Shigella

INTRODUCTION:
  • Shigella is an enterobacteriaceae
  • Gram negative, Non motile,Facultative anaerobes, Non spore forming, Non capsulated, Non lactose fermenting except S. sonnei.
  • Catalase positive except S. dysenteriae type1 and Oxidase negative.
  • People with blood group O has increased susceptibility to shigella
  • Deoxycholate citrate agar and xyloselysindeoxycholate agar is useful selective medium (Shigella do not have black center in this medium as Salmonella).
CULTURAL CHARACTERISTIC:
  • Temperature range for growth 10-40°C(Optimum temp 37 °C)
  • In Nutrient Agar (After overnight culture) 2 mm in diameter, circular, convex, smooth and translucent.
  • In MacConkey agar:Colorless Colony except S. sonnei (Pink colony)
  • In Shigella-Salmonella agar: Colorless Colony
  • Viability: Death point (56°C for 1 hour), 1% phenol for 30 min,
  • Viable in water for days and in ice for 1-6 months, In faeces it dies within few hour due to acidity produced by growth of coliform.
BIOCHEMICAL REACTION:
  • Catalase positive except S. dysenteriae type 1.
  1. Methyl red positive
  2. VP negative
  3. Urease negative
  4. Mannitol ferment 
  5. Citrate negative
  6. Oxidase negative
  7. No production of H2S
  8. S. sonnei is a late lactose fermenter.
CLASSIFICATION:
  1. Based on biochemical (fermentation of mannitol)and antigenic characteristics.
  • Subgroup A: S. dysenteriae: 15 serotypes
  • Subgroup B: S. flexneri: 8 serotypes.
  • Subgroup C: S. boydii: 19 serotypes
  • Subgroup D: S. sonnei: only one serotypes(lactose non-fermeter)
TOXONOMY:
  1. Family Enterobacteriaceae
  • Shigella dysenteriae: most serious form of bacillary dysentery (Shiga toxin)
  • Shigella flexneri: shigellosis in underdeveloped countries
  • Shigella sonnei: shigellosis in developed countries
  • Shigella boydii : Less frequently isolated from dysentry patients.
CLINICAL SYMPTOMS:
  • Shigella Associated with hemolytic uremic Syndrome
  • Ranges from asymptomatic infection to severe bacillary dysentery
  • Two-stage disease: watery diarrhea changing to dysentery with frequent small stools with blood and mucus, tenesmus, cramps, fever
  • Early stage:
  1. Watery diarrhoea attributed to the enterotoxic activity of Shiga toxin
  2. Fever attributed to neurotoxic activity of toxin 
  • Process involves:
  1. Ingestion
  2. Non-invasive colonization and cell multiplication
  3. Production of the enterotoxin by the pathogenic bacteria in the small intestine;
  • Second stage:
  1. Adherence to tissue invasion of large intestine
  2. Typical symptoms of dysentery
  3. Cytotoxic activity of Shiga toxin increases severity
PATHOGENESIS:
  • SOURCE : MAN: CASE OR CARRIER
  • MODE OF SPREAD: CONTAMINATED FINGERS, FOOD, FLIES, FOMITES
  • PERSON TO PERSON TRANSMISSION
  • Gut pathology is due to toxin
  • INFECTIVE DOSE: 10-100 VIABLE BACILLI
  • HIGHEST CONCENTRATION IN STOOL DURING EARLY/ACUTE INFECTION 103 TO 109 VIABLE BACILLI PER GRAM OF STOOL
LAB DIAGNOSIS:
  • Sampling: fresh stool, mucus flakes and rectal swabs
  • Selenite F broth(0.4%) is used as enrichment and transport media (for 9-12hours)
  • Total blood count reveals anemia and thrombocytopenia, and schistocytes
  • Increase blood urea nitrogen(BUN)
  • Invasive test for shigella is Rabbit ileal loop
  • Culture media: Non Selective Bromocresolpurpe lactose agar, Low selective MacConkey agar, High selective Deoxycholate citrate agar and SS agar.
  • Hektoen agar is the Selective medium for shigella
TREATMENT CONTROL:
  • Ciprofloxacin, Fluoroquinol, Azithromycin, Pivmecillinam, Ceftriaxone
  • Preventing infected individuals from handling food
  • Thoroughly washing hands after changing and disposing of an infant’s diaper 
  • Disinfecting surfaces handled by infected individuals
  • Not allowing infected children to play in community swimming areasIf traveling, consuming boiled or filtered water, fruits peeled by self, and hot meals
  • Proper storage of food
Exam Question of:
  • Blood per rectum, dehydration, decreasing urine output with rising blood urea nitrogen (BUN),Total blood count reveals anemia and thrombocytopenia, and schistocytes are suggestive of hemolytic-uremic syndrome (HUS), a complication of the Shiga toxin 
  • Ciprofloxacin is drug of choice in dysentery due to shigella
  • People with blood group O has increased susceptibility to shigella
  • Non motile organism is Shigella
  • Shigella are be divided into subgroup on the basis of ability to ferment mannitol
  • Selenite F broth is an enrichment media for Shigella
  • Shigella ferment mannitol like E.coli
  • Shigella Associated with hemolytic uremic Syndrome
  • Infective dose for shigella is 10 - 100 bacilli 
  • Gut pathology is due to toxin
  • Subgroup D: S. sonnei: only one serotypes(lactose non-fermeter)
  • Invasive test for shigella is Rabbit ileal loop
  • Shigella produces verocytotoxin
  • Most common cause of dysentery is Shigella 
  • Selective medium for shigella is Hektoen agar

Don't Forget to Solve all the previous Year Question asked on Shigella