Bacteriology-Escherichia coli | Quick Revision Notes for NEET-PG

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  1. Gram-negative rods, non-capsulated(some capsulated-pathogenic)motile by peritrichous flagella with 80% strains possessing flagella
  2. Commensals in the gut(MC-Bacteroides fragilis-Anaerobic)
  3. Gut flora- anaerobes v/s aerobs – 1000:1
  4. Indole +ve
  5. MethylRed+ve
  6. VP(Voges Praus) test -ve
  7. Citrate test -ve
  8. Ferments glucose-produce acid and gas

Diarrhoeagenic E. Coli – 6 types

  1. EPEC(Enteropathogenic E. coli)
  2. ETEC(Enterotoxigenic E. coli)
  3. EIEC(Enteroinvasive E. coli)
  4. EHEC(Enterohaemorrhagic E. coli)
  5. EAEC(Enteroaggregative E. Coli)
  6. DAEC(Diffusely-adherent E. Coli)


  • Attach to brush border cells of enterocytes by bundle forming pilli(plasmid coded) & disrupt brush border microvilli leading to a decrease in surface area of absorption resulting in diarrhea
  • Lesions-A/E type (attaching and effacing type)
  • Gene responsible for the production of A/E lesion- eae i.e. Escherichia coli  attaching and effacing gene
  • Non-toxigenic
  • Non-invasive
  • Does not ferment ribitol
  • Cause diarrhea in infants and children and sporadic diarrhea in adults


  • The most common cause of traveler’s diarrhea
  • They attach to enterocytes with the help of colonization factor antigen/CFA
  • It produces 2 types of toxins
    1. Heat-labile toxin/LT -acts by increasing cAMP levels(More antigenic-same as cholera toxin)
    2. Heat stable toxin/ST- acts by increasing cGMP levels leading to increased secretion of Na+, K+, HCO3into the lumen
  • Non-invasive
  • Toxigenic


  • It invades the enterocytes with the help of VMA (Virulence marker antigen-plasmid mediated) and causes dysentery followed by diarrhea.
  • Locally invasive(so blood culture is not possible)
  • Non-pathogenic
  • It is aka atypical E. coli due to 3 reasons
    1. Non-motile
    2. Non-lactose fermentors
    3. Ferments glucose with the production of acid but no gas(anaerobic)

 These features are possessed by Shigella and it causes a disease similar to EIEC

  • Test for invasiveness is done by Sereny’s test
    • We inject culture suspension filled with EIEC into the lower conjunctival sac of guinea pig and rabbits
    • A positive test is indicated by severe mucopurulent keratoconjunctivitis


  • It produces VTEC(verocytotoxin)/shiga-like toxin which inhibits 28s subunit of 60s ribosomes leading to inhibition of protein synthesis. Organisms producing Shiga  like toxin are known as STEC (Shiga-like toxin E. coli)
  • Disease caused by EHEC
    1. HC (Haemorrhagic colitis)
    2. HUS (hemorrhagic uremic syndrome).It is characterized by
      1. Microangiopathic hemolytic anemia
      2. Thrombocytopenia
      3. Renal failure
      4. encephalopathy
  • Food implicated in this-beef hamburger


  • It causes diarrhea in developing countries
  • It produces EAST 1(enetro-aggregative heat-stable enterotoxin 1) toxin
  • The exact mechanism of action is not known
  • It shows ‘stacked brick appearance’.


E. coli- Most common cause of UTI(urinary tract infection). It causes 60% of community-acquired UTI and 50% of hospital-acquired UTI

Most common nosocomial infection-UTI

Most common nosocomial pathogen-E. coli

Diagnosis-Samples of be collected-

  1. Best specimen –suprapubic aspirate
  2. Most common specimen- early morning Midstream urine

Kass concept of significant bacteriuria- A count of 105 CFU/ml (colony forming units) or more is indicative of significant bacteriuria.

Culture media for urine culture (Both the media are negative for swarming by Proteus and both differentiate between lactose fermentors and non-lactose fermentors)

  1. CLED agar(Cysteine lactose electrolyte deficient )-better than mc Conkey’s agar because
    •  if allows the growth of G+ cocci and candida but Mc Conkey does not allow their growth
    • Electrolyte deficiency in this media is responsible for no swarming by Proteus
    • Pigmentation of Pseudomonas aeriginosa is better appreciated on CLED agar
  2. Mc Conkey’s agar-Ferments lactose and produce pink colonies


  1. Never treat asymptomatic significant bacteriuria because resistance to antibiotics develop except in conditions like
    1. Pregnancy
    2. Urinary tract obstruction due to caliculi/malignancy
    3. Neutropenia

E. coli- causes neonatal septicemia and neonatal meningitis(such strains are capsulated and possess K‘/capsular antigen)

E. coli is the Most common cause of-

  1. Intra-abdominal infection
  2. Peritonitis
  3. Cholangitis
  4. Septicemia in old people(focal of infection being urinary tract)
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