MICROBIOLOGY
as significant contributors to STEC- related illnesses. These strains produce Shiga toxins and have been associated with outbreaks linked to raw vegetables, sprouts, and dairy products.12
Unlike
O157:H7, non-O157 strains are often underdiagnosed due to the limitations of traditional diagnostic methods. This underscores the need for advanced diagnostics to accurately identify these pathogens and mitigate their impact. Laboratories are reporting significant rises in non-O157 STEC infections causing food-borne illnesses globally. The increased recognition of these strains and their economic impact due to food recalls highlights the need for enhanced diagnostic and surveillance systems.
n EIEC – the dysentery-like pathogen Enteroinvasive E. coli (EIEC) shares similarities with Shigella spp. in its ability to invade and damage intestinal epithelial cells. EIEC infections are typically associated with poor sanitation and contaminated food or water, leading to dysentery-like symptoms.
EIEC invades intestinal epithelial cells using its type III secretion system, which allows the bacteria to enter host cells, replicate intracellularly, and spread to adjacent cells. This process triggers inflammation, ulceration, and significant destruction of the intestinal mucosa. Clinically, EIEC infections present as bloody diarrhoea accompanied by fever, abdominal cramps, and urgency to defaecate. The disease can persist for several days, particularly in individuals with weakened immune systems, posing a significant challenge in resource-limited settings.
n EAEC – persistent diarrhoea in vulnerable populations
Enteroaggregative E. coli (EAEC) is 5% 6% 1%
2% 6%
Uncomplicated UTI 11%
2% 2%
8% Complicated UTI 3%1%1% 75%
characterised by its ability to adhere to the intestinal mucosa in a stacked- brick formation, forming biofilm-like aggregates. EAEC is increasingly recognised as a cause of persistent diarrhoea in children, travellers, and immunocompromised individuals. Pathogenesis is mediated
by aggregative adherence fimbriae (AAF), which allow EAEC to form thick biofilms on the intestinal surfaces. The bacterium also produces cytotoxins and enterotoxins that damage the mucosal barrier and trigger inflammatory responses.
The clinical presentation of EAEC includes watery diarrhoea, often containing mucus, which can persist for weeks. Prolonged infections are particularly concerning in malnourished children, as they can lead to significant weight loss, nutrient deficiencies, and impaired growth.
n UPEC – a leading cause of UTIs Uropathogenic E. coli (UPEC) is an opportunistic pathogen from the gut microbiota and is the predominant cause of Gram-negative urinary tract infections (UTIs), accounting for up to 90% of community acquired cases. UPEC infections are particularly common in women and individuals with urinary catheters or underlying conditions. UPEC utilises a range of virulence factors to colonise and infect the urinary tract: n Adhesins: type 1 fimbriae and P fimbriae allow UPEC to adhere to and invade the urinary epithelium.
n Toxins: haemolysin and cytotoxic necrotising factor (CNF) contribute to tissue damage and inflammation.
n Iron acquisition systems: UPEC produces siderophores to acquire iron in the nutrient-limited urinary tract.
2% 3% 7% 65%
Clinically, UPEC infections range from mild cystitis to severe pyelonephritis. Symptoms include dysuria, haematuria, fever, and flank pain. If left untreated, UPEC can cause bacteraemia and sepsis, particularly in vulnerable populations.
n NMEC – risk to newborns Neonatal meningitis E. coli (NMEC) is a leading cause of meningitis in neonates, often acquired during childbirth. The virulence of NMEC is largely attributed to its K1 capsule, a polysaccharide structure that helps the bacterium evade immune defences and cross the blood-brain barrier.
Once in the cerebrospinal fluid, NMEC
produces siderophores to acquire iron, facilitating its survival and proliferation. Clinical symptoms include fever, irritability, lethargy, and seizures. If untreated, NMEC can cause severe neurological complications, including brain damage and ultimately death, underscoring the importance of early diagnosis and intervention.
Pathogenicity of E. coli Table 1 and the above text, describing the opportunistic and pathogenic strains of E. coli associated with human and animal disease, demonstrates the remarkable diversity in their mechanisms of infection, ranging from toxin production to intracellular invasion. These strains are responsible for significant morbidity and mortality worldwide, particularly in vulnerable populations such as children, the elderly, and immunocompromised individuals.
Understanding the pathogenesis
of each E. coli pathotype is essential for developing targeted therapies, improving diagnostics, and implementing preventive measures. As global food safety, water quality, and healthcare
Uropathogenic E. coli K. pneumoniae P. mirabilis P. aeruginosa
S. saprophyticus Enterococcus spp. GBS S. aureus
Candida spp. Fig 1. Causes of uncomplicated and complicated UTI infections.13 24 FEBRUARY 2025
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Gram
negative bacteria
Gram positive bacteria
fungi
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