Escherichia coli

One-Sentence Definition

Escherichia coli is a Gram-negative rod of the Enterobacterales; common gut Normal Microbiota member and leading cause of UTI, as well as diarrheal pathotypes and invasive extraintestinal disease.

Taxonomy & Morphology

• Gram-negative bacillus; usually lactose-fermenting (MacConkey pink)
• Facultative anaerobe
• Gram Stain: pink rods

Pathotypes / Virulence

• ExPEC / UPEC: UTI, bacteremia — adhesins (P fimbriae), siderophores
• Intestinal: ETEC, EPEC, EHEC/STEC (Shiga toxin — often phage-borne Transduction), EIEC, EAEC
• LPS endotoxin → sepsis pathophysiology

Clinical Syndromes

• Cystitis, pyelonephritis, bloodstream infection
• Traveler’s diarrhea / bloody diarrhea (STEC — HUS risk)
• Neonatal meningitis (K1 strains)

Diagnosis

• Urine/blood/stool culture as appropriate
• STEC: culture + Shiga toxin immunoassay/PCR (do not just use sorbitol MacConkey alone)
• Antimicrobial Susceptibility Testing for extraintestinal isolates

Treatment Notes (conceptual)

• Syndrome- and AST-directed; avoid antibiotics in many STEC cases (HUS risk — clinical rule)
• Empiric choices depend on local ESBL rates

AMR

• ESBL (blaCTX-M etc.) and carbapenemases on Plasmids via Conjugation
• Major One Health AMR indicator organism

Related Notes

• Normal Microbiota · Klebsiella pneumoniae · MOC - Antimicrobial Resistance (AMR)
• Diseases: Acute Cystitis · Acute Pyelonephritis · Sepsis · (Community-Acquired Pneumonia less common)
• Methods: Culture and Isolation · PCR · Whole-Genome Sequencing

Learning Aids

Diagrams

• Figure - Horizontal Gene Transfer (ESBL/carbapenemase plasmids)
• Flora vs disease: think Normal Microbiota + Pathogen

Clinical Example

Example

Case: Young woman with dysuria; urine culture >10⁵ CFU/mL lactose-fermenting Gram-neg rod, AST pending.
Question: Most likely organism and why antibiotics still need AST?
Answer: Escherichia coli UTI is statistically likely; local ESBL rates mean empiric choice can fail — confirm with Antimicrobial Susceptibility Testing. Contrast with STEC diarrhea where antibiotics may be harmful.

Videos

Video	Why watch
HGT overview	How resistance genes jump into E. coli

Learning Media Hub

Active Recall

1. Why can E. coli be both flora and pathogen?
2. What complication is feared with STEC?
3. How do ESBL plasmids spread between species?