Historical informationDiscovered over 100 years ago by a Japanese scientist named Kiyoshi Shiga,Since its discovery, there have been several epidemics of shigellosis (caused by the S. dysenteriae bacteriumOutbreak in the Mexican-Guatemalan border caused the deaths of 20,000 people
Reservoirs:Human intestines are the natural habitat and reservoir for Shigella ,thus the bacteria are present in the stools of an infected person up to a week or two after the alleviation of symptoms
Morphology Shigellae are Gram negative, rods.Unlike salmonellae and many otherenterobacteria, shigellae are non-motile. Non-sporing Noncapsulate
Medically Important sppSubgroup A: Shigella dysenteriaeContains 12 distinct serotypesSerotype 1 was formerly called S. shigaSerotype 2 was formerly called S. schmitziiSubgroup B: Shigella flexneriContains 6 related serotypes and 4 serotypesdivided into subserotypes.Subgroup C: Shigella boydiiContains 18 distinct serotypesSubgroup D: Shigella sonneiContains one serotype
Culture &chaacteristics:Shigellae are aerobes and facultativeanaerobes.They grow between 10–45 ºC with anoptimum temperature of 37 ºC.Specimens must be cultured with theminimum of delay. A selective medium is required toisolate Shigella species from faeces.
XLD agar: Shigellae produce red-pinkcolonies, 2–4 mm in diameter,without black centresDCA and MacConkey agar:Shigellaeproduce non-lactose fermenting palecoloured 1–2 mm diameter colonies.On prolonged incubation, S. sonneiforms pink colonies
.Salmonella-Shigella (SS) agar: Despite its name, this medium isnot suitable for isolating shigellae asit is inhibitory to most strains
Routes of transmissionFaecal-oral route with poor sanitation,unhygienic conditions, overcrowding,facilitating the rapid spread of infection.Only a few organisms are required tocause disease.Houseflies help to transfer shigellaefrom faeces to food. The
Reactions of shigellae● Lactose negative (S. sonnei is a late lactose andsucrose fermenter)● H2S negative● Urease negative● Oxidase negative● Citrate negative● Lysine decarboxylase (LDC) negative● Ornithine decarboxylase (ODC) negative exceptS. sonnei which is ODC positive● Beta-galactosidase (ONPG) negative. S. sonneiand up to 15% of Sd 1 strains and minority ofS. boydii strains are ONPG positive
Colicins typingColicins are bactericidal macromoleculeswhich have narrow spectrum activity and theyare produced by Sh.sonneii (16)colicins kill sensitive bacteria in 3 definedsteps :1. Adsorption onto a specific receptor at the surface of the bacterium.2. Translocation across the outer membrane.3. Killing activity.
Virulence factorsseveral toxins (endotoxin, enterotoxin, andcytotoxin)its ability to induce endocytosis into host cells, andintracellular growth. Each variation of the bacteria is able to penetratelarge intestine epithelial cells and multiply within.S. dysenteriae, however, causes more severesymptoms due to the production of the Shiga toxin.This enables it to kill host cells by inhibiting proteinsynthesis
Pathology: ShigellosisDeveloping countries:Sh. flexneri is endemic (always present) in mostcommunitiesSh. dysenteriae type 1 often occurs in an epidemic patternThese two species of Shigella generally produce the mostsevere illness.Developed countries:Sh. sonnei is the most common and is the least virulentSh. boydii causes disease of intermediate severity is leastcommon, except in the Indian sub-continent.
Fecal-oral transmissionperson-to-person, fomites, food, water,Waterborne and water-washedInfectious dose: low; as few as 10 cells toinfectIncubation period: 1 to 7 days; typically,1-3 daysDuration of illness: untreated: severe symptoms for about two weeks Antibiotic treatment shortens illness and prevent spread to others
Shigellosis - Complicationssevere anorexia (loss of appetite)hypoproteinaemia (a low concentration of blood protein)hyponatraemia (a low concentration of blood sodium)dilation of the large intestineseizuresanaemiakidney damagepersistent diarrhoea
Laboratory diagnosisSpecimens:Collection of SpacimenCultureIdentificaionSerologyMolecular characterization -
Antimicrobial suseptibility testingAmpicillin,Septra,Nalidixic acidCiprofloxacin. Since manyShigella are becoming resistant to antibiotics, theyshould be avoided as a means of treatment for mildcases , as most will recover without help.
Prevention and Control: Handwashing, especially afterdefacationImproved sanitation and hygiene Improve water, wastetreatment/disposal and food sanitation Reduce overcrowding, etc. No effective vaccine