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 solun
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 cell Penetrate 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