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.
    • On gram stain, 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% 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, typically lactose negative.
    • Indole testing in all E. coli ~ 99% (+).
    • Catalase positive, oxidase negative.
• Major resistance concerns[16]:
  • Plasmid-mediated AmpC β-lactamases (e.g., CMY)
  • Extended-spectrum β-lactamases (e.g., CTX-M)
    • Now worldwide concern in community-acquired UTIs and also bloodstream infections
    • In some Asian countries rates of ESBL E. coli have been reported as high as 55-79%.
    • Rates are lower in North America but rising.
  • Carbapenemases (e.g., New Delhi metallo-β-lactamase, Klebsiella pneumonaie carbapenemase and OXA-48)