MICROBIOLOGY


Pathogenic Strain Enterotoxigenic


E. coli (ETEC)


Enteropathogenic E. coli (EPEC)


Key Features


Watery diarrhoea, common in travellers


Affects infants, forms attaching lesions


Shiga Toxin-Producing Severe colitis, E. coli (STEC)


HUS risk


Enteroinvasive E. coli (EIEC)


Enteroaggregative E. coli (EAEC)


Uropathogenic E. coli (UPEC)


Neonatal meningitis E. coli (NMEC)


Dysentery-like illness


Persistent diarrhoea, biofilm formation


Leading cause of UTIs


Virulence Factors


Heat-Labile Toxin (LT), Heat-Stable Toxin (ST)


Bundle-Forming Pili (BFP), Intimin, Haemolysins Type III Secretion System


Aggregative Adherence Fimbriae (AAF), Cytotoxins


Type 1/P Fimbriae, Haemolysin, Siderophores


Affects newborns, crosses K1 Capsule, blood-brain barrier


Siderophores Table 1. Pathogenic strains of Escherichia coli and their characteristics.


setting, E. coli is extensively used as an indicator organism revealing faecal contamination of water, food, and the environment.5


of chloride ions and water into the intestinal lumen.


Emerging research also


highlights E. coli’s probiotic potential, with certain engineered strains shown to reduce inflammation, combat infections, and deliver therapeutic compounds; thus making E. coli a promising candidate for therapeutic interventions in gut dysbiosis and other inflammatory conditions.6


Pathogenic strains and human health


While many E. coli strains are harmless commensals that coexist with humans, certain pathogenic strains have evolved sophisticated mechanisms to cause a wide variety of diseases. These strains, collectively known as E. coli pathotypes, are defined by their virulence factors, which enable them to adhere, invade, evade immune responses, and disrupt host tissues.7


Understanding the


pathogenic potential of E. coli is essential to develop effective prevention and treatment strategies, particularly in the context of global health.8


Table 1


briefly describes the key aspects of each pathogenic group, and the text following provides a more detailed account.8


n ETEC – travellers’ diarrhoea Enterotoxigenic E. coli (ETEC) is a major cause of diarrhoea, particularly in travellers to developing regions and children living in low-resource settings. ETEC infections are often associated with poor sanitation and contaminated water supplies. The virulence of ETEC is largely attributed to its production of two toxins: n Heat-Labile Toxin (LT): LT resembles cholera toxin and activates adenylate cyclase in intestinal epithelial cells. This causes an overproduction of cyclic AMP, leading to the secretion


n Heat-Stable Toxin (ST): ST activates guanylate cyclase, increasing cyclic GMP levels. This disrupts sodium and water absorption, compounding the dehydration caused by LT.


Clinically, ETEC infections are characterised by watery, non-bloody diarrhoea, often accompanied by abdominal cramps and nausea. While most cases are self limiting, severe dehydration and electrolyte imbalances can occur, particularly in young children and the elderly, posing significant health risks.


n EPEC – a threat to infants Enteropathogenic E. coli (EPEC) primarily affects infants and young children, particularly in developing countries.9 EPEC infections are associated with chronic, watery diarrhoea that can lead to malnutrition and developmental delays in children.


The pathogenic mechanism of EPEC involves a unique ability to damage the intestinal epithelium. Using its bundle- forming pili (BFP), EPEC adheres tightly to the enterocytes of the small intestine. It then deploys its type-III secretion system (T3SS) to inject effector proteins into host cells, leading to the formation of attaching and eroding lesions. These lesions are marked by: destruction of microvilli (the loss of the absorptive surface of the intestine which impairs nutrient absorption) and cytoskeletal rearrangement (changes in the host cell structure compromise its function and lead to inflammation). Clinically, EPEC causes persistent diarrhoea accompanied by vomiting and fever. In prolonged cases, malabsorption due to epithelial damage contributes to stunted growth and developmental delays, making EPEC a critical focus of paediatric health initiatives.


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n STEC – a deadly food-borne pathogen Shiga toxin-producing E. coli (STEC) are among the most dangerous E. coli pathotypes, known for causing severe outbreaks of food-borne illness. STEC includes the infamous O157:H7 serotype, as well as an increasing number of non-O157 strains, commonly referred to as the ‘big six’ strains.10


The characteristic


of STEC infections is the production of Shiga toxins (Stx1 and Stx2), which inhibit protein synthesis in host cells by targeting the 28S rRNA.11


These toxins are


responsible for the severe complications associated with STEC infections, including: n Haemorrhagic colitis: severe inflammation of the colon causing bloody diarrhoea, intense abdominal cramps, and vomiting.


n Haemolytic uraemic syndrome (HUS): In approximately 5-10% of STEC cases, particularly in children and older adults, Shiga toxins enter the bloodstream and damage endothelial cells in the kidneys, which in turn may lead to acute kidney failure, haemolytic anaemia, and thrombocytopenia. HUS is life-threatening and often requires intensive medical intervention, including dialysis.10,11 STEC infections are primarily transmitted through the consumption of undercooked meat, unpasteurised dairy, contaminated produce, and water. Direct contact with livestock, particularly cattle, also serves as a significant source of infection. Given its severity, STEC remains a focus for global food safety and public health programmes.


n Non-O157 STEC


Non-O157 STEC strains as indicated with the big six above (O26, O45, O103, O111, O121, and O145), have emerged


23


Clinical Manifestations Watery, non-bloody


diarrhoea, dehydration Watery diarrhoea,


Type III Secretion System (T3SS) malabsorption, vomiting Shiga Toxins (Stx1, Stx2),


Bloody diarrhoea, HUS (kidney failure)


Bloody diarrhoea, fever, abdominal pain


Watery diarrhoea, mucus, Contaminated food/water, weight loss


poor hygiene


Cystitis, pyelonephritis, sepsis


Meningitis, fever, lethargy, seizures


Ascending infection, person-to-person


Maternal transmission during childbirth


Transmission


Contaminated food/water, poor sanitation


Contaminated food, person-to-person


Undercooked meat, unpasteurised milk


contaminated produce, direct livestock contact


Contaminated food/water


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