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Chapter: Microbiology and Immunology: Bacteriology: Yersinia

Yersinia enterocolitica

Y. enterocolitica is a well-recognized enteric bacterium thatcauses distinctive clinical manifestations, ranging from asymptomatic infections to life-threatening sepsis, especially in children.

Yersinia enterocolitica

Y. enterocolitica is a well-recognized enteric bacterium thatcauses distinctive clinical manifestations, ranging from asymptomatic infections to life-threatening sepsis, especially in children. Schleifstein and Coleman were first to describe Y. enterocolitica in 1939. Y. enterocolitica is a Gram-negative coc-cobacillus that is motile at 22°C but not at 37°C. This bacil-lus resembles Y. pseudotuberculosisin being motile at 22°C, but differs from it in fermenting sucrose and cellobiose and decar-boxylating ornithine. Many strains of Y. enterocolitica are VP and indole positive. They do not ferment rhamnose or meli-biose. They are oxidase negative and nonlactose fermenting.

Y. enterocolitica are aerobes and grow at an optimum tem-perature of 22–29°C. Y. enterocolitica grows well at a pH of 5–9; hence the increased incidence of Y. enterocolitica infection is seen in patients who take antacids and H2 blockers. The bacteria require iron to survive; hence Yersinia sepsis has been reported in children following accidental iron overdose and in hemo-chromatosis, clinical states of iron overload.

They grow well on commonly used basic enteric media. On MacConkey agar, they produce pinpoint colorless colonies after 24 hours of incubation. On blood agar, they form nonhe-molytic smooth and translucent colonies measuring 2–3 mm in diameter after 2 days of incubation at 22°C. Cefsulodin-irgasan-novobiocin (CIN) agar is a frequently used selective medium for Y. enterocolitica.

The antigenic structure of Y. enterocolitica is distinct. Y. enterocolitica possesses 34 different O antigen factors and19 H factors. On the basis of these antigens, more than 60 O serotypes have been reported. Most human isolates belong to serotypes O:3, O:5, 27; O:8 and represent the most virulent worldwide causes of human yersiniosis.

Virulence of Y. enterocolitica is chromosomal or plasmid mediated.

Humans acquire infection by ingestion of food or water contaminated with the bacteria. On ingestion, the bacteria adhere to the mucosa of terminal ileum, penetrate, and mul-tiply in Peyer patches. The bacteria induce an inflammatory infiltrate in the lamina propria. The outer membrane protein is an important virulence factor that mediates adherence and invasion of the organism.

The bacteria may then spread to the mesenteric lymph nodes and may cause bacteremia, or form abscesses and pain in the right lower quadrant that simulate appendicitis. Although Y. enterocolitica produce a heat-stable enterotoxin similar to thatof Escherichia coli, the enterotoxin does not contribute to the pathogenesis of the disease.

Y. enterocolitica causes enterocolitis primarily in youngchildren, pseudoappendicitis syndrome, and extraintestinal infections.

·           Enterocolitis is the most common clinical form of the disease, which occurs primarily in young children, with a mean age of 24 months. The incubation period is short, varies from 4 to 6 days. The condition manifests as watery, mucoid diarrhea in majority of patients; fever, colicky abdominal pain, bloody stools; and leukocytes in the stool. Diarrhea usually lasts from 1 day to 3 weeks. Most cases are self-limited.

·           Y. enterocolitica infection causes mesenteric lymphadenitiswith terminal ileitis. The condition, known as pseudoap-pendicitis syndrome, is more common in older children and young adults, and mimics appendicitis. It is characterized by fever, abdominal pain, tenderness in the right lower quad-rant, and leukocytosis. The pseudoappendicitis syndrome is also caused by infection with Y. pseudotuberculosis.

·           Extraintestinal infections, such as cellulitis, conjunctivitis, meningitis, osteomyelitis, pharyngitis, pneumonia, and uri-nary tract infection caused by Y. pseudotuberculosis are rare.

Laboratory diagnosis is made by isolation of Y. enterocolitica from stool by culture. Stool culture is positive within 2 weeks of onset of disease. Tube agglutination, ELISA, and radioim-munoassays are frequently used tests to detect antibodies in sera for diagnosis of the condition. Antibody titers rise 1 week after onset of illness and reach maximal levels at 2 weeks; raised antibody levels can be found for years after infection. Antibody titers more than 1:128 are suggestive of a previous infection with Y. enterocolitica.

Good nutrition and hydration are the mainstays of treat-ment. The role of antibiotic therapy in uncomplicated acute colitis and mesenteric adenitis is not established. Antibiotic therapy is indicated only for patients (a) with septicemia, (b) with focal extraintestinal manifestations, and (c) with enterocolitis who are immunocompromised. Tetracycline, aminoglycosides, and trimethoprim–sulfamethoxazole are the antibiotics of choice.

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