Morphology & Culture Characteristics of Escherichia coli (E. coli)

MORPHOLOGY AND CULTURE CHARACTERISTICS OF ESCHERICHIA COLI (E.COLI)

MORPHOLOGY OF ESCHERICHIA COLI (E. COLI)

ShapeEscherichia coli is a straight, rod shape (bacillus) bacterium.

Size – The size of Escherichia coli is about 1–3 µm × 0.4–0.7 µm (micrometer).

Arrangement Of CellsEscherichia coli is arranged singly or in pairs.



MotilityEscherichia coli is a motile bacterium. Some strains of E. coli are non-motile.

FlagellaEscherichia coli is a flagellated bacterium with peritrichous flagella arrangement.

Spores – The Escherichia coli is a non–sporing bacterium.

Capsule – Capsules are present in some strains of the E. coli which can easily be demonstrated using India ink preparation, appear as a clear halo in a dark background.

Gram Staining Reaction – Escherichia coli is a Gram -ve (Negative) bacterium.

morphology of escherichia coli - morphology of e.coli
MORPHOLOGY OF ESCHERICHIA COLI (E.COLI)

CULTURE REQUIREMENTS OF ESCHERICHIA COLI (E. COLI)

Special requirementsEscherichia coli or E. coli have no complex nutritional requirements and readily grow in an ordinary media like Nutrient Agar medium (NAM). Commonly the NAM & MacConkey Agar medium is used for the cultivation of Escherichia coli in Laboratory.

 Optimum temperatureEscherichia coli ranges from 10–40°C but the optimum temperature for most of the strains is 37°C and usually cultivated at this temperature in laboratories.

⇒ Optimum pHE. coli can survive at 4.5– 9.5 pH but the maximum growth observed at 7.0 i.e. neutral pH. Also, the pH requirements vary as per the strain of Escherichia coli.

Check out the Morphology & Culture Characteristics of Clostridium botulinum

MORPHOLOGY AND CULTURE CHARACTERISTICS OF CLOSTRIDIUM BOTULINUM

⇒ Oxygen requirementsEscherichia coli (E. coli) is an aerobic bacterium i.e. grow best in the presence of oxygen and it is also a Facultative anaerobic organism i.e. can grow in a low oxygen environment.

⇒ There are various culture media used for the cultivation of Escherichia coli  (E. coli) in the laboratory and most commonly the Nutrient Agar medium and MacConkey Agar medium is used, the other media are as follows –

  • Columbia Horse Blood Agar medium
  • Sheep Blood Agar medium
  • Violet Red Bile Agar medium
  • Cysteine Lactose Electrolyte DEficient Agar (CLED) medium
  • Eosin Methylene Blue Agar (EMB Agar) Medium (Selective medium).
  • Sorbitol MacConkey Agar medium (Selective medium).
  • Modified Tryptic Soy Broth (Selective medium)
  • The liquid medium (Nutrient Broth medium, TSB medium, etc.).

 The Eosin Methylene Blue Agar (EMB Agar) medium which is a Selective medium for Escherichia coli contains Eosin dye and Methylene Blue dye which inhibits the growth of many gram-positive bacteria, inhibits the growth of Shigella and Salmonella species and supports the growth of Escherichia coli.



 Similarly, the Sorbitol MacConkey Agar medium which is also a selective medium for Escherichia coli can be prepared by incorporating Sorbitol in the Traditional MacConkey Agar Base medium instead of Lactose, make it selective for the cultivation of pathogenic E. coli in the laboratory.

The modified Trypticase Soy Broth medium which is also a selective medium for the E. coli and for coliform bacteria, is less commonly used for the Cultivation of E. coli in laboratory, can easily be prepared by suspending bile salt, Dipotassium Hydrogen phosphate & Antibiotics like Vancomycin in the Traditional Tryptic Soy broth medium for the cultivation of Escherichia coli in laboratory.

Check out the Preparation of MacConkey Agar medium in Laboratory

PREPARATION OF MACCONKEY AGAR MEDIUM (MAC) IN LABORATORY

CULTURE CHARACTERISTICS OF ESCHERICHIA COLI (E. COLI)

 

Cultural CharacteristicsNutrient Agar Medium (NAM)Eosin Methylene Blue (EMB) Agar mediumMacConkey Agar mediumBlood Agar Medium
ShapeCircularCircularCircularCircular
Size1-3 mm2-3 mm2-3 mm1-3 mm
ElevationConvexConvexConvexConvex
SurfaceSmooth (fresh isolation) ; Rough (repeated subculture) ; mucoid (capsulated strains)Smooth (fresh isolation) ; Rough (repeated subculture) ; mucoid (capsulated strains)Smooth (fresh isolation) ; Rough (repeated subculture) ; mucoid (capsulated strains)Smooth (fresh isolation) ; Rough (repeated subculture) ; mucoid (capsulated strains)
ColorGreyish whiteGreen metallic sheenPinkGreyish white
StructureTranslucent –OpaqueOpaqueOpaqueTranslucent –Opaque
Hemolysis---------------β-Hemolysis (in some strains)
EmulsifiabilitySmooth form – Easily emulsifiable; Rough forms – Autoagglutinable, hence do not emulsify easilySmooth form – Easily emulsifiable; Rough forms – Autoagglutinable, hence do not emulsify easilySmooth form – Easily emulsifiable; Rough forms – Autoagglutinable, hence do not emulsify easilySmooth form – Easily emulsifiable; Rough forms – Autoagglutinable, hence do not emulsify easily

In liquid culture media like Trypticase soy broth or Nutrient broth, the growth of the bacterium occurs as a turbidity in the broth medium with a heavy deposits that disperses in the medium on shaking, which is further analyzed for the morphology (under the microscope), gram reaction, biochemical tests, and Escherichia coli specific tests.

In Blood Agar medium, some of the strains show beta hemolysis, especially which are isolated from the pathologic conditions whereas those which are isolated from normal persons may or may not show hemolysis on blood agar.

escherichia coli on blood agar medium - e. coli on blood agar medium - escherichia coli on blood agar
COLONY OF ESCHERICHIA COLI ON BLOOD AGAR MEDIUM

In MacConkey Agar medium, the colonies of Escherichia coli are pink colored due to the lactose fermentation which is of great importance in differentiating E. coli from other Bacteria present in the specimen, especially from Gram-positive bacteria and Salmonella species which are non–lactose fermentors and gives colorless colonies on MacConkey agar medium.

escherichia coli on macconkey agar medium - e.coli on macconkey agar - e coli on macconkey agar
COLONY OF ESCHERICHIA COLI ON MACCONKEY AGAR MEDIUM

In Eosin Methylene Blue Agar (EMB) medium, the colonies of Escherichia coli grows with a green metallic sheen which is due to the Metachromatic property of dyes (Eosin & Methylene blue) and the lactose fermenting property of E. coli which changes the pH of the medium to Acidic. Hence making the medium more selective for E. coli and made the identification much easier.

ESCHERICHIA COLI CULTURE ON EOSIN METHYLENE BLUE AGAR (EMB) MEDIUM

That’s All about the Morphology & Culture Characteristics of Escherichia coli (E. Coli)



Further Reading:

Escherichia coli – Microbe wiki

E. coli – Wikipedia

Escherichia coli – Textbook of Bacteriology

E. coli – Healthline

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