Yersinia pestis

 Yersinia pestis

GENERAL CHARACTERISTICS

• Gram’s classification – Gram negative
• Shape – Rod shape (Bacilli)
• Motility – Non-motile
• Capsule – Absent
• Endospores – Absent
• Respiration –  Facultative anaerobic
• Optimum Temperature – 28 to 30℃
• Optimum pH – 7.2 to 7.6
• Habitat – Y. pestis has two main habitats in the stomach of proventriculus of various flea species at ambient temperature or  the blood or tissues of a rodent host at body temperature.
• Alexandre Yersin (Swiss physician and bacteriologist) discovered the Yersinia pestis in 1894.

PATHOGENIITY OF Yersinia pestis

DISEASE TRANSMISSION

         Plague is an infectious disease caused by the bacteria Yersinia pestis, a zoonotic bacteria, usually found in small mammals and their fleas. It is transmitted between animals through fleas. Humans can be infected through the bite of infected vector fleas.

INCUBATION PERIOD

1 to 3 days 

VIRULENCE FACTORS OF Yersinia pestis

•  Capsule.
•  Lipopolysaccharide
•  Yersinia Outer Proteins (Yops).
•  Plasminogen Activator (Pla).
•  Pesticin.
•  Hemagglutinin Proteins.

 

PATHOGENESIS OF Yersinia pestis

• Yersinia pestis is primarily a rodent pathogen, with humans being an accidental host when bitten by an infected rat flea.
• Plague is a disease that affects humans and other mammals. It is caused by the bacterium, Yersinia pestis. Humans usually get plague after being bitten by a rodent flea that is carrying the plague bacterium or by handling an animal infected with plague.
• While growing in the flea, Y. pestis loses its capsular layer. Most of the organisms are phagocytosed and killed by the polymorphonuclear leukocytes in the human host.  A few bacilli are taken up by tissue macrophages.
• The macrophages are unable to kill Y. pestis and provide a protected environment for the organisms to synthesize their virulence factors.
• The organisms then kill the macrophage and are released into the extracellular environment, where they resist phagocytosis by the polymorphs.The Y. pestis quickly spread to the draining lymph nodes, which become hot, swollen, tender, and hemorrhagic. 

CLINICAL DISEASES CAUSED BY Yersinia pestis

1). Bubonic Plague

• Bubonic Plague is the most common form of plague and typically manifests with sudden onset of symptoms.
• Bubonic plague is usually transmitted through the bite of an infected flea. The bacteria enter the host’s bloodstream, leading to lymph node infection.
• Fever, chills, headache, and muscle aches are common systemic symptoms. 

2). Septicemic Plague

• Septicemic Plague occurs when the bacteria spread to the bloodstream, leading to a rapidly progressing and severe systemic infection.
• Septicemic plague can develop from untreated bubonic plague or be the primary form of infection. It can also result from direct exposure to infected tissues or fluids.
• Septicemic plague is life-threatening condition that can lead to multiple organ failure, shock and death.

 3). Pneumonic Plague

• Pneumonic Plague is a severe form of respiratory disease, characterized by the infection of the lungs.
• Pneumonic Plague can transmitted from person to person through respiratory droplets.
• Symptoms include high fever, cough, chest pain, and difficulty breathing. Respiratory distress can rapidly progress to severe Pneumonia.   

LABORATOR DIAGNOSIS OF Yersinia pestis

MICRSCOPIC EXAMINAION  

• Gram staining – Pink colored Gram negative rod shaped bacteria.
• Motility test – Non- motile.

COLONY MORPHOLOGY ON CULTURE MEDIUM

• Blood agar – Yersinia pestis colonies are on blood agar are typically Non-hemolytic or gamma hemolytic.
• MacConkey agar – White colored non-lactose fermenting colonies
•  Selective medium :  Yersinia CIN Agar – Colonies on CIN agar often appears as pale to bright yellow colonies. The colonies may have a yellow center and with a translucent or clear edge. 

BIOCHEMICAL TESTS

• Catalase test –  Positive
• Oxidase test –   Negative
• Urease test –   Negative
• Indole test –   Negative
• Methyl Red (MR) test –    Negative
• Voges Proskauer (VP) test –   Negative
• Citrate utilization test –  Negative
• Starch Hydrolysis –  Negative
• Casein Hydrolysis –   Negative
• Nitrate Reduction – Variable

 

ANTIBIOTIC THERAPY AND PREVENTION

• Streptomycin often considered the first-line treatment. It is effective against Yersinia pestis. Gentamicin is preferred as another option.
• Doxycycline and Ciprofloxacin are the alternative antibiotics.
• Vaccination is the preventive measure for Yersinia pestis.
• Use of insecticides, environmental sanitation and measures to reduce contact with rodents.