Shigellae are Gram-negative, non-motile, non-spore forming, rod-shaped highly infectious bacteria of the Enterobacteriaceae family. Shigella species include; Shigella sonnei, S. boydii, S. flexneri, and S. dysenteriae.
Shigella species are classified by four species (serogroups):
- Serogroup A: Shigella dysenteriae (12 serotypes)
Type 1- Shiga’s bacilli
Type 2- Schimitz bacilli
- Serogroup B: Shigella flexneri (6 serotypes)
- Serogroup C: Shigella boydii (18 serotypes)
- Serogroup D: Shigella sonnei (1 serotype)
*S. dysentriae is the most virulent but S.sonnei is most common isolated species worldwide.
Shigellosis (also called bacillary dysentery).
- S. dysenteriae type 1 causes the most severe disease and responsible for cases in which hemolytic uremic syndrome (HUS) develops.
- S. sonnei produces the mildest form of shigellosis; usually watery diarrhea.
- S. flexneri and S. boydii infections can be either mild or severe.
Routes of transmission: Humans are the only host of Shigella. Fecal-oral route is the primary means of human-to-human transmission. The five “Fs” of disease transmission are:
- Food- ingestion ( water, food, drinks)
- Fomites-(taps, toilet seats etc)
Pathogenesis: Some strains of Shigella produce enterotoxins and Shiga toxin. Shigella species are tolerant to low pH and are able to transit the harsh environment of the stomach. Pathogenic mechanism of Shigellosis is complex which involves:
- Enterotoxic/cytotoxic diarrheal prodrome
- Cytokine-mediated inflammation of the colon
- Necrosis of the colonic epithelium
- Shigellas are taken up by M cells and transported beneath the epithelium. Macrophage takes up shigellas, die and release the bacteria.
- The bacteria enter the inferior and lateral aspects of the epithelial cells by inducing endocytosis. The endosomes are quickly lysed leaving shigellas free in the cytoplasm.
- Actin filaments quickly form a tail pushing the shigellas into next cell.
- Shigella multiply in the cytoplasm and infection spread to next cell.
- Infected cells die and slough off, intense response of acute inflammatory cells (neutrophils), bleeding and abscess formation.
Infectious dose (ID): As few as 10 to 200 cells can cause disease, depending on the age and condition of the host.
Freshly passed stool is the preferred sample. The sample must reach laboratory within 2 hours of collection. If delay is anticipated transport media should be used to retain the viability of the organism.
Macroscopic examination: scanty fecal matter, bright red in colour, and presence of mucus
Microscopic examination: Plenty of pus cells, RBCs & a dead background
(no motile bacteria)
Transport Media for Shigella
- Buffered Glycerol water,
- Cary-Blair transport media,
Transported in the cool box or refrigerated if delay is likely.
The sample can be plated directly in either MacConkey Agar or Deoxycholate Citrate Agar (DCA) or Xylose Lysine Desoxycholate (XLD) Agar, Salmonella Shigella Agar or after enrichment in Selenite F broth (incubated at 37o C)
- Shigellae form non-lactose fermenting pale colored or colorless colonies on MacConkey’s agar or EMB agar or DCA. S. sonnei forms pink colonies.
- Xylose Lysine Desoxycholate (XLD) Agar: Shigellae produce red-pink colonies without black centers.
- Salmonella-Shigella (SS) agar: Despite its name, this medium is not suitable for isolating shigellae as it is inhibitory to most strains.
On Triple Sugar Iron (TSI) agar test, they cause an alkaline slant and an acid butt, with no gas and no hydrogen sulphide. Confirmation of the organism as Shigella and determination of its group are done by slide agglutination test.
Biochemical properties of Shigellae
- Catalase +ve (Shigella dysenteriae type 1 is always catalase -ve)
- Lactose negative (S. sonnei is a late lactose and sucrose fermenter)
- Hydrogen sulphide negative
- Urease test: Negative
- Oxidase test: Negative
- Citrate utilization test: negative
- Lysine decarboxylase (LDC) test: Negative
- Ornithine decarboxylase (ODC) test: Negative except S. sonnei which is ODC positive.
- Beta-galactosidase (ONPG) test: Negative
S. sonnei and upto 15% Shigella dysenteriae type 1 strains and minority of S. boydii strains are ONPG positive.
- Acid produced from a number of Carbohydrates.
- Glucose is fermented with acid only (gas is produced by few strains only)
- Mannitol is fermented by all except S.dysenteriae
- Lactose & sucrose are not fermented except by S.sonnei
Serology of Shigella species
- Serological identification is dependent upon the O antigens.
- Shigella are non-motile organisms have no H antigens.
- K-antigens present on the surface may interfere with serotyping.
- Prevalence of the species type depends on the geographical location.
- Group A & B are common in developing countries.
- After isolation all 4 A, B, C, & D polyvalent antisera have to be used to determine the groups then specific monovalent antisera is to be used to identify particular serogroup.