SHIGELLA
➧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.
Taxonomy:
➤ 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.
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