Genus shigella

Shigellosis

1. GENUS SHIGELLA
Causes Shigellosis
(dysentery): blood & mucus
in stool. Named after
Shiga : isolated organism in
1896.
Dr V S Vatkar
Asso Prof
Microbiology Dept
D Y Patil Medical College

2. Scientific Classification
• Kingdom : Bacteria
• Phylum : Proteobacteria
• Class : Gamma Proteobacteria
• Order : Enterobacteriales
• Family : Enterobacteriaceae
• Genus : Shigella
• Species : S.dysentery, S.sonnei, S.flexneri, S.boydii

3. MORPHOLOGY
Short, GNB, size: 0.5×
1-3µm
Non motile
Non capsulated
Some strains : type 1
fimbria (S.flexneri)

4. CULTURAL CHARACTERISTICS
Aerobes & facultative anaerobes.
Optimum temp : 37ºC. S.sonnei : grow at
10ºC & 45ºC.
Grow on ordinary media less readily.
Colonies on B.A. & N.A.= smooth, grayish or
colourless, translucent, 2-3mm dia. S.sonnei:
slightly larger, more opaque, 2 antigenic
forms called PHASES.

5. M.A: pale yellowish (NLF), S.sonnei: LLF on prolong
incubation.
DCA medium: selective medium for faeces. Small,
pale colonies. S.sonnei: pink papillae on prolong
incubation.
XLD medium: red colonies. Less inhibitory to
S.dysentery, S.flexneri than DCA medium
H E agar: green colonies
S-S agar: colourless colonies

6. Peptone water & Nutrient broth : uniform
turbidity on overnight incubation. Fimbriated
strains form surface pellicle on prolong
incubation.
Enrichment broth : Selenite F broth:
S.sonnei, S.flexneri serotype 6 grow well,
other Shigella strains inhibited. Weakly
inhibitory GN broth : less inhibitory

8. Colonies on Congo Red Agar

10. BIOCHEMICAL REACTIONS
Ferment Glucose : acid
no gas (S.flexneri type 6
produces acid & gas).
Do not ferment salicin,
adonitol, inositol &
lactose (S.sonnei: LF).
Reduces Nitrates to
Nitrites
Inhibited by KCN
Urease: - ve
Indole +ve: some
memb of Gr A,B & C,
not Gr D.
M-R : +ve
Catalase +ve: except
S.dysentery type 1
H2S : - ve
Citrate : - ve

11. Antigenic structure
1 or more Major Ag. Large no of Minor
Somatic Ag
Some strains : K Ag
Fimbrial Ag : S.flexneri
Identification of Shigella: made by
combination of Agenic & biochemical
properties.

12. VIABILITY
Viable in faeces for few hrs, but survive for few days
in faeces kept in Buffered glycerol solu
n
or preserved
at 4ºC
Survive for several days (5-20) on :
Faeces dried on cloth
Soiled lavatory seats, fomites
In cool, damp & dark conditions
Cult. remain viable for many yrs on Dorset egg agar

13. RESISTANCE
Killed at 56ºC in 1 hr & 1% phenol : 30 min
In ice: last for 1-6 mths
Die rapidly on drying
S.sonnei : most resistant spp than other
shigella

14. VIRULENCE FACTORS
Shiga toxin: S.dysentery type 1: known as
Verotoxin or shiga like toxin. MOA : act like
toxin produced by EHEC. Responsible for
complications like Hemorrhagic colitis &
Hemolytic Uremic Syndrome.
Shigella enterotoxin 1: S.flexneri ,
S.dysentery type 1. Chromosomally
encoded, responsible for early diarrheal
phase.

15. Shigella enterotoxin 2 : S.flexneri ,
S.dysentery : encoded on plasmid,
responsible for early diarrheal phase.
Type II secretion system(T2SS):
S.dysentery : encoded by gene, show
similarity to genes of ETEC & cholera toxin.
Function: unknown.

16. Invasion plasmid Ag: responsible for
attachment & penetration of bacilli to the
mucosal epith cells of colon, also resist
phagocytosis.
Intracellular spread proteins: increases cell
to cell spread of bacteria.

17. Invasive property of Shigella demonstrated
by : ability to penetrate HeLa cells & Hep-2
cells and by Congo Red binding test. Plasmid
mediated:proteins present on OMP
responsible for cell penetration. These
proteins are known as Virulence Marker Ag
(VMA). Detected by ELISA.

18. CLASSIFICATION
Antigenically divided into 4 grs. On the basis
of serologically somatic O Ag & on
biochemical characteristics.
S.sonnei: classified by Colicin typing.
Subgroup A :S.dysentery type 1, described
by Shiga. Consist of 12 serotypes. Type 1
bacilli: Shigella shiga (synonym) : mannitol
non fermenter, Indole –ve, Catalase –ve.

19. 3 types of toxins produce by it :
1) Neurotoxin: damages endothelial cells of
small bl vs of CNS, causes polyneuritis,
meningitis, coma. 2) Enterotoxin 1 & 2: fluid
accumulation in lumen 3) Cytotoxin :
causes cytopathic changes in Vero cells
Type 2 bacilli : S.schmitzi: Indole +ve,
ferment Sorbitol & Rhamnose.

20. Subgroup B: S.flexneri
Flexner (1900): described mannitol
fermenting Shigella from Philippines.
6 serotypes (1-6) & 2 Agenic variants X & Y.
Several subtypes: 1a, 1b, 2a,2b, 3a, 3b,
4a,4b,5a, 5b. Biochemically heterogeneous
& Agenically more complex
Serotype 6: Indole –ve.

