SHIGELLA

SHIGELLA

Species

1. Non-mannitol-fermenters

Shigella dysenteria

2. Mannitol-fermenters

Shigella flexneri

Shigella boydii

Shigella sonnei

Characteristics

  • gram -
  • Rod
  • Non-encapsulated
  • Non-motile
  • Non-spore forming
  • Facultative anaerobic
  • Facultative intracellular
  • Obligate pathogen
  • All ferment glucose, some ferments mannitol
  • They do not form acetyl-methylcarbinol,
  • Does not hydrolyze urea or liquefy gelatin
  • Citrate negative
  • TSIA (Alkaline slant over acid butt)
  • IMVIC V + - -

Habitat and Transmission

  • Shigella species are found only in the human intestinal tract.
  • Carriers of pathogenic strains can excrete the organism up to two weeks after infection and occasionally for longer periods.
  • Shigellas are killed by drying.
  • Shigellas are transmitted by the fecal-oral rout.
  • The highest incidence of Shigellosis occurs in areas of poor sanitation and where water supplies are polluted.

Cultural Characteristics:

  • All members of Shigella are aerobic and facultative anaerobes.
  • Grow readily in culture media at pH 6.4 to 7.8 at 10 oC - 40 oC, with optimum of 37 oC.
  • After 24 hours incubation, Shigella colonies reach a diameter of about 2 mm.
  • The colonies are circular, convex, colorless, but moderately translucent with smooth surface, and entire edges.
  • Small tangled hair-like projections can sometimes be seen at one or more points on the periphery of the colony. In XLD they appear pinkish to reddish colonies while in Heaktoen Enteric Agar (HEA), they give green to blue green colonies.
  • If a number of typical colonies present onto the original plate, a tentative diagnosis can be made by direct slide agglutination with polyvalent Shigella antiserum. In all instances, diagnosis should be confirmed by additional biochemical tests and by specific type agglutination.

Reservoirs

  • Humans (only reservoir, not normal flora)

Transmission

  • Direct contact
  • Fecal-oral
  • Vectorial (flies)
  • Contaminated water
  • Contaminated food

Toxins

  • Shiga toxin (analogous to verotoxin, causes hemorrhagic bacillary dysentery and hemolytic-uremic syndrome)
  • LPS

Diseases

  • Bacillary Dysentery
  • Low-grade fever, abdominal cramps, abdominal pain, vomiting and purulent hemorrhagic diarrhea
  • Spontaneously resolves in < 1 week
  • Primarily occurs in children and elderly
  • Analogous to hemorrhagic colitis
  • Caused by Shigella infection of the GI tract " production and secretion of shiga toxin" necrosis of the enterocytes Hemolytic-Uremic Syndrome

PATHOGENIC DETERMINANTS:

1. O antigen: The ability to survive the passage through the host defenses may be due to O antigen.

2. Invasiveness: Virulent shigella penetrate the mucosa and epithelial cells of the colon in an uneven manner. Intracellular multiplication leads to invasion of adjacent cells, inflammation and cell death. Cell death is probably due to cytotoxic properties of shiga toxin that interfere with protein synthesis. The cellular death and resulting phagocytosis response by the host accounts for the bloody discharge of mucus and pus and shallow ulcers characteristic of the disease.

3. Other toxins: It has a protein toxin which may be neurotoxic, cytotoxic, and enterotoxic. The enterotoxic property is responsible for watery diarrhea.

PATHOGENICITY:

Shigella dysentery’s is set apart from other dysentery bacilli by its capacity to form a powerful exotoxin, it is associated with epidemics of bacillary dysentery. It is the only dysentery bacillus that is pathogenic to laboratory animals.

In man, shigellosis begins with symptoms of acute gastro-enteritis which is accompanied by abdominal pain and diarrhea. As it progresses, diarrhea becomes more frequent and is usually accompanied colicky pain. Later diarrhea losses its fecal characteristic and is followed by mucus with pus and blood. The disease is usually accompanied by fever and marked prostration. It is also known that children are more frequently attacked than adult persons and the symptoms are more severe.

Treatment

1. Water and electrolytes replacement

2. Antibiotic therapy is required to eliminate the organism. Due to the emergence of resistant strains of Shigella, antibiotic sensitivity must be performed on any Shigella isolate to determine suitable antibiotics:

Sulfonamides, tetracycline, Chloramphenicol, ampicillin and streptomycin are known to be effective against shigella.

Immunity:

• Short lived; Preparation of oral live attenuated vaccine is on the way to stimulate mucosal IgA.

Prevention

• Sanitary precautions

• Good personal hygiene (hand washing)