E. coli in Gram Stain: Introduction, Pathogenic Strains and Lab Diagnosis

E. coli in gram stain

E. coli in gram stain

E. coli in Gram stain showing gram-negative rods having a size of about 2.0 µm long and 0.25-1.0 diameter as shown above picture.

Introduction of Escherichia coli 

E. coli is a Gram-negative, aerobe, and facultative anaerobic, rod-shaped bacteria. The optimal temperature for growth is 36-17°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), fimbrial (F), and flagellar (H) antigens. e.g.  E. coli O157:H7

Pathogenic strains

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


  1. Enterotoxigenic E. coli: It is also known as traveler’s diarrhea. The infection leads to watery diarrhea which may last up to a week. Symptoms include abdominal cramps, sometimes nausea, and headache.  It establishes itself by adhering to the epithelium of the small intestine via colonization factor antigens. This is followed by the expression of heat-stable (ST) or heat-labile (LT) enterotoxins. These toxins increase adenylate cyclase> CAMP levels> secretion of chloride ions and water.
  2. Enteropathogenic E. coli:  Following ingestion, organisms adhere to the epithelial cells of the intestine causing watery or bloody diarrhea. Adherence is mediated by the EPEC adherence factor (EAF) and intimin- a non-fimbrial adhesin. EPEC attaches to and alters the integrity of the intestine. Bloody diarrhea is associated with attachment and an acute tissue-destructive process. EPEC does not produce toxins. Their virulence mechanism involves the formation of attaching and effacing lesions followed by interference with host cell signal transduction. This strain is most commonly associated with kids.
  3. Enteroinvasive E. coli: Transmitted through feco-oral route. Following ingestion, organisms invade epithelial cells of the intestine resulting in a mild form of dysentery. Illness is characterized by the presence of blood and mucus in the stools of infected individuals. Characteristic features of EIEC are their ability to induce entry into epithelial cells and disseminate from cell to cell. EIEC infection can occur through contaminated food or water or through mechanical factors such as flies. The genes required for entry are clustered on a virulence-associated invasion plasmid in EIEC strains.
  4. Enterohaemorrhagic E. coli: This is a strain of E. coli that produces cytotoxins that disrupt protein synthesis within host cells. These toxins are also called verocytotoxins or Shiga- like toxins. Entero-haemorrhagic E. coli are pathogenic to humans. They produce verocytotoxins that form attaching and effacing lesions on epithelial cells. Infection occurs via the feco-oral route. Symptoms range from mild diarrhea to severe bloody diarrhea. Complications include hemolytic uremic syndrome (HUS) which can lead to death if untreated. Common serotype E. coli O157:H7
  5. Enteroaggregative E. coli (EaggEC): It causes chronic watery diarrhea and vomiting, mainly in children, Due to the bacteria adhering to the tissue often in stacks.

Laboratory Diagnosis of E. coli


It depends on the site of infection and includes-

  • Urine
  • pus
  • Feces
  • CSF
  •  Blood for culture


E. coli is a Gram-negative usually motile rod. Inactive strains ( Alkascens-Dispar) are non-motile. A minority of strains have capsules.

Culture characteristics on MacConkey agar,  CLED  agar, and Sorbitol MacConkey agar

E. coli ferments lactose, producing smooth pink colonies on MacConkey agar and yellow colonies on CLED agar.

E. coli (VTEC) O157  is non-sorbitol fermenting and therefore produces colorless colonies. Most other E.  coli strains and other enterobacteria ferment sorbitol.

Biochemical test

TSI test

Acid slant/ acid butt

No H2S production

Gas formation – shown by most strains

SIM test 

Indole test: Positive

Motility: Most (most strains except A-D)

H2S: Negative

Citrate Utilization test: Negative

Urea hydrolyzation test: Negative

Lysine decarboxylase test: Positive

Reduce nitrate to nitrite

Further Readings

  1. Cowan & Steel’s Manual for identification of Medical Bacteria. Editors: G.I. Barron & R.K. Felthani, 3rd ed 1993, Publisher Cambridge University Press.
  2. Bailey & Scott’s Diagnostic Microbiology. Editors: Bettey A. Forbes, Daniel F. Sahm & Alice S. Weissfeld, 12th ed 2007, Publisher Elsevier.
  3. Clinical Microbiology Procedure Handbook, Chief in editor H.D. Isenberg, Albert Einstein College of Medicine, New York, Publisher ASM (American Society for Microbiology), Washington DC.
  4. Colour Atlas and Textbook of Diagnostic Microbiology. Editors: Koneman E.W., Allen D.D., Dowell V.R. Jr, and Sommers H.M.
  5. Jawetz, Melnick and Adelberg’s Medical Microbiology. Editors: Geo. F. Brook, Janet S. Butel & Stephen A. Morse, 21st ed 1998, Publisher Appleton & Lance, Co Stamford Connecticut.
  6. 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.
  7. Textbook of Diagnostic Microbiology. Editors: Connie R. Mahon, Donald G. Lehman & George Manuselis, 3rd edition2007, Publisher Elsevier.
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