İnsan bağırsağında yaşayan Shigella basilleri, maymunlar dışında sadece insanlarda hastalık meydana getirir. Shigella cinsi içinde Shigella dysenteriae (en ağır seyirli olan), Shigella flexneri, Shigella boydii, Shigella sonnei olmak üzere 4 tür bakteri bulunur.


Morfolojisi

Shigella hareketsiz, sporsuz, kapsülsüz, gram negatif (-) basildir. Hareketsiz olmaları ile Salmonellalardan ayrılır; bakteriyolojik boyalarla kolay boyanır, Shigella flexneri'nin bazı tipleri dışında, Shigella basillerinde fimbria yoktur.


Üreme Özellikleri

Shigella, aerob veya fakültatif anaerob bakteridir. Optimal üreme ısısı 37oC‟dir; ancak 8-40 oC‟de de üreyebilir. Optimal pH‟sı 7.2-7.4‟dür. Gaita içinde bulunan koliform bakteriler tarafından hızla asitleştirildiği için birkaç saatte ölür. Dış ortam koşullarında oldukça dirençlidir. Isıya, güneş ışığına ve antiseptiklere dirençli değildir. Klorlu sularda harap olur. Adi besiyerinde kolay ürer, buyyonda homojen bulanıklık yapar. Jelozde yuvarlak, hafif kabarık, düzgün yüzeyli, saydam, E.coli kolonilerine benzer S tipi koloni oluşturur. Shigella sonnei kolonileri, diğerlerine göre daha büyüktür. Endo, EMB, MacConkey besiyerlerinde renksiz koloni oluşturur. S.sonnei hariç, laktoz negatiftir (-).TSİ besiyerinde asit yapar, gaz oluşturmaz, H2S yapmaz, üre ve sitrat negatiftir. Shigella dysenteriae hariç, diğer Shigellalar mannit (+) tir.
                                       

Morphology   
➧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.



➤ Family Enterobacteriaceae
➤ All Enterobacteriaceae
➧ferment glucose
➧reduce nitrates (NO3 to NO2 or all the way to N2)
➧are oxidase negative


 Four species:
➧Shigella dysenteriae: causes most serious form of bacillary dysentery
➧Shigella flexneri: most common cause of shigellosis in underdeveloped countries
➧Shigella sonnei: most common cause of shigellosis in developed countries
➧Shigella boydii


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
➧Process involves:
1-Ingestion
2-Noninvasive 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 throughout the developing nations of the world
➧ Major cause of bacillary dysentery (severe second stage form of shigellosis) in pediatric age group (1-10 years old) via fecal-oral transmission
➧ Estimated 15%-20% of pediatric diarrhea in United States
➧ Leading cause of infant diarrhea and mortality (death) in developing countries
➧ Shigella occurs naturally in higher primates, so shigellosis is spread from human to human via the fecal-oral route primarily by contaminated hands (ingestion is portal of entry)
➧ Less frequently, transmission by ingestion of contaminated food or water
➧ Outbreaks usually occur in close communities; Daycare facilities account for a large proportion of cases that occur in the U.S.
➧ Secondary transmission occurs frequently
➧ Low infectious dose (102-104 CFU) with 1-3 day incubation period
➧ Carriage of the organism persists for approximately one month following convalescence



Laboratory Identification:
➧ Closely related to Escherichia
➧ Four distinct species differentiated on basis of serogrouping and biochemical analysis
➧ Stool specimens and rectal swabs should be cultured soon after collection or placed in appropriate transport medium (e.g., Cary-Blair medium)
➧ Readily isolated on selective/differential agar media (e.g., XLD, SS, and brilliant green agar
➧ Lactose nonfermenter


Treatment, Prevention & Control:
➤Dehydration is problem to attend
➤Treat carriers, major source of organisms; Ampicillin and trimethoprim-sulfamethoxazole are effective antibiotics that reduce duration of carriage
➤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.




INSTAGRAM
https://www.instagram.com/microbiologylab_turkey/
FACEBOOK 
https://www.facebook.com/mikrobiyolojilab/
TWITTER
https://twitter.com/MicrobioLabTurk