Shigella

Question 1

1) classification

Answer 1

Family: Enterobacteriaceae
Genus: shigella

Pathogenic species:

S. Dysenteriae ; S. Flexneri; S. boydii; S. Sonnei

Low infectious dose 10^3

Transmitted by the four “f’s”:
Food, finger, faeces, flies

Occur only in humans and monkeys

Question 2

2) morphology

Answer 2

• gram negative
• short rods
• noncapsulated
• nonmotile
• non spore forming

Question 3

3) growth characteristics

Answer 3

• facultatively anaerobes but grows best in aerobic conditions
• non-fastidious
• do not ferment lactose
• highly communicable: infectious dose 10^3

Question 4

4) antigenic structure

Answer 4

• somatic O antigens (lipopolysacharride)

Question 5

5) virulence factor

Answer 5

• invasion of host mucosal epithelial cells
• endotoxin
• exotoxin

Endotoxin- autolysis of shigella releases their toxic Lipopolysaccharide - probably contribute to irritation of bowels

S.dysenteriae exotoxin:
Affects the gut and CNS
Cause diarrhea
Inhibit sugar and amino acid absorption in the intestine

Question 6

6) disease

Answer 6

Bacillary dysentery

Affect mainly children less than 10 yrs

Question 7

7) microbiological diagnosis

Answer 7

Specimen:

Stool mucus flecks
Rectal swab

DME:
Gram-stain is not useful > pathogens cannot be differentiated from normal bowel flora.
Methylene blue stain that reveals many WBCs indicates invasive vowel disease.

Culture:

• differential media (MacKonkey’s, EMB agar):
  Lactose negative colonies
• selective media (SS agar):
  Colourless colonies with no blackening

Biochemical tests:
Nonmotile
Do not ferment lactose
Ferment glucose with no gas

Serologic tests:
Rapid slide agglutination tests

Question 8

8) treatment and prevention

Answer 8

• ciprofloxacin, ampicillin, doxycycline and trimethoprime sulfamethoxazole
• children: azithroymycin
• multiple drug resistance transmitted by plasmids

Antimicrobial susceptibility tests