Microbiology: Escherichia coli Overview

Escherichia coli (E. coli)

• E. coli is a facultative anaerobe, gram-negative rod that is part of the normal flora of the human gut.
• It is a lactose fermenter, with greenish, metallic sheen colonies on EMB agar.
• It can ferment lactose producing yellow slant and yellow butt, and gas, on TSI agar.
• Indole positive: Tryptone broth with a red ring when Kovac’s reagent is added.

Pathogenic Strains of E. coli

• There are five pathogenic strains of E. coli: enterotoxigenic (ETEC), enteroinvasive (EIEC), enterohemorrhagic (EHEC), enteropathogenic (EPEC), and enteroaggregative (EAEC).

Enterotoxigenic E. coli (ETEC)

• ETEC produces two enterotoxins: heat-labile toxin (LT) and heat-stable toxin (ST).
• These toxins cause watery diarrhea by inducing water and electrolytes into the lumen of the small intestines.

Laboratory Diagnosis of Pathogenic E. coli

• Identification of pathogenic E. coli strains relies on culture methods, biochemical tests, and molecular detection.

Enrichment Media & Techniques

• Specialized media such as Salmonella Shigella Agar (SSA), Selenite F Broth, Tetrathionate Broth, and Gram-negative Broth (GN broth) are utilized to isolate enteric pathogens from stool samples.
• These media inhibit the growth of coliforms while promoting the growth of enteropathogens.

Salmonella and Shigella Isolation

• Stool or rectal swabs are inoculated into enrichment media like Selenite F Broth or Tetrathionate Broth followed by subculturing on selective media like SSA.
• Salmonella is isolated from Tetrathionate Broth, while Shigella is isolated from Selenite F Broth.

Differentiation of E. coli Strains

• Identifying the specific strain of pathogenic E. coli is important for appropriate treatment and infection control.
• The differentiation of the strains can be achieved using serological tests and molecular methods.

Coli O157: H7

• This is a non-motile strain that synthesizes the K1 antigen and Verotoxin or Shiga-like toxin.
• Verotoxin damages vascular endothelial cells and can cause hemorrhagic colitis.
• This strain has been linked to foodborne outbreaks, particularly in meat products like hamburgers.

Other Important Microorganisms

• Other notable Gram-negative rods that may be encountered in the laboratory include Proteus, Providencia, and Morganella.
• They are non-lactose fermenters and rapid urease producers. These organisms are commonly associated with urinary tract infections.

Proteus

• Proteus is a motile bacterium with a distinctive foul odor and "chocolate cake" or "burnt chocolate" colonies on agar plates.
• It produces the enzyme tryptophanase and can be identified via the IMVIC reaction (Indole, Methyl Red, Voges-Proskauer, Citrate).
• It is commonly found in the gut and is associated with urinary tract infections.

This quiz covers essential information about Escherichia coli, including its characteristics, pathogenic strains, and the laboratory diagnosis methods used for identification. Test your knowledge on the role of E. coli in the human gut and the implications of its pathogenic variants.