SHIGELLA SPECIES

 (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