L23 - Medical Microbiology Laboratory (Shigella) by Hussein A. Abid

1. Medical Microbiology Laboratory
Gram Negative Bacilli – Enterobacteriaceae
(Shigella spp.)
Hussein A. Abid
Medical Laboratory Scientist
Member at American Society of Microbiology
Chairman of Iraqi Medical Laboratory Association
Teacher at Middle Technical University

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Enterobacteriaceae
 A large Family of aerobic bacterial flora of intestine of
humans and other animals.
 Its members are non-spore forming, non acid-fast,
gram negative bacilli.
 Capsule (variable)
 Motility (variable)
 Aerobic and facultatively anaerobic, grow readily on
ordinary media
 Ferment glucose, reduce nitrates to nitrites, catalase
positive but oxidase negative.

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CLASSIFICATION ACCORDING TO LACTOSE
FERMENTATION
Lactose Non-fermentativeLactose Fermentative
Salmonella spp.
Shigella spp.
Proteus spp.
Yersinia spp.
E. coli
Klebsiella spp.
Enterobacter spp.
Citrobacter spp.
Culture media used to differentiate lactose fermenter from
lactose non fermenter are:
1. MacConkey’s (Mac) agar medium
2. Eosin Methylene Blue (EMB) agar medium
3. Salmonella-Shigella (SS) agar medium
4. Triple Sugar Iron (TSI) agar medium

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TAXONOMY
Scientific nameRank
• EnterobacteriaceaeFamily
• ShigellaGenus
• S. boydii
• S. dysenteriae
• S. flexneri
• S. sonnei
Species
(medically important spp.)

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Shigella spp.
 Gram-negative, non-motile bacilli, facultative anaerobes
 Non-spore former, non-capsulated
 Non-lactose fermenter (NLF)
 Deoxycholate citrate agar (DCA) is selective media (pale color
colonies)
 Primarily a parasite of the digestive tract of humans
 Infective dose (ID) is small (102 – 104 CFU)
 Produce a diarrhea-inducing enterotoxin
 Cause a severe form of dysentery called shigellosis
 Bacillary dysentery: passing blood & mucus in the stool

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Shigella spp.
Clinical specimens:
 Stool
 Rectal swabs from an ulcer (buffered glycerol saline –
transport medium)

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Shigella (microscopy)
 Gram negative (-ve) rods
Shigella dysenteriaeShigella flexneri

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Shigella (culture)
 MacConkey’s (Mac) agar: medium sized, non-lactose
fermenter colonies.

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Shigella (culture)
 Salmonella-Shigella (SS) agar: small to medium sized
(2 mm), colorless, round, opaque, elevated

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Shigella (culture)
 Xylose Lysine Deoxycholate (XLD) agar: Shigella
species produce red colonies on XLD agar, no H2S
production, negative for xylose utilization.

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Shigella (culture)
 Hektoen Enteric (HE) agar: Shigella species produce
green to blue green colonies.
S. flexneri

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BIOCHEMICAL TESTS
1. Catalase: positive (+ve)
2. Oxidase: negative (-ve)
3. Motility: non-motile
4. Triple Sugar Iron (TSI): K/A, gas (-), H2S (-)
5. Indole, MR, VP, Citrate (IMViC): +/-, +, -, -
6. Urease test: negative (-ve)
6. Mannitol fermentation:
 Fermentative: S. flexneri, S. boydii, S. sonnei
 Non-fermentative: S. dysenteriae

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BIOCHEMICAL TESTS
Api 20E
Mannitol fermentation
+ve+ve-ve-ve

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SHIGELLA DYSENTERY
-ve-ve
 Must be differentiated from dysentery-like diseases
caused by:
 E coli (EIEC)
 Campylobacter jejuni
 Entamoeba histolytica
 Balantidium coli