- 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.
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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.
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- 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.
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On nutrient agar
- Colonies appear small.
- Seem delicate, transparent discs.
- Become opaque if incubation is continued further.
On blood agar
- Colonies are generally dark brown.
- This is due to absorption of hemin pigment.
On MacConkey agar
- Colonies are found colorless.
- Flocculent growth occurs at the bottom.
- Little or no turbidity noticed.
- Formation of delicate pellicle may occur later.
- Shows characteristics growth i.e., stalactites growth in ghee broth.
- Colonies are found hanging down into the broth from the surface.
- 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.
- pYV (plasmid of Yersinia virulence): helps in encoding the following components of the T3SS.
- Ysc (Yersinia secretion proteins): helps in constructing the secretion organelles.
- Yops (Effector Yersinia outer proteins): helps in interfering with host cell signaling.
- Translocators: helps in delivering the effectors across the host cell membrane.
- Chaperones: helps in transportation of the effectors and translocators
- 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.
- 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.
- YopE and YpkA: responsible in disrupting actin microfilaments. Thus, Y. pestis will avoid phagocytosis.
- YopM: by migrating to the nucleus interferes with the eukaryotic cell cycle.
- 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