Microbiology

1. Shigella
Buga Rudolf
HTC LECTURE SERIES
1

2. Introduction
•Shigella are Gram negative, non-motile,
none sporing, rod shaped bacteria,
closely related to Escherichia coli
•Causative agent of human shigellosis
•Also cause disease in other primates but
not in other mammals
2

3. Classification
•Shigella species are classified into four sero –
groups:
oSero-Gp A: Shigella dysenteriae (12
seroptypes) is very virulent
o Sero-Gp B: Shigella flexneri (6 serotypes,X
& Y variants)
oSero-Gp C: Shigella boydii (18 serotypes)
oSero-Gp D: Shigella sonnei (1 serotype, 2
Phases [1&2]).
3

4. Classification
•Gp A – C are physiologically similar
•Shigella sonnei (Gp D) can be differentiated on the
basis of biochemical metabolism assays
Habitat
•These species are found only in the intestines of
humans
•Rarely found in the faeces of healthy convalescent
carriers ( or permanently in biliary tract of carriers
of S.typhi).
4

5. Diagnosis
1. Fresh faeces with mucous
2. Wet saline preparation examination: Numerous
RBCs and polymorphs/pus cells
3. Culture:
• Aerobic/facultatively anaerobic + Optimum
temperature 370 C
• Solid media: Mc, DCA/XLD
• Enrichment medium: Selenite F broth
5

6. Diagnosis…
4. Identification:
• Primary identifying tests:
• UCI (urease test, citrate, indolase test)
• SIM (sulphide production, indole production,
motility test)
• TSI (triple sugar, iron)
• GLMS (Glucose, Lactose, Maltose, Sucrose)
• Slide agglutination: Poly- and Monoclonal
antisera typing
6

7. Pathogenesis
•Shigella infection is typically faecal – oral
route
•As few as 10 bacterial cells can cause an
infection depending on age and condition of
the host
•Dysentery caused results in the destruction of
the epithelial cells of the intestinal mucosa in
the caecum and rectum
7

8. Pathogenesis…
• Some strains produce enterotoxin and shiga toxin,
similar to the verotoxin of Escherichia coli O157:H7.
• Common symptoms: diarrhoea, fever, nausea, vomiting ,
stomach cramps and straining to have bowel movement
oStool may contain blood, mucous or pus
oIn rare cases young children may have seizures
oSymptoms may take as long as a week to show up, but
mostly 2 to 4 days
oSymptoms last for several days or for weeks
8

9. Antibiotics/Control
•Proper sanitation
•Antimicrobial agents
oVaried drug susceptibility
o Use: Amp, TE, Chlor, Cephalosporins,
Fluoroquinolones and Nalidixic acid.
9