Escherichia coli

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Background
Pathogenesis

Background

Escherichia coli (E. coli) is a Gram-negative, rod-shaped, facultative anaerobic bacterium. It is a member of the Enterobacteriaceae family and a component of the normal gut flora in humans and animals. It is oxidase-negative, lactose-fermenting, and some strains produce toxins, including exotoxins and endotoxins. Many strains are harmless, but pathogenic variants are significant causes of both community-acquired and healthcare-associated infections.

E. coli infections lead to a variety of diseases in humans including:

Diarrhoeal illnesses
UTIs
Neonatal meningitis

Scanning electron micrograph of Escherichia coli, grown in culture and adhered to a cover slip. Credit: NIAID

Transmission
E. coli is transmitted through various routes depending on the clinical syndrome:

Faecal-oral transmission is common for enteric disease, typically through ingestion of contaminated food or water.
Peripartum exposure during childbirth is the main transmission route for neonatal infections.
Endogenous spread from gut flora to the urinary tract or surgical sites is typical for UTIs, wound infections, and intra-abdominal sepsis.
Nosocomial transmission is particularly relevant in intensive care units and catheterised patients.

Serotypes
E. coli may be classified according to the expressed antigens which may trigger an immune response:

Antigen	Origin	Notes
O	Lipopolysaccharide layer
K	Capsule	Neonatal meningitis secondary to E. coli is usually caused by a serotype that contains the capsular antigen K-1
H	Flagellin

E. coli O157:H7 is a particular strain associated with severe, haemorrhagic, watery diarrhoea. It has a high mortality rate and can be complicated by haemolytic uraemic syndrome. It is often spread by contaminated ground beef.

Pathogenesis

E. coli has a variety of virulence factors that contribute to its pathogenicity across different systems:

Pili (fimbriae): Adhesion to mucosal surfaces, particularly uropathogenic strains that target urinary tract epithelium (e.g. P fimbriae).
Capsule (K antigen): Protects against phagocytosis, particularly important in neonatal meningitis strains.
Endotoxins (lipopolysaccharide): Induce systemic inflammatory responses and septic shock.
Exotoxins: Various toxins including heat-stable and heat-labile enterotoxins (ETEC), Shiga toxin (STEC/EHEC), and cytotoxins (EIEC).
Flagella: Confer motility, aiding ascent through the urinary tract.

E Coli virulence in UTIs
Uropathogenic E. coli (UPEC) strains possess adaptations that enhance their ability to cause infection:

Adhesins: Pili or fimbriae (especially P fimbriae) enable attachment to uroepithelial cells, avoiding urinary flushing.
Motility: Flagella assist in ascending infection from urethra to bladder and kidneys.
Siderophores: Sequester iron to support bacterial growth in the host environment.
Toxins: May damage uroepithelial cells and contribute to inflammation and symptoms.

Sites of infection and clinical syndromes
E. coli causes a wide range of infections, which can be broadly categorised into enteric, urinary, CNS, and systemic infections:

Site	Details
Urinary Tract Infections (UTIs)	The most common cause of uncomplicated UTIs in adults (70–95% of cases) Usually from endogenous gut flora. Also a leading cause of catheter-associated UTIs Uropathogenic strains may ascend to cause pyelonephritis
Central Nervous System Infections	E.coli is a major cause of neonatal meningitis and septicaemia (more common in preterm or low-birth-weight infants) Infection is typically acquired during labour. Post-operative meningitis can occur following neurosurgery, especially when prosthetic devices (e.g. shunts) are used
Gastrointestinal Disease	Several pathotypes cause diarrhoea: ETEC (enterotoxigenic E. coli): major cause of traveller’s diarrhoea. EHEC/STEC (e.g. E. coli O157:H7): Bloody diarrhoea; produces Shiga toxin (associated with haemolytic uraemic syndrome (HUS), see below) EIEC (enteroinvasive): Causes dysentery-like illness. EPEC (enteropathogenic): Important in infantile diarrhoea in developing countries
Other Infections	Wound infections: especially post-operative or following trauma Intra-abdominal sepsis: e.g. peritonitis, appendicitis (often part of polymicrobial infectionn) Neonatal and hospital-acquired pneumonia Bacteraemia (septicaemia): particularly in immunocompromised, elderly, or catheterised patients

Treatment
Treatment depends on the site and severity of infection and local antimicrobial resistance patterns.

Site	Treatment
Uncomplicated UTIs	First-line therapy includes: Nitrofurantoin Trimethoprim Pivmecillinam (pending resistance data)
Pyelonephritis or complicated infections	First-line therapy includes: Cefalexin Co-amoxiclav Trimethoprim Ciprofloxacin
Neonatal meningitis	Empirical treatment options include: IV Benzylpenicillin and gentamicin IV Cefotaxime and gentamicin IV Cefotaxime or ceftriaxone and ampicillin
Diarrhoeal illness	Usually supportive; antibiotics are generally not indicated except in invasive disease. Antibiotics avoided in EHEC infections due to risk of precipitating HUS