MORPHOLOGY OF ESCHERICHIA COLI (E. COLI)
Shape – Escherichia 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 Cells – Escherichia coli is arranged singly or in pairs.
Motility – Escherichia coli is a motile bacterium. Some strains of E. coli are non-motile.
Flagella – Escherichia 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.
CULTURE REQUIREMENTS OF ESCHERICHIA COLI (E. COLI)
⇒ Special requirements – Escherichia 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 the Laboratory.
⇒ Optimum temperature – Escherichia 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 pH – E. 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.
⇒ Oxygen requirements – Escherichia 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.
CULTURE CHARACTERISTICS OF ESCHERICHIA COLI (E. COLI)
Cultural Characteristics Nutrient Agar Medium (NAM) Eosin Methylene Blue (EMB) Agar medium MacConkey Agar medium Blood Agar Medium
Shape Circular Circular Circular Circular
Size 1-3 mm 2-3 mm 2-3 mm 1-3 mm
Elevation Convex Convex Convex Convex
Surface 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) Smooth (fresh isolation) ; Rough (repeated subculture) ; mucoid (capsulated strains)
Color Greyish white Green metallic sheen Pink Greyish white
Structure Translucent –Opaque Opaque Opaque Translucent –Opaque
Hemolysis ----- ----- ----- β-Hemolysis (in some strains)
Emulsifiability Smooth form – Easily emulsifiable; Rough forms – Autoagglutinable, hence do not emulsify easily Smooth form – Easily emulsifiable; Rough forms – Autoagglutinable, hence do not emulsify easily Smooth form – Easily emulsifiable; Rough forms – Autoagglutinable, hence do not emulsify easily Smooth 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.
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.
In Eosin Methylene Blue Agar (EMB) medium, the colonies of Escherichia coli grow 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.
That’s All about the Morphology & Culture Characteristics of Escherichia coli (E. Coli)