MICROBIOLOGY

• Aerobic, Gram-negative (GN) rod, E. coli is a member of Enterobacteriaceae: human strains may be: (1) commensal bowel flora; (2) intestinal pathogenic (enteric/diarrheagenic); (3) extra-intestinal pathogenic.
  • The predominant Gram-negative in the composition of normal human colonic flora.
    • Motile, flagellated, non-spore-forming.
    • Upon Gram staining, they tend to be plump rods [Fig]
• E. coli is easy to grow from sterile specimens.
  
  • Stool cx: only if severe diarrhea (may need reference lab to ID) or suspect O157:H7 (cx all bloody diarrhea) use sorbitol-MacConkey agar or perform Shiga toxin EIA.
  • ~ 90% of strains ferment lactose (i.e., lactose-fermenter, D-glucose fermentation produces mixed acids that trigger an indicator such as methyl red)
    • Some diarrheagenic E. coli strains, including many of the EIEC strains, are typically lactose-negative.
    • Indole testing in all E. coli ~ 99% (+).
    • Catalase positive, oxidase negative.
• Major resistance concerns:
  • Plasmid-mediated AmpC β-lactamases (e.g., CMY)
  • Extended-spectrum β-lactamases (e.g., CTX-M, TEM and SHV β-lactamases)
    • A worldwide concern, including community-acquired UTIs and also bloodstream infections
      • In some Asian countries, rates of ESBL E. coli have been reported to be as high as 55-79%.
      • Rates are lower in North America but rising.
    • Most labs judge the presence of ESBL through ceftriaxone non-susceptibility, MIC ≥2 μg/mL.
  • Carbapenemases (e.g., New Delhi metallo-β-lactamase, Klebsiella pneumonaie carbapenemase and OXA-48)
    • Less common than ESBL.