Yersinia pestis: Characteristics and Structure

• Yersinia pestis is a short, pleomorphic, Gram-negative rod.
• It often exhibits bipolar staining, where only the two opposite poles of the microorganism stain.
• Special stains like Wright, Giemsa, Wayson, or methylene blue reveal single cells, pairs, or short chains in clinical samples.
• Y. pestis is catalase positive and either microaerophilic or facultatively anaerobic.

Cultural and Biochemical Characteristics

• Y. pestis is non-motile and grows as a facultative anaerobe on many bacteriologic media.
• It is readily isolated from sterile blood or lymph node aspirates on sheep blood agar.
• Growth is faster when agar plates are incubated at 28°C.
• At 37°C, colonies on sheep blood agar may be smaller compared to those at 28°C.
• Colonies are typically gray to white, sometimes opaque, and 1-1.5 mm in diameter with irregular edges.
• It does not produce hemolysis.

Antigenic Structure

• All Yersiniae possess lipopolysaccharides with endotoxic activity upon release.
• Y. pestis and Y. enterocolitica produce antigens and toxins that act as virulence factors.
• These bacteria have type III secretion systems, a membrane-spanning complex.
• This complex allows them inject proteins directly into the host cell's cytoplasm.
• Virulent Yersiniae produce V and W antigens.

Clinical Findings: Black Death

• Plague's clinical manifestations depend on the route of exposure.
• Three forms of the disease include bubonic, pneumonic, and septicemic plague.
• Bubonic plague is the most common form.
• It has an incubation period of 2-7 days, with a sudden onset of high fever and painful lymphadenopathy.
• Tender lymph nodes (buboes) appear in the neck, groin, or axillae.
• Pneumonic plague commonly involves greatly enlarged lymph nodes.
• Septicemic plague can occur spontaneously or as a complication of untreated bubonic plague.
• Y. pestis multiplies intravascularly, which is seen in blood smears.
• Patients typically experience a sudden onset of high fever, chills, and weakness.
• This progresses rapidly to septic shock with disseminated intravascular coagulation and hypotension (septic shock).
• Symptoms also include altered mental status, renal and cardiac failure.
• Bleeding into skin and organs may occur, and vomiting and diarrhea may arise during the early stages of septicemic plague.
• Terminally, pneumonia and meningitis signs can appear.

Pathogenesis and Pathology

• A flea feeding on a rodent infected with Y. pestis ingests the organisms, which multiply in its gut.
• Coagulase then helps block the flea's proventriculus, preventing food passage.
• The "blocked" and hungry flea bites ferociously.
• Aspirated blood, contaminated with Y. pestis, is vomited into the bite wound.
• Inoculated organisms may be phagocytosed by polymorphonuclear cells and macrophages.
• Y. pestis organisms can be killed by polymorphonuclear cells but multiply in macrophages.
• Because the bacteria are multiplying at 37°C, they produce antiphagocytic protein.
• This allows them to resist phagocytosis, rapidly reaching the lymphatics.
• An intense hemorrhagic inflammation develops in enlarged lymph nodes, which may undergo necrosis and become fluctuant.

Diagnostic Laboratory Tests

• Blood is taken and aspirates of enlarged lymph nodes are taken for smear and culture.
• Acute and convalescent sera are examined for antibody levels.
• Sputum is cultured in pneumonia cases, and cerebrospinal fluid is taken for meningitis.

Smears

• Wright, Giemsa, or Wayson stains are useful when staining material from suspected buboes.
• They are also useful with positive blood culture results because of the organism's striking bipolar appearance (safety pin shape).
• This bipolar appearance is not evident on direct Gram stains.
• More specific methods use fluorescent antibody stains targeting the capsular F1 antigen.

Culture

• All materials are cultured on blood, chocolate, and MacConkey agar plates, and in brain-heart infusion broth.
• Growth on solid media may be slow, requiring over 48 hours.
• Blood culture results are often positive in 24 hours.
• Y. pestis creates non-lactose-fermenting colonies on MacConkey agar and grows better at 28°C than at 37°C.
• The organism is catalase positive and indole, oxidase, and urease negative.
• It is also non-motile.
• Immunofluorescence or lysis by a specific Y. pestis bacteriophage is a definitive way to identify cultures.
• All cultures are highly infectious and must be handled carefully inside a biological safety cabinet.