Yersinia enterocolitica characteristics

• Yersinia enterocolitica is a gram-negative bacillus within the Enterobacteriaceae family.
• The Yersinia genus is named after Alexander Yersin, its discoverer.
• Enterocolitica refers to the intestine and colon, indicating the diarrheal illness it causes called yersiniosis.

Morphology and Physiology

• Possesses a thin peptidoglycan layer, staining pink with Safranin dye during Gram staining.
• Appears as a small, pink rod under a microscope.
• Motile at 25 degrees Celsius but non-motile at 37 degrees Celsius.
• Facultative anaerobe, thriving in both aerobic and anaerobic conditions.
• Facultative intracellular, surviving both outside and inside cells.
• Non-spore-forming and oxidase-negative.

Growth on Media

• Grows well on MacConkey agar but does not ferment lactose, forming colorless colonies.
• On cefsulodin-irgasan-novobiocin (CIN) agar, it forms white colonies with a deep red center, resembling bull's-eye colonies, after 24 hours.
• Does not produce hydrogen sulfide, so no black precipitate forms in the triple sugar iron (TSI) test.

Virulence Factors

• Adhesins like YadA and Ail facilitate attachment to gut epithelial cells.
• Type three secretion system (T3SS) delivers Yersinia outer proteins (Yops) into host cells.
• Yops block the secretion of pro-inflammatory cytokines (TNF-alpha, IL-8) and inhibit macrophage activation, suppressing the host's inflammatory response and avoiding phagocytosis.
• Can replicate inside Peyer's patches and spread to mesenteric lymph nodes.
• Uses siderophores to chelate iron from host cells for survival.
• Produces an enterotoxin called Yst, potentially involved in diarrheal disease, but its role is not well-defined.

Transmission and Diseases

• Transmitted through the fecal-oral route, often from contaminated milk or pork.
• Causes gastrointestinal infection called yersiniosis, impacting the ileum (terminal ileitis), right colon (enterocolitis), and mesenteric lymph nodes (mesenteric lymphadenitis).
• Mesenteric lymphadenitis can mimic appendicitis, known as pseudoappendicitis.
• In immunocompromised individuals or those with iron overload conditions, it can cause septicemia.
• Post-infection sequelae, such as reactive arthritis, can occur, especially in individuals with the HLA-B27 antigen.
• Reactive arthritis develops 1-3 weeks post-infection, possibly due to bacterial antigens depositing in the joints, causing inflammation.
• Erythema nodosum, inflammation of fat cells under the skin, can occur due to a delayed hypersensitivity reaction to bacterial antigens.

Symptoms

• Enterocolitis: fever, diarrhea (with blood and mucus), and abdominal pain.
• Pseudoappendicitis: fever, abdominal pain, and tenderness in the lower right quadrant.
• Septicemia: fever, chills, hypotension, and tachycardia.
• Reactive arthritis: joint pain and swelling.
• Erythema nodosum: red or purple painful lesions, mainly on the legs.

Diagnosis

• Identifying bacteria in stool cultures.
• Reactive arthritis diagnosis involves identifying inflammatory cells or bacteria in synovial fluid.
• Serological tests (tube agglutination, ELISA) can confirm infection by detecting increased antibody titers in serum, but culture method is easier.
• Ultrasound or CT scans help exclude appendicitis in pseudoappendicitis cases.

Treatment

• Third-generation cephalosporins, trimethoprim-sulfamethoxazole, aminoglycosides, fluoroquinolones, and tetracyclines.