21. Serogroup C: S.boydii
Boyd (1931): discovered this strain from
India.
Resembles with S.flexneri biochemically but
not Agenically.
18 serotypes: yet identified

22. Serogroup D: S.sonnei
Described by Sonne (1915) in Denmark.
Indole –ve, Catalase +ve
Colonies: S to R variation, called Phase I &
Phase II (may present in patients & carriers,
loss of variability in subcult.)
On subcult. produce phase II type of
colonies.
Causes mildest type of infection.

23. PATHOGENESIS
Causes bacillary dysentery, Shigellosis
Mode of infection: i) thr’ contaminated food
& water. 10-100 bacilli capable of causing inf
(survival in gastric acidity). ii) thr’ fomites.
Incubation period: short upto 48hrs
(1-7days)
Source of inf: human beings (cases), less
often carriers.

26. Organism reaches to terminal ileum & colon
Attaches to epith.cells of villi of large intestineMultiply in them
Spread laterally to infect adjacent cellPenetrate into lamina propria
Inflammatory reaction with capillary
thrombosis
Necrosis of epith cells.
Cells: soft & friable Serpinginous ulcer
Bleeding from mucosa

27. Endotoxins irritates bowel causes diarrhoea.
Ulcer surface: covered with pseudomemb
made up of fibrin, leucocytes, cell debris,
necrosed epith & bacteria.
Clinical features: loose, scanty, faeces
frequently with blood & mucus
Abdominal cramps, tenesmus.
Fever, vomiting may present.

28. complications
Hemorrhagic colitis
Hemorrhagic Uremic Syndrome
(S.dysentery type 1) convulsions in children.
Reiter’s Syndrome: conjunctivitis, toxic
neuritis, arthritis(3% pt suffered with
S.flexneri inf)
Intussusception in children

29. Dysentery carriers
Acquired after Ac.attack of dysentery. Person
excretes bacilli for few days in faeces.
Diagnosis done : culture on DCA or S-S
agar.
Healthy carriers may be seen.

30. Laboratory Diagnosis
Specimen: fresh stool, rectal swab
Inoculate immediately or transport media:
Sach’s buffered glycerol saline / Selenite F
broth.
Microscopy: to exclude amoebic dysentery.
Culture : loopful of sample is inoculated in
Enrichment broth: Weakly inhibiting GN
broth less inhibitory than Selenite F broth.
Incubated at 37ºC

31. Subcultured on M.A./ DCA/ XLD/ S-S agar
after 6hrs & 18-24hrs.
Biochemical reactions : for primary
identification of organism.
Slide agglutination test: serological test.
Polyvalent sera : available for spp
identification.

32. Tube agglutination test: done if
biochemical reactions & slide agglutination
tests are atypical.
Ag is prepared. Dilutions : like Widal test
Incubate for 4 hrs at 50ºC
Agglutination titer of serum is observed.

33. Colicin typing of S.sonnei
Helpful in epidemiological study : done by
phage typing & colicin typing.
Bacteriosins produced by these bacilli have a
wide range of activity against enteric bacilli &
on this basis S.sonnei strains : subdivided
into 17 colicin types against 15 indicator
strains characterized by production of
specific colicin.

34. Bacteria confused with shigella
Salmonella: NLF, non fermenter,
agglutinated by polyvalent O & H sera.
Alkalescens Dispar gr: NLF, non gas
producers, non-motile
Hafnia: NLF, motile, citrate +ve.
Providencia: NLF, motile, deamination of
phenyl alanine
Plesiomonas shigelloides: NLF,
anaerogenic

35. Treatment & prophylaxis
Uncomplicated shigellosis: self limiting
Ac cases ORT: no vomiting. I/V fluids to
correct dehydration.
Ampicillin, Trimethoprim-Sulfamethoxazole,
Ciproflox, Norflox, Nitrofurantoin are useful.
Improvement in personal hygiene
Environmental sanitation
No effective vaccine.

36. VACCINES under trial
Live attenuated strains:
i) two prototype attenuated vaccine strains of
S.flexneri 2a & S.dysentery type 1 . Phase 1
trials recently begun in USA (Baltimore).
Stimulates secretory IgA Ab (sIgA Ab).
ii) Attenuated mutant S.flexneri 2a deve in
France. Double attenuation: capacity to
survive intra & intercellularly.

37. iii) Lypopolysaccharide in live attenuated vector :
under deve. in Mexico.
iv) strains used as a carrier for expression of O-Ag.
Ist generation cholera vaccine(CH3), expresses both
encoded Inaba & heterologous (S.sonnei) O-
serotype.
Subunit vaccine:
i) Conjugate vaccine: S.sonnei-rEPA (recombinant
exoprotein A) & S.flexneri 2a-rEPA. Parenteral
under trial in Israel.

38. ii) Proteosome vaccine: proteosome :
multimolecular vesicle form from Neisseria
OMP, used as mucosal vaccine to induce
systemic & mucosal immune response. e.g.
after oral / nasal immunization in mice/guinea
pig with S.flexneri or S.sonnei proteosome,
high levels of Ab against LPS.
Under phase I trial.

39. Shigella nucleoprotein (ribosomal)
vaccine:
Lyophilized Shigella vaccine: stable, low
cost, under development.
Non-covalent complex of O-polysaccharide &
ribosomal particles from Shigella.
Single does :S/C
IgA Ab secretion.

41. Mode Of Action of LT
Toxin binds to Gm1 receptors of intestinal epithelium with subunit
B subunit A activated


activates Adenyl
cAMP cyclase.


increase outflow of
water & electrolytes leads to DIARRHOEA
in intestinal lumen

* MOA of Cholera toxin is same as LT