Download Shigella

Shigella
Dr Sabrina Moyo
Department of Microbiology and
Immunology
Definition
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An enterobacteriaceae
Gram negative bacilli.
Readily growth O2 + An O2.
Metabolically active, fermenting a variety of
substrates.
• Mostly non-motile, non sporing, non acid
fast, 2-4um x 0.4 -0.6um rounded ends.
Morphology & Physiology
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Small Gram-negative, facultatively anaerobic, coliform
bacillus
• Non-motile (no H antigen)
• Possess capsule (K antigen) and O antigen
• K antigen not useful in serologic typing, but can
interfere with O antigen determination
• O antigens: A, B, C, D correspond respectively to the
four species
• Non-lactose fermenting
• Bile salts resistant: trait useful for selective media
ferment glucose
reduce nitrates (NO3 to NO2 or N2)
are oxidase negative
Taxonomy
Family Enterobacteriaceae
1. Shigella dysenteriae: most serious form
of bacillary dysentery
2. Shigella flexneri: shigellosis in
underdeveloped countries
3. Shigella sonnei: shigellosis in developed
countries
4. Shigella boydii
Taxonomy
• Classification: 4 groups or species on
the based on difference in O antigen
and some biochemical reactions
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Group A - S. dysenteriae 1-10
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group B - S. flexneri 1-6
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group C - S. boydii 1-15
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group D - S. sonnei 1
Clinical Syndromes (Shigellosis)
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Ranges from asymptomatic infection to
severe bacillary dysentery
• Two-stage disease: watery diarrhea
changing to dysentery with frequent small
stools with blood and mucus, tenesmus,
cramps, fever
Early stage:
• Watery diarrhea attributed to the
enterotoxic activity of Shiga toxin
• Fever attributed to neurotoxic activity of
toxin
Clinical Syndromes
Process involves:
 1. Ingestion
 2. Non-invasive colonization and cell multiplication
• 3. Production of the enterotoxin by the pathogenic
bacteria in the small intestine;
Second stage:
• Adherence to and tissue invasion of large intestine
• Typical symptoms of dysentery
• Cytotoxic activity of Shiga toxin increases severity
Epidemiology
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Shigellosis is a major cause of diarrheal
disease (developing nations)
• Major cause of bacillary dysentery (severe
second stage form of shigellosis)
• Leading cause of infant diarrhea and
mortality (death) in developing countries
Epidemiology
• Shigella occurs naturally in higher primates
• Spread from human to human via the fecaloral route
• Less frequently, transmission by ingestion
of contaminated food or water
• Outbreaks usually occur in close
communities;
• Secondary transmission occurs frequently
Epidemiology
• Low infectious dose (102-104 CFU) with 1-3
day incubation period
• Carriage of the organism persists for
approximately one month following
convalescence
Pathogenesis & Immunity
• Invasiveness involves attachment
(adherence) and internalization
• Controlled by a multi-gene virulence
plasmid
• Organisms penetrate through colonic
mucosa
• invade and multiply in the colonic
epithelium
• Not beyond the epithelium into the lamina
propria
Pathogenesis & Immunity
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Bacterial cells preferentially attach to and
invade into M cells in Peyer's patches of
small intestine
• M cells typically transport foreign antigens
from the intestine to underlying
macrophages,
• Shigella can lyse the phagocytic vacuole
(phagosome) and replicate in the
cytoplasm
Pathogenesis & Immunity
• Exotoxin (Shiga toxin) is neurotoxic,
cytotoxic, and enterotoxic, encoded by
chromosomal genes,
• Enterotoxic effect: Shiga toxin adheres to
small intestine receptors
• Blocks absorption (uptake) of electrolytes,
glucose, and amino acids from the intestinal
lumen
Pathogenesis & Immunity
• Cytotoxic effect: B subunit of Shiga toxin
binds host cell glycolipid in large intestine,
• Inactivate the 60S ribosomal subunit,
• Inhibit protein synthesis, causing cell
death, microvasculature damage to the
intestine, and hemorrhage (blood and
fecal leukocytes in stool)
• Neurotoxic effect: Fever, abdominal
cramping are considered signs of
neurotoxicity
Laboratory Identification:
• Closely related to Escherichia
• Species (serogrouping and biochemical
analysis
• Stool specimens and rectal swabs should be
cultured soon after collection or placed in
appropriate transport medium (Cary-Blair
medium)
• Readily isolated on selective/differential agar
media (XLD, SS, and brilliant green agar
• Lactose nonfermenter
Treatment, Prevention & Control:
• Dehydration is problem to attend
• Treat carriers, major source of organisms;
Cirpflxacin , Erythromycin
• Antibiotic resistance is a major problem
• Proper sewage disposal and water
chlorination
• Oral vaccines of Shigella: E. coli hybrids or
Shigella mutants offers immunity for six
months to one year
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THANK YOU!
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