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1. Sharq Elneil College
School of Medical Laboratory Sciences
Department of Microbiology
Medical Bacteriology course
shigella
U.Mahadi Hassan Mahmoud
mahadi2010sd@yahoo.com
Bsc, Msc, MIBMS Microbiology

2. Kiyoshi Shiga
7 February 1871

3. Historical information
Discovered over 100 years ago by a
Japanese scientist named Kiyoshi Shiga,
Since its discovery, there have been several
epidemics of shigellosis (caused by the S.
dysenteriae bacterium
Outbreak in the Mexican-Guatemalan
border caused the deaths of 20,000 people

4. Reservoirs:
Human intestines are the
natural habitat and reservoir for
Shigella ,
thus the bacteria are present in
the stools of an infected person
up to a week or two after the
alleviation of symptoms

5. Morphology
 Shigellae are Gram negative, rods.
Unlike salmonellae and many other
enterobacteria, shigellae are non-motile.
 Non-sporing
 Noncapsulate

8. Medically Important spp
Subgroup A: Shigella dysenteriae
Contains 12 distinct serotypes
Serotype 1 was formerly called S. shiga
Serotype 2 was formerly called S. schmitzii
Subgroup B: Shigella flexneri
Contains 6 related serotypes and 4 serotypes
divided into subserotypes.
Subgroup C: Shigella boydii
Contains 18 distinct serotypes
Subgroup D: Shigella sonnei
Contains one serotype

9. Culture &chaacteristics:
Shigellae are aerobes and facultative
anaerobes.
They grow between 10–45 ºC with an
optimum temperature of 37 ºC.
Specimens must be cultured with the
minimum of delay.
 A selective medium is required to
isolate Shigella species from faeces.

10. XLD agar: Shigellae produce red-
pink
colonies, 2–4 mm in diameter,
without black centres
DCA and MacConkey agar:
Shigellae
produce non-lactose fermenting pale
coloured 1–2 mm diameter colonies.
On prolonged incubation, S. sonnei
forms pink colonies

11. .
Salmonella-Shigella (SS) agar:
 Despite its name, this medium is
not suitable for isolating shigellae as
it is inhibitory to most strains

15. Routes of transmission
Faecal-oral route with poor sanitation,
unhygienic conditions, overcrowding,
facilitating the rapid spread of infection.
Only a few organisms are required to
cause disease.
Houseflies help to transfer shigellae
from faeces to food. The

16. Reactions of shigellae
● Lactose negative (S. sonnei is a late lactose and
sucrose fermenter)
● H2S negative
● Urease negative
● Oxidase negative
● Citrate negative
● Lysine decarboxylase (LDC) negative
● Ornithine decarboxylase (ODC) negative except
S. sonnei which is ODC positive
● Beta-galactosidase (ONPG) negative. S. sonnei
and up to 15% of Sd 1 strains and minority of
S. boydii strains are ONPG positive

17. Colicins typing
Colicins are bactericidal macromolecules
which have narrow spectrum activity and they
are produced by Sh.sonneii (16)
colicins kill sensitive bacteria in 3 defined
steps :
1. Adsorption onto a specific receptor at the
surface of the bacterium.
2. Translocation across the outer membrane.
3. Killing activity.

18. Virulence factors
several toxins (endotoxin, enterotoxin, and
cytotoxin)
its ability to induce endocytosis into host cells, and
intracellular growth.
 Each variation of the bacteria is able to penetrate
large intestine epithelial cells and multiply within.
S. dysenteriae, however, causes more severe
symptoms due to the production of the Shiga toxin.
This enables it to kill host cells by inhibiting protein
synthesis

19. Pathology: Shigellosis
Developing countries:
Sh. flexneri is endemic (always present) in most
communities
Sh. dysenteriae type 1 often occurs in an epidemic pattern
These two species of Shigella generally produce the most
severe illness.
Developed countries:
Sh. sonnei is the most common and is the least virulent
Sh. boydii causes disease of intermediate severity is least
common, except in the Indian sub-continent.

20. Fecal-oral transmission
person-to-person, fomites, food, water,
Waterborne and water-washed
Infectious dose: low; as few as 10 cells to
infect
Incubation period: 1 to 7 days; typically,
1-3 days
Duration of illness:
untreated: severe symptoms for about two
weeks
Antibiotic treatment shortens illness and
prevent spread to others

21. Shigellosis - Complications
severe anorexia (loss of appetite)
hypoproteinaemia (a low concentration of blood
protein)
hyponatraemia (a low concentration of blood
sodium)
dilation of the large intestine
seizures
anaemia
kidney damage
persistent diarrhoea

22. Laboratory diagnosis
Specimens:
Collection of Spacimen
Culture
Identificaion
Serology
Molecular characterization
-

23. Antimicrobial suseptibility testing
Ampicillin,
Septra,
Nalidixic acid
Ciprofloxacin. Since many
Shigella are becoming resistant to antibiotics, they
should be avoided as a means of treatment for mild
cases , as most will recover without help.

24. Prevention and Control:
 Handwashing, especially after
defacation
Improved sanitation and hygiene
 Improve water, waste
treatment/disposal and food sanitation
 Reduce overcrowding, etc.
No effective vaccine

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