Shigella

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Contents

Introduction

Shigella is a term for the genus of gram negative bacteria that cause bacillary dysentery (Shigellosis, Marlow syndrome) spread usually by faecal–oral contamination.

The genus is named after Kiyoshi Shiga, who first discovered it in 1897.

Aetiology

The first member of this group which is particularly adapted to primates (indeed probably man over the last 250,000 years) was discovered in 1897 by Kiyoshi Shiga.

  1. Shigella dysenteriae (Serogroup A)
    • 12 serotypes based on O-antigen component of the capsule lipopolysaccharide)
    • Usual cause of epidemics
  2. Shigella flexneri (Serogroup B)
    • 6 serotypes based on O-antigen component of the capsule lipopolysaccharide
    • 60% of cases in the developing world
  3. Shigella boydii (Serogroup C)
    • 23 serotypes based on O-antigen component of the capsule lipopolysaccharide)
  4. Shigella sonnei (Serogroup D)
    • 1 serotype based on O-antigen component of the capsule lipopolysaccharide
    • 75% of cases in the developed world
    • Unique beta-D-galatosidase and ornithine decarboxylase biochemical reactions assay
    • Spread from Europe about 1500 AD

Some strains of E coli such as E. coli O157:H7 are genetically almost identical to shigella and the differentiation is essentially an artifact promoted by the early discovery that lactose fermentation is able to be done by shigella and not E. coli.

Structure

Rod-shape, Gram-negative bacterium without flagella. The outer membrane is filled with lipopolyscharride (LPS) with the O-antigen component a virulence factor.

Genetics

Have a highly dynamic circular DNA genome about 4Mb in size with the same replication origin and terminus as those of E. coli.

All Shigella species have plasmids that increase microbial toxicity to their host and that produce the Shiga toxin.

Clinical

The most highly adapted serogroup, Shigella sonnei rarely causes fatal dysentery. Most Shigella species are found intracellularly during infection. They polymerize actin and use this to move withon cells.

Transmission

Either water or food contamination. Usually in the 3rd world. However infection is readily transmitted from child to child and outbreaks of Shigella sonnei and Shigella flexneri have been linked to person-to-person spread among men who have sex with men

Investigations

Usually demonstrated on stool culture.

Treatment

Rehydration and broadspectrum antibiotics

Prevention

Good hygiene practice.

Notification

Statutorily notifiable

External links

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