E. coli: Introduction, Identification Features, Keynotes, and Escherichia coli Footages

Introduction of E. coli

Escherichia coli is a member of the family, Enterobacteriaceae and it is a Gram-negative, aerobe, and facultative anaerobic, rod-shaped bacterium. The optimal temperature for growth is 36-37°C with most strains growing over the range 18-44 °C. It is a commensal that is found inhabiting the lower intestine of our body. A small proportion of E. coli strains are pathogenic.  The harmless strains produce vitamin K and prevent the colonization of the intestine by pathogenic bacteria. It is classified into serotypes based on cell wall (O), capsular (K), fimbria (F), and flagellar (H) antigens. e.g.  E. coli O157:H7

Pathogenic strains

  1. Enterotoxigenic Escherichia coli (ETEC)
  2. Enteropathogenic E.coli (EPEC )
  3. Enteroinvasive E. coli (EIEC)
  4. Enterohaemorrhagic E. coli (EHEC)
  5. Enteroaggregative E. coli ((EAEC))

Identification Features of E. coli

Biochemical reactions of E. coli in TSI, SIM, urea and citrate agar
Fig. Biochemical reactions of Escherichia coli in TSI, SIM, urea, and citrate agar
Basic FeaturesProperties
1. Gram StainingGram-Negative Rods (GNRs)
2. SporeNon-Sporing
3. CapsuleCapsulated (many isolates)
4. MotilityMotile
5. PigmentNegative
6. Growth in potassium cyanide (KCN) mediumNegative
7. Catalase testPositive
8. Oxidase testNegative
9. Nitrate reduction testPositive
10. MR (Methyl Red) testPositive
11. VP (Voges- Proskauer) assayNegative
12. OF (Oxidative-Fermentative) testFermentative
13. Gas from GlucosePositive
14. H2S productionNegative
15. Indole formationPositive
16. Urease/ urea hydrolysis testNegative
17. Citrate/ citrate utilizationNegative
18. DNase testNegative
19. Glucose fermentationPositive 
20. Maltose fermentationPositive
21. Lactose fermentationPositive
22. Sucrose fermentationNegative
23. Xylose fermentationPositive
24. Mannitol fermentationVariable
25. Acetate UtilizationPositive
26. ONPG (β-galactosidase)Positive
27. Phenylalanine Deaminase (PDA)/PPA TestNegative
28. Lipase testNegative
29. Esculin Hydrolysis testVariable
30. Lysine Decarboxylase TestPositive
31. Ornithine Decarboxylase TestVariable
32. Arginine Dihydrolase TestNegative
Table: Properties of E. coli

Escherichia coli Footages

Gram negative rods (GNRs) of Escherichia coli in Gram staining of culture

Gram negative rods (GNRs) of Escherichia coli in Gram staining of culture
Fig. Gram-negative rods (GNRs) of Escherichia coli in Gram staining of culture

E. coli growth on MacConkey medium

E. coli growth on MacConkey medium
Fig. Escherichia coli growth on MacConkey medium

E. coli colony morphology on blood agar

E. coli colony morphology on blood agar
Fig. Escherichia coli colony morphology on blood agar

E. coli Colony Characteristics on Chocolate Agar

E. coli Colony Characteristics on Chocolate Agar
Fig. Escherichia coli Colony Characteristics on Chocolate Agar

Indole positive Escherichia coli in MIU medium

Indole positive E. coli in MIU medium
Fig. Indole positive Escherichia coli in MIU medium

Indole positive Escherichia coli in peptone water

Indole positive E. coli in peptone water
Fig. Indole positive Escherichia coli in peptone water

Escherichia coli metallic green sheen on eosin methylene blue (EMB) agar demonstration

Escherichia coli metallic green sheen on eosin methylene blue agar (EMB) agar demonstration
Fig. Escherichia coli metallic green sheen on eosin methylene blue (EMB) agar demonstration

Sorbitol MacConkey Agar (SMAC) expressing sorbitol fermenting colony of Escherichia coli and the non-sorbitol fermenting colony of Escherichia coli

Sorbitol MacConkey Agar (SMAC) expressing sorbitol fermenting colony of E.coli and non-sorbitol fermenting colony of E.coli
Fig. Sorbitol MacConkey Agar (SMAC) is expressing sorbitol fermenting colony of E.coli (fecal flora) and the non-sorbitol fermenting colony of Escherichia coli (pathogenic strain) that may be E. coli O157:H7 strain.

Escherichia coli on CLED agar

Escherichia coli on CLED agar
Fig. Escherichia coli on CLED agar of urine sample showing pink colony

Further Readings

  • https://www.sciencedirect.com/science/article/pii/B97801241584670000
  • https://universe84a.com/sorbitol-macconkey-agar-smac-introduction/
  • Cowan & Steel’s Manual for identification of Medical Bacteria. Editors: G.I. Barron & R.K. Felthani, 3rd ed 1993, Publisher Cambridge University Press.
  • Bailey & Scott’s Diagnostic Microbiology. Editors: Bettey A. Forbes, Daniel F. Sahm & Alice S. Weissfeld, 12th ed 2007, Publisher Elsevier.
  • Mackie and Mc Cartney Practical Medical Microbiology. Editors: J.G. Colle, A.G. Fraser, B.P. Marmion, A. Simmous, 4th ed, Publisher Churchill Living Stone, New York, Melborne, Sans Franscisco 1996.
  • Textbook of Diagnostic Microbiology. Editors: Connie R. Mahon, Donald G. Lehman & George Manuselis, 3rd edition2007, Publisher Elsevier.

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