Escherichia coli

Mr. Parag Chauhan

Escherichia coli is a versatile bacterium with significant clinical importance. While many strains are harmless and part of the normal gut flora, pathogenic strains can cause serious infections, ranging from gastrointestinal illness to urinary tract infections. Understanding its characteristics, pathogenic mechanisms, and effective laboratory identification is crucial for managing E. coli-related diseases. Continuous surveillance and research into antibiotic resistance are necessary to address the challenges posed by this bacterium.

General Character

  • Genus: Escherichia
  • Species: Escherichia coli (E. coli)
  • Family: Enterobacteriaceae
  • Gram Staining: E. coli is a Gram-negative bacterium, appearing pink due to its thin peptidoglycan layer and outer membrane.
  • Shape and Arrangement:
    • Shape: E. coli are rod-shaped (bacilli).
    • Arrangement: Typically found as single cells, but can also occur in pairs or chains.
  • Oxygen Requirements: E. coli is a facultative anaerobe, capable of growing in aerobic and anaerobic conditions.

Morphology

  • Cell Wall Structure:
    • It comprises a thin layer of peptidoglycan, surrounded by an outer membrane containing lipopolysaccharides (LPS), contributing to its pathogenicity and immune evasion.
  • Flagella:
    • Many strains possess flagella, which provide motility, allowing E. coli to move toward favourable environments (chemotaxis).
  • Capsule: Some strains have a polysaccharide capsule that helps protect against phagocytosis.

Cultural Characteristics

  • Growth Media

    • MacConkey Agar: Selective and differential medium that inhibits Gram-positive bacteria; E. coli ferments lactose, producing pink colonies.
    • Blood Agar: Supports growth and allows observation of hemolytic activity.
    • XLD Agar (Xylose Lysine Deoxycholate): Useful for distinguishing E. coli from other enteric pathogens.
  • Colony Appearance
    • On MacConkey agar, E. coli typically produces pink colonies due to lactose fermentation.
  • Temperature and pH Range
    • The optimal growth temperature is around 37°C, with a pH range of 6.0 to 7.5 being favourable.

Biochemical Reactions

  • Catalase Test: E. coli is catalase-positive, producing bubbles when added hydrogen peroxide.
  • Oxidase Test: E. coli is oxidase-negative, distinguishing it from other Gram-negative bacteria.
  • Lactose Fermentation: E. coli ferments lactose, producing acid and gas, which can be detected on lactose-containing media.
  • Indole Production: Most E. coli strains are indole-positive, indicating the ability to convert tryptophan into indole.
  • Methyl Red Test: Positive, indicating mixed acid fermentation.
  • Voges-Proskauer Test: Negative, indicating the absence of acetoin production.

Pathogenicity

  • Virulence Factors

    • Adhesins: Enable adherence to the intestinal epithelium, facilitating colonization.
    • Toxins:
      • Shiga Toxin (produced by E. coli O157): Can cause severe gastrointestinal disease and hemolytic uremic syndrome (HUS).
      • Enterotoxins (e.g., heat-labile and heat-stable toxins) can cause diarrhoea, especially in enterotoxigenic E. coli (ETEC).
    • Capsule and Biofilm Formation: Help protect against host immune responses.

  • Clinical Infections

    • Gastroenteritis: Caused by pathogenic strains such as enterotoxigenic E. coli (ETEC), enteropathogenic E. coli (EPEC), and enterohemorrhagic E. coli (EHEC).
    • Urinary Tract Infections (UTIs): Uropathogenic E. coli (UPEC) is a leading cause of UTIs.
    • Sepsis: This can occur in immunocompromised patients.
    • Meningitis: Particularly in neonates, caused by certain serotypes.

 

Laboratory Diagnosis

  • Specimen Collection: Commonly involves stool samples for gastroenteritis or urine samples for UTIs.
  • Microscopic Examination:
    • Gram staining reveals Gram-negative bacilli.
  • Culture Techniques:
    • Inoculation on selective media like MacConkey or XLD agar, followed by incubation.
  • Biochemical Testing:
    • Confirmatory tests for lactose fermentation, indole production, and other biochemical reactions to identify E. coli.
  • Molecular Methods: PCR and other nucleic acid amplification techniques can identify specific pathogenic strains, especially in outbreaks.

 

Antibiotic Resistance

  • Emergence of Resistance: E. coli has shown increasing resistance to multiple antibiotics, including penicillins and fluoroquinolones.
  • Multidrug-Resistant Strains: Especially concerning hospital-acquired infections and among strains producing extended-spectrum beta-lactamases (ESBLs) or carbapenemases.
  • Treatment Options: The selection of antibiotics based on susceptibility testing is crucial due to varying resistance patterns.

 

Prevention

  • Hygiene Practices: Proper handwashing and food safety measures (cooking food thoroughly, avoiding cross-contamination) are vital to prevent E. coli infections.
  • Vaccination: Currently, no vaccines exist for E. coli, but research is ongoing, particularly for pathogenic strains like EHEC.
  • Surveillance: Monitoring food sources and outbreaks can help manage risks associated with E. coli infections.