1. SHIGELLA Important Properties: @ Gram-negative, Lactose negative rods.
@ Distinguished from Salmonella by:
# No gas from glucose fermentation
# Do not produce H2S.
# Non-motile.

3. SPECIES:
Shigellae are divided into the following:
@ Group (A): S. dysenteriae, serotypes 1-10
@ Group (B): S. flexneri, serotypes 1-6.
@ Group (C): S. boydii, serotypes 1-15.
@ Group (D): S. sonnei, only one serotype.

5. Antigens:
@ All shigellae have polysaccharide O
antigens in their cell walls.
@These antigens are used to divide the
Shigella genus into 4 groups: A, B, C, & D
Disease:
Shigella causes enterocolitis (dysentery).

7. Pathogenesis:
@ Shigella has a very low ID50
@ ID50 (the 50 % infectious dose) is the
number needed to cause infection in half
the hosts.
@ LD50 (the 50 % lethal dose) is the number
needed to kill half the hosts.
@ Ingestion of as few as 100 organisms
causes disease

9. @ Shigellosis is a disease of humans only.
@ It leads to bloody diarrhoea (dysentery)
by invading the mucosa of the distal
ileum and colon.
@ This leads to inflammation and ulceration
@ Shigella will not penetrate the intestinal
wall or enter the bloodstream
(unlike Salmonella)

11. @ Some Shigella strains produce an
enterotoxin, but its role in pathogenesis
is uncertain.
@ The evidence for this:
@ Shigella strains not producing
enterotoxin can still cause dysentery.
@ Non-invasive Shigella strains are non-
pathogenic.

13. Clinical Findings:
@ First fever, abdominal cramps, then
watery diarrhea, which later contains
blood and mucus.
@ The severity of the disease depends on:
# The species (S. dysenteriae is the most
severe and S. sonnei is the mildest)
# The age of the patient (children and
elderly most affected).
@ Antibodies are not protective

15. Laboratory Diagnosis:
@ Shigellae form non-lactose-fermenting
colonies on Mac Conkey & EMB agar.
@ On TSI agar, they give an alkaline slant
and an acid butt, without gas or H2S.
@ Confirmation by slide agglutination

17. @ Methylene blue stain of feces detects
polymorpho-nuclear cells (PMNs).
# If present, an invasive organism is
involved, e.g. Shigella, Salmonella,
Campylobacter
# If absent, a toxin-producing organism is
involved, e.g. V cholerae, E coli,
Clostridium perfringens.

19. Epidemiology:
@ Shigella is transmitted from person to
person by asymptomatic carriers.
@ The four F's are the main sources of
infection (Fingers, Food, Faeces, Flies)
@ Portal of entry is the faecal-oral route.
@ Food-borne outbreaks outnumber water-
borne outbreaks by 2 to 1.
@ Children under 10 yrs account for half of
Shigella positive stool cultures.

21. Treatment:
@ Main therapy is fluid & electrolyte
replacement
@ In mild cases, no antibiotics. In severe
cases, ciprofloxacin is the best.
@ Incidence of drug resistance is high,
hence sensitivity tests are performed.
@ Antiperistaltic drugs (Lomotil) are not used so as not to prolong the symptoms

23. Prevention:
@ Interrupt the fecal-oral transmission by:
# Proper sewage disposal.
# Chlorination of water.
# Hand washing by food handlers.
# No vaccine, or antibiotic prophylaxis.

25. Salmonella and Shigella
Important Features of
Salmonella and Shigella
Feature
Shigella
S. typhi
Other Salmonellae
Reservoir
Humans
Animals, Eggs, Poultry
Infectious dose
Low
High
Diarrhoea as
a prominent feature
Yes NO
Invasion of blood stream
Chronic carrier state
Infrequent
Lactose fermentation
H2 S production
Vaccine available