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Characteristics: Gram-negative, bacilli, E. coli bacteria are rod-shaped cells, typically measuring between 1.0 and 2.0 micrometers in length, and approximately 0.5 micrometers in radius. Nevertheless, their size is not fixed and can vary based on the medium in which they exist. Cells that grow more rapidly tend to be larger in size,growth both-aerobic-and-anaerobic, catalase-positive, oxidase-negative, indole-positive, urease-negative, TSI no production of H2S, motility nonmotile, vancomycin-resistant, colistine-susceptible, motility-peritrichous, flagella. E. coli has various virulence factors that contribute to its pathogenicity: 1- Pili: These hair-like structures aid the bacteria in attaching to the cells of the jejunum and ileum during intestinal infections and to the urinary tract epithelium during urinary tract infections. 2- Capsule: This outer layer prevents phagocytosis (a type of immune response where cells are engulfed) and plays a significant role in enabling the bacteria to cause systemic infections. 3- Endotoxin (Lipopolysaccharide): This is a key contributor to many symptoms of gram-negative sepsis, including fever, low blood pressure, and a condition called disseminated intravascular coagulation (DIC), where small blood clots develop throughout the bloodstream. 4- Exotoxins: Among them is the enterotoxin, which targets the cells of the jejunum and ileum, resulting in diarrhea. Other exotoxins include verotoxin and Shiga-like toxin.
Description: Escherichia coli, often abbreviated as E. coli, is a type of bacteria primarily living in the lower intestines of warm-blooded organisms, including humans. Generally, E. coli is harmless and contributes to a healthy gut flora, even aiding in vitamin K production. However, some strains can cause serious foodborne illnesses. E. coli is a Gram-negative, rod-shaped bacterium, and it is facultatively anaerobic, meaning it can survive in both oxygen-rich and oxygen-poor environments. Most infections occur when an individual s defenses are compromised, allowing normally benign E. coli to establish infection. Certain E. coli strains have become harmful by gaining elements that increase their virulence, which can be acquired through mechanisms like plasmids, transposons, bacteriophages, or pathogenicity islands. There are four categories of pathogenic intestinal E. coli, classified based on the mechanism by which they cause diarrhea. These include: enterohemorrhagic (EHEC) or verotoxigenic (VTEC), enterotoxigenic (ETEC), enteropathogenic (EPEC), and enteroinvasive (EIEC) or enteroaggregative (EAEC).
Manifestation: E. coli is the predominant cause of urinary tract infections (UTIs), both within the community and healthcare settings, accounting for more than 75% of all UTI cases. It is also the second most common cause of sepsis induced by Gram-negative bacteria. Newborns can contract a perinatal infection from E. coli during natural birth, if the bacteria are present in the mother s birth canal. This is a leading cause of neonatal meningitis. Enterotoxigenic E. coli, also known as ETEC, is a frequent culprit behind the diarrhea of traveler, characterized by watery stools. Bloody diarrhea and a condition known as hemolytic uremic syndrome (HUS) are often caused by enterohemorrhagic strains of E. coli, which produce a shiga toxin.
Laboratory diagnosis: I - colony morphologies: 1- MacConkey Agar: E. coli typically ferments lactose and forms pink colonies on MacConkey Agar, giving them a shiny appearance with diameters between 0.5 to 1 mm after an overnight incubation. However, on blood agar plates, the appearance of colonies can range from grey to white, transparent to opaque, and from a raised convex shape to flat. Notably, certain E. coli strains, such as those belonging to the Alkaligens-Dispar group, may not ferment lactose on MacConkey agar. For instance, E. coli O157:H7, which does not ferment sorbitol, is distinguishable from other E. coli strains through this trait. 2- EMB Agar: When cultured on Eosin Methylene Blue (EMB) agar, E. coli produces colonies with a distinctive green metallic sheen. II- Biochemical Tests: 1- Indole positive: produces indole from tryptophan, 2- It is motile, 3- It decarboxylates lysine. 4- It uses acetate as the only source of carbon, 5- Catalase test positive, 6- Oxidase test negative, 7- Nitrate reduction test positive, 8- Methyl-Red (MR) test positive, 9- Voges-Proskauer (VP) test negative, 10- Citrate utilization test negative, 11- Acetate utilization test positive, 12- Indole test positive, 13- Pyrrolidonyl-β-naphthylamide (PYR) test negative, 13- H2S production test no, 14- Urease test negative, 15- Oxidative-fermentative (OF) test fermentative, 16- MUG test positive, 17- TSI reactions Acid/Acid, Gas, 18-Motility motile, 19-Phenyl Pyruvic acid (PPA) test negative, 20- Lysine decarboxylation test +, 21- Arginine decarboxylation test -/+ (strain variability), 22- Ornithine decarboxylation test +/- (strain variability), 23- ONPG +, 24- Sugar fermentation test: Glucose-Yes, Sucrose-No, Lactose-Yes, Mannitol-Yes. III- Antimicrobial susceptibility testing: Antimicrobial susceptibility testing for E. coli, especially strains producing Extended-spectrum ß-lactamases (ESBL), is performed via the disc-diffusion method. ESBLs are ß-lactamases that can break down third-generation cephalosporins and aztreonam but are blocked by clavulanic acid. Resistance to third-generation cephalosporins among E. coli is primarily due to ESBL production. The Clinical and Laboratory Standards Institute (CLSI) guidelines state that any E. coli strain with an inhibition zone size of ≤ 22 mm with ceftazidime, ≤ 25 mm with ceftriaxone, and ≤ 27 mm with cefotaxime, should be shortlisted for ESBL confirmation. ESBL confirmation involves two phenotypic methods: the combination disc method and the double disc synergy method. Both methods examine the difference in zone diameters with and without clavulanic acid, and an increase of ≥ 5 mm is a confirmatory sign of ESBL production. Uropathogenic Escherichia coli (UPEC) is responsible for a majority of urinary tract infections (UTIs) both in the community and hospitals. Compared to normal E. coli, UPEC strains possess additional virulence factors that enable them to overcome host defenses and establish infection in the urinary tract. These fact
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