The genus Shigella exclusively infects human intestine.  Shigella dysenteriae is the causative agent of bacillary dysentery or shigellosis in humans. It is a diarrheal illness which is characterized by frequent passage of blood stained mucopurulent stools. The four important species of the genus Shigella are:  Shigella dysenteriae Shigella flexneri Shigella sonnei Shigella boydii.

1. SHIGELLA
Dr. Roma Goyal

2. INTRODUCTION
• The genus Shigella exclusively infects human
intestine.
• Shigella dysenteriae is the causative agent of
bacillary dysentery or shigellosis in humans.
• It is a diarrheal illness which is characterized by
frequent passage of blood stained mucopurulent
stools.
• The four important species of the
genus Shigella are:
– Shigella dysenteriae
– Shigella flexneri
– Shigella sonnei
– Shigella boydii.

3. MORPHOLOGY
• Short GNB
• 1-3 μm x 0.5 μm
• Nonmotile
• Noncapsulate & nonsporing
• Fimbriae – may be present

4. CULTURAL CHARACTERISTICS
 Facultative anaerobes
 pH 7.4
 Temp – 100 -400 C
 Grow on ordinary media
 Colonies: small, <2 mm; circular, convex
smooth & translucent
 MacConkey Agar, Salmonella-Shigella agar&
DCA: colourless (except Sh sonnei)
 Wilson & Blair BS medium: growth inhibited

6. RESISTANCE
• 56oC X 1 hour
• 1% phenol X 30 mins
• Water & ice 1-6 months
• 1-6 weeks on dried stools
• Acidic stools – few hours
• Antibiotics: multiple resistance- streptomycin,
ampicillin, chloramphenicol

7. BIOCHEMICAL CHARACTERISTICS
• Non lactose fermenter (Sh sonnei- late)
• MR +
• Mannitol : Subgroup A –ve, Rest +ve
• Sucrose & H2S –ve
• Indole : S. dysentriae serotype 2 +, S flexneri
serotype 6, S sonnei ALWAYS NEGATIVE
• Catalase -ve Sh dysentriae type 1, Sh flexneri
4a

8. CLASSIFICATION
• Shigella are classified into four Subgroups/
species based on biochemical and Serological
characteristics:
– Subgroup A: Shigella dysentriae
– Subgroup B: Shigella flexneri
– Subgroup C: Shigella boydii
– Subgroup D: Shigella sonnei

9. Subgroup A
Sh.dysentriae
• 12 serotypes, mannitol -ve
• Type 1- Sh.shiga (Shiga’s bacillus)
– Most virulent
– Catalase –ve
– Indole –ve
– Produces exotoxins: Enterotoxins, Neurotoxin,
cytotoxin
• Type 2 –Sh. smitzi- Indole +ve
• Type 3-7 Large Sach’s group

10. Subgroup B
Sh.flexneri
• Most common in India
• 6 serotypes
• Mannitol +ve
• Biochemically and antigenically
heterogenous

11. Subgroup C
Sh. boydii
• Least common
• Biochemically similar to Sh. Flexneri
• 15 serotypes
• Mannitol +ve

12. Subgroup D
Sh. sonnei
• Least virulent
• Late lactose and sucrose fermenter
• Most common species in West
• Indole –ve
• Antigens – 2 phases (I & II)
• Two different colony morphotypes
• Subclassified into 17 colicine types

14. VIRULENCE FACTORS
• Shigella dysenteriae produces 3 types of
toxins:
–Endotoxin
–Exotoxin
–Verocytotoxin.

15. Various toxins of Shigella
Toxins Mode of Action
Endotoxin It is released after autolysis, it has irritating
effect on intestinal wall which causes
diarrohea and intestinal ulcers
Exotoxins It is a powerful toxin and acts as well as
neurotoxin.
As Entertoxin: it induces fluid accumulation.
As Neurotoxin: it damages the endothelial
cells of small blood vessels of CNS which
results in coma and death.
Vero cytotoxin It acts on vero cells

16. PATHOGENESIS
• Infective dose is low 10 - 100 bacilli
• Source of Infection – Patient or carriers
• Route of entry – faecal – oral route
• Site of infection – Large intestine Incubation
Period – Less than 48 hours (1–7 days)
• Mode of transmission – Food, finger, faeces and
flies

17. Sh. Dysenteriae causes bacillary dysentery. The pathogen enters
into the host by the ingestion of contaminated food
The bacilli reaches large intestines and adheres to the epithelial
cells of villi. Multiples and produces toxins. Which stimulates an
inflammatory reaction and causes extensive tissue destruction.
Which leads to necrosis of epithelial cells.
The necrosed epithelial, becomes soft and friable and leads to
ulcers. Abdominal cramps and pain are caused by the distruption of
the intestine.
The degeneration of intestinal villi and local erosion causes bleeding,
heavy mucous secretion resulting in BACILLARY DYSENTERY.

20. CLINICAL MANIFESATION
• Frequent passage of loose, scanty faeces
containing blood and mucus.
• Abdominal cramps and tenesmus (straining
to defecate).
• Fever and vomiting.
• Hemolytic uremic syndrome (It is a
condition caused by the abnormal
destruction of red blood cells).

21. LAB DIAGNOSIS
• Specimens: Fresh stool is collected.
• Direct Microscopy: Saline and Lugol’s iodine
preparation of faeces show large number of pus
cells, and erythrocytes.
• Culture: For inoculation, it is best to use mucus
flakes (if present in the specimen) on MacConkey
agar and SS agar. After overnight incubation at
37°C, the plates are observed for characteristic
colonies, which is confirmed by Grams staining and
biochemical reactions.

22. • Serotyping: Identification confirmed by
slide agglutination with polyvalent &
monovalent sera. Tube agglutination if
doubtful. Boil 1hr x 100o C and look for
agglutination if masking K Ag present.
Tube Agg- in saline Mercuric iodide soln
(Mackie McCartney)
• Serology for epidemeology

23. • DNA Colony Hybridization
– DNA probe directed against various plasmid-
encoded virulence genes
• PCR- Primer directed against plasmid-
encoded virulence gene. Problems: living vs.
dead cells, plasmid stability
• Latex particle agglutination - clinical samples
• ELISA - clinical samples

24. TREATMENT AND PREVENTION
• Uncomplicated shigellosis is a self – limiting
condition that usually recovers spontaneously.
• In acute cases, oral rehydration therapy (ORT)
is done.
• In all severe cases, the choice of antibiotic
should be based on the sensitivity of
prevailing strain.
• Many strains are sensitive to Nalidixic acid and
Norfloxacin.

25. • The precautions for Shigella are same as that of
any food and water-borne diseases.
• WASH YOUR HANDS thoroughly before and after
a meal.
• Wash your hands properly after a bowel
movement.
• Ensure the water that you drink is clean and the
fruits and vegetables are fresh.
• Ensure products such as milk, chicken, and fish
that have a higher tendency to spoil must be kept
at a proper temperature and also cooked well

26. When to suspect for Shigella???
• A person who has severe diarrhoea, which means 20
or more bowel movements in a day, A patient
with mild diarrhoea may wait.
• If child develops bloody diarrhoea or diarrhoea
severe enough to cause weight loss and dehydration.
• If the infected person is running a fever of 101
degree F (38ᵒC) or higher.