Escherichia coli (E. coli) is a member of the Enterobacteriaceae family and is part of the normal gut flora of humans and animals.
While most strains are harmless commensals, certain strains are important pathogens in the urinary tract, gastrointestinal tract, and bloodstream.

🔬 Characteristics

Gram-negative rods, aerobic and facultatively anaerobic.
Ferments glucose, oxidase-negative, catalase-positive.
Reduces nitrates → nitrites (helpful for dipstick UTI diagnosis).
Lactose-fermenter → pink colonies on MacConkey agar.

⚔️ Virulence Factors

P fimbriae & Type 1 pili – allow adhesion to urothelial surfaces.
Lipopolysaccharides (LPS) – trigger cytokine release, septic shock, and DIC.
K1 capsule – associated with neonatal meningitis and sepsis.
Enterotoxins – heat-labile (↑cAMP) and heat-stable (↑cGMP) in ETEC diarrhoea.

🌍 Source

Normal human intestinal flora; also found in animals (esp. cattle).
Transmission: faeco-oral spread via contaminated meat, water, or direct person-to-person contact.

🧪 Other Actions

Produces Vitamin K in the colon – important for coagulation. Broad-spectrum antibiotic use may suppress this, occasionally contributing to bleeding risks.

🩺 Pathogenicity

Cystitis: Dysuria, frequency, urgency. Women at higher risk. 💡 Advice: void urine post-intercourse to reduce risk.
Pyelonephritis: Flank pain, fever, rigors. Risk of Gram-negative sepsis if untreated.
ETEC (Traveller’s diarrhoea): Watery diarrhoea via heat-labile and heat-stable enterotoxins.
EPEC: Causes chronic infantile diarrhoea → malnutrition in low-resource settings.
EHEC (O157:H7): Shiga-like toxin → bloody diarrhoea. 🚨 Complication: Haemolytic Uraemic Syndrome (HUS) → triad of AKI, thrombocytopenia, haemolytic anaemia. Antibiotics contraindicated.
EIEC: Dysentery-like illness with fever, cramps, and bloody diarrhoea (invades colonic mucosa).
Neonatal sepsis/meningitis: K1 strains a major cause of neonatal morbidity and mortality.

🔎 Investigations

Urine dipstick: Nitrites + leukocyte esterase.
Urine culture: ≥10³ CFU/mL significant in symptomatic UTI.
Stool culture: Detects EHEC (O157:H7) in suspected bloody diarrhoea outbreaks.
Blood cultures: Needed in suspected sepsis.

🧬 Resistance

Beta-lactamase production → resistance to penicillin and ampicillin.
ESBL (Extended Spectrum Beta-Lactamases) and carbapenemase-producing E. coli (CPE) are rising threats in UK hospitals.

💊 Sensitivities

Trimethoprim (but resistance increasing in UK, esp. community UTIs).
Cephalosporins (3rd generation e.g. cefotaxime, ceftriaxone).
Ciprofloxacin and aminoglycosides (gentamicin) – hospital use, guided by sensitivities.

🩺 Management

Uncomplicated UTI: Oral nitrofurantoin (first-line in UK), or trimethoprim (if low resistance risk).
Pyelonephritis/sepsis: IV cephalosporin or aminoglycoside, escalate if ESBL suspected.
EHEC: 🚨 No antibiotics – risk of HUS. Supportive fluids & monitoring renal function.
HUS: Involve renal/ID team early. May require dialysis, plasma exchange, or intensive care.

📌 Key Facts

Most common cause of community-acquired UTIs worldwide.
E. coli O157:H7 linked to undercooked beef, unpasteurised milk, contaminated salad/veg.
Exam pearl: “Child with bloody diarrhoea → develops oliguria, thrombocytopenia, anaemia” → think HUS from EHEC.

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Escherichia coli (E. coli)

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