Yersinia Pestis: Plague Microbiology

• Plague is caused by the gram-negative coccobacillus Yersinia pestis.
• Yersinia pestis displays bipolar staining with Wright-Giemsa and Wayson stains.
• It is non-spore forming and highly motile due to flagella.
• This bacterium is a facultative anaerobe and can live and multiply both inside and outside of host cells.
• Yersinia pestis is oxidase and urease negative, and it is a non-lactose fermenter.
• It grows on MacConkey agar (colorless colonies), sheep blood agar, and chocolate agar.
• Rodents, prairie dogs, livestock, and rabbits are carriers.
• Fleas serve as vectors, regurgitating bacteria into the bite wound.
• Humans are hosts, and transmission can occur through contaminated surfaces and water droplets.
• The enzootic cycle (sylvatic) takes place between fleas and rats, while the epizootic cycle (urban plague) is an outbreak amongst animals.

Virulence Factors

• Capsular fraction 1 (F1) and LcrV antigen contribute to virulence allowing survival.
• F1 is a surface protein that forms a capsule around Yersinia pestis, aiding in the evasion of phagocytosis.
• LcrV antigen is part of the Type III secretion system.
• Plasminogen activator protease degrades complement proteins C3b and C5a.
• The Type III secretion system injects Yersinia outer proteins (Yops) into macrophages, blocking proinflammatory cytokine secretion.
• Siderophore molecules help Yersinia pestis capture iron for survival.
• It contains endotoxin and exotoxin.
• V and W antigens allow the organism to survive and grow intracellularly.
• Macrophages burst, leading to the spread of bacteria.

Forms of Plague

• Bubonic, septicemia, and pneumonic are the three forms of plague.

Bubonic Plague

• The incubation period is under 7 days.
• Symptoms include high fever, chills, and fatigue.
• It spreads to cells near lymph nodes, causing swollen lymph nodes (buboes) in the groin or axilla.

Septicemia Plague

• Septicemia plague starts in lymph nodes and enters the bloodstream.
• It secretes endotoxins, leading to hypotension, malaise, and purpuric skin lesions.
• It causes excessive production of thrombin, leading to disseminated intravascular coagulation (DIC).
• Symptoms include tissue necrosis, multiorgan failure, and death.

Pneumonic Plague

• Pneumonic plague starts with bacteria spreading to the lungs via the bloodstream or through inhalation of droplets from another patient.
• The incubation period is 1-3 days.
• Symptoms include fever, headaches, weakness, and a rapidly developing pneumonia with shortness of breath, chest pain, cough, and hemoptysis.

Diagnosis

• Rapid development of a swollen and painful lymph gland + flea bite + travel to western U.S. or endemic area are indicators.
• Collect samples from blood, pus from bubo, or sputum.
• Diagnosis involves fluorescent staining, rapid antigen test for capsular F1 antigen, and chest x-ray.

Treatment

• Treatment should begin immediately, before confirming diagnosis.
• Treatment options include a combination of streptomycin and tetracycline (such as doxycycline), streptomycin alone, or levofloxacin.