SHIGELLA SPECIES

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 (Shigella species) Mode of transmission,mechanisms by which microorganisms cause disease and diagnosis





Define Shigella 
A Gram negative rod. Its an enterobactericiae.
  List the species of Shigella
  • Shigella dysenteriae-sero group A
  • Shigella flexineri-sero group B
  • Shigella boydii-sero group C
  • Shigella sonnei-sero group D

 Mention the species of medical importance
  • Shigella dysenteriae-sero group A
  • Shigella flexineri-sero group B
  • Shigella boydii-sero group C
  • Shigella sonnei-sero group D
  NB:Shigella dysentriae is the commonly isolated and causes bacillary dysentery





Describe the normal habitat Shigella
—  It forms the normal flora of the intestinal tract of humans and animals.

Describe the morphology Shigella 
  • Gram –ve rods
  • Measures 2-4 um in length and 0.6um in width.
  • Non- motile
  • Non-capsulated
  • Non-sporing.

 Describe the mode of transmission and pathogenicity of Shigella species

 Shigella species is transmitted :-
  • By ingestion of faecal material orally
PATHOGENICITY-
Shigella species causes:- bacillary dysentery(shogellosis)
  • Shigella dysentriae causes infections including toxaemia,bacteraemia,inflammation and ulceration of large intestines, haemorrhage, abdominal pain and high fever.
  • Shigella  dysentiriae and Shigella flexineri can lead to leukaemoid reaction developing 5-10 days after incubation due to endotoxin
  • Shigella sonnei is not very pathogenic thus infections are rarely serious

 Explain the toxins of Shigella 
  • It produces endotoxin
 Describe the laboratory diagnosis Shigella 
1.SPECIMENS
  • Stool(Fresh)-It contains blood, pus and mucus. Specimen have an alkali PH. When examined microscopically, RBC and pus cell are usually found.

2.CULTURE CHARACTERISTICS
·         Shigellae are aerobe and facultative anaerobes.
·         They grow between 10-450c with an optimum of 370c.
·         Shigellae can grow in the following media:-
·         XLD
·         DCA
·         SS
·         Macconkey

(i)XLD AGAR(xylose Lysine Deoxycholate)
It produces red-pink colonies,(NLF)2-3mm in diameter without black centers.
(ii)DCA AGAR(Deoxycholate Citrate Agar)
Pink colonies are seen(NLF).The colonies are small measuring 1-1.5mm in diameter.Sh.sonnei is a late LF(lactose fermentor).
(iii)MACCONKEY AGAR
NLF colourless colonies are seen.measures 2-3mm in diameter.
Sh.dysentriae is a late lactose fermenter and appears pink in color

3.BIOCHEMICAL TESTS
The species have varying Biochemical reactions
  • Lactose –ve                
  • H2s –ve                                               
  • Oxidase –ve
  • Citrate –ve
  • TSI-R(slope)/Y(butt) ,No gas, No H2s
  • MR +ve
  • Nitrate reductase      +ve
  • indole     -/+ve(varies)
  • Urease –ve
SUMMARY OF BIOCHEMICAL TESTS REACTION
I-indole=  +VE
M-methyred= +VE
V-voges prousker= -VE
C-Citrate= -VE
4.PREVENTION
  • Seek medical treatment
  • Hygiene
  • Isolation of the infected
  • Screening before admission
  • Proper cooking and storage of food
 USE DRUG EFFECTIVELY 



Key Points 
Define Escherichia
A Gram negative rod in the Genus enterobactericiae
 List the species of Shigella
  • Shigella dysenteriae-sero group A
  • Shigella flexineri-sero group B
  • Shigella boydii-sero group C
  • Shigella sonnei-sero group D
BIOCHEMICAL TESTS
FOR  Shigella:-Perform Biochemical tests to identify the organisms
Perform the following tests for Shigella:-
  • Lactose –ve                
  • H2s –ve                                               
  • Oxidase –ve
  • Citrate –ve
  • TSI-R(slope)/Y(butt) ,No gas, No H2s
  • MR +ve
  • Nitrate reductase      +ve
  • indole     -/+ve(varies)
  • Urease –ve

Evaluation 

  • List 3 species of Shigella
  • Mention 3 Biochemical tests
  • Mention the appearance of colonies on blood Agar and macconkey

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