Shigella

Dr Sabrina Moyo

Department of Microbiology and Immunology

Definition• 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

•   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

• Group A - S. dysenteriae 1-10

• group B - S. flexneri 1-6

• group C - S. boydii 1-15

• group D - S. sonnei 1

Clinical Syndromes (Shigellosis)•   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•   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 fecal-oral 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

•   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

• THANK YOU!

Reference:

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