L20 - Medical Microbiology Laboratory (Enterobacteriaceae - I)

1. Medical Microbiology Laboratory
Gram Negative Bacilli – Enterobacteriaceae
(E. coli)
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
• EscherichiaGenus
• E. coliSpecies
(medically important spp.)

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E. coli
 Gram negative bacilli (1-3 x 0.4-0.7 µm)
 Aerobic and facultative anaerobic
 Single, pairs
 Motile by peritrichous flagella
 Found in some (capsules, fimbriae, immobility)
 Non spore forming
 It causes:
1) Urinary tract infections (UTI) 2) Diarrhea
3) Pyogenic infections 3) Septicemia

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E. coli
Clinical specimens:
 Stool (ETEC, EIEC, EPEC, VTEC)
 Urine
 CSF (mainly in neonates)
 Pus
 Blood for culture
 Cervical swabs (from women with puerperal sepsis or septic
abortion)
 Synovial fluid
 Ascitic fluid

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E. coli (microscopy)
 Gram negative (-ve) straight rods, scattered

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E. coli (culture)
 Nutrient agar: colonies are large, thick, greyish white,
moist, smooth, opaque or partially translucent discs.

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E. coli (culture)
 Blood agar: many strains especially pathogenic ones
are hemolytic on blood agar.

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E. coli (culture)
 MacConkey agar medium: colonies are bright pink due
to lactose fermentation, medium sized, 3-6 mm, round,
opaque, elevated with characteristic odor (smell)

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E. coli (culture)
 Broth: general turbidity, heavy deposit.

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BIOCHEMICAL TESTS
1. Catalase: positive (+ve)
2. Oxidase: negative (-ve)
3. Motility: motile
4. Triple Sugar Iron (TSI): A/A, gas (+), H2S (-)
5. Indole, MR, VP, Citrate (IMViC): + + – –
6. Urease test: negative (-ve)
7. Api 20E

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BIOCHEMICAL TESTS
Motility test medium

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BIOCHEMICAL TESTS
A/A, gas (+)

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BIOCHEMICAL TESTS

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BIOCHEMICAL TESTS

17. Recorded results for API 20E strip inoculated with E.coli
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BIOCHEMICAL TESTS

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AUTOMATED IDENTIFICATION SYSTEMS
1 Microscan Walkaway

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AUTOMATED IDENTIFICATION SYSTEMS
2
3
Phoenix
Vitek 2