related with virulent strains.
  • Pesticin I inhibits other strains of Yersinia like Y. enterocolitica, E. coli, etc.

iv) Plague toxin

  • Two classes of toxins fall under this group.
  • These are found in culture filtrates or cell lysates.
  • Endotoxin and murine toxins are the toxins of these group.
  • Endotoxin is lipopolysaccharide in nature which is similar to those produced by other enteric bacteria.
  • Murine toxins are active in rats and mice.
  • In humans, murine toxins role has not been known.

v) Type III secretion system

  • Utilization of Type III secretion system (T3SS): helps in evading host immune responses.
  1. pYV (plasmid of Yersinia virulence):  helps in encoding the following components of the T3SS.
  2. Ysc (Yersinia secretion proteins): helps in constructing the secretion organelles.
  3. Yops (Effector Yersinia outer proteins): helps in interfering with host cell signaling.
  4. Translocators: helps in delivering the effectors across the host cell membrane.
  5. Chaperones:  helps in transportation of the effectors and translocators
  6. Regulatory components: helps in regulating the system.

These all processes results in host immune cell death via apoptosis.

vi) Yop effectors

  • Different Yop effectors YopH, YopE, YpkA, and YopM contribute to the virulence of  pestis, while YopJ’s role is still unclear.
  1. YopH: responsible for disruption of focal complexes. It helps in inhibiting pro-inflammatory signaling. This in turn helps Y. pestis avoid host phagocytosis and inflammation.
  2. YopE and YpkA: responsible in disrupting actin microfilaments. Thus, Y. pestis will avoid phagocytosis.
  3. YopM: by migrating to the nucleus interferes with the eukaryotic cell cycle.
  4.  YopJ: responsible for inhibiting MAPK and NF-κB signaling pathways. This induces apoptosis of macrophages and thus contributes to anti-inflammation.

vii) Plasminogen activator (Pla) protein

  • The adhesion and invasion of the bacterium to the extracellular matrix of host tissues is made easy.
  • Pla inititaes the activation of plasminogen into plasmin.
  • This in turn causes proteolysis and damage to host tissues.
  • In addition, this protein also increases Y. pestis’s ability to invade epithelial cells.

viii) Unidentified surface component

  • Some unknown surface component also acts as virulence factors.
  • Hemin and basic aromatic dyes are absorbed in culture media.
  • This results in the formation colored colonies.

ix) Synthesis of purine

  • Ability to synthesize purine is also associated with virulence factors.





Yersinia pestis: Introduction and morphology