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Last updated on April 17, 2014 at 9:31 EDT

Escherichia coli

Escherichia coli is a Gram-negative rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms. Most strains are harmless; however, some such as O157:H7 can cause food poisoning in humans and are often responsible for product recalls. The normal flora of the gut normally contains the harmless strains and often provide K2 to the body.

They are not always confined to the intestine and have the ability to survive briefly outside of the body. It grows easily and its genetics are simple and easily manipulated through a process of metagenics making it an ideal model for studying.

Theodor Escherich discovered E. Coli in 1885. E. coli is Gram-negative, facultative anaerobic and non-sporulating. Cells are typically rod-shaped and are about 2 micrometres (μm) long and 0.5 μm in diameter, with a cell volume of 0.6 – 0.7 (μm)3. It can live on a wide variety of substrates and uses mixed-acid fermentation in anaerobic conditions, producing lactate, succinate, ethanol, acetate and carbon dioxide.

Optimal growth occurs at 37°C but some laboratory strains can multiply at temperatures of up to 49°C. Aerobic and anaerobic respiration can drive growth. Any strain that possess flagella can swim and are motile.

E. coli possesses the ability to transfer DNA via bacterial conjugation, transduction or transformation, which allows genetic material to spread horizontally through an existing population. This process leads to the spread of the gene encoding shiga toxin from Shigella to E. coli.

Only 20% of the genome is common to all strains. From an evolutionary point of view genus Shigella are actually E. coli strains “in disguise”. E. coli is a sub-group within the species that has unique characteristics that distinguish it from other E. coli strains. The differences are detectable only at the molecular level; however, they may result in changes to the physiology or lifecycle of the bacterium.

Different strains are host-specific, making it possible to determine the source of fecal contamination in environmental samples. Knowing what E. coli strains are present helps researchers decide whether the contamination originated from human, another mammal or a bird. Strains of E. coli evolve through the natural biological process of mutation and through horizontal gene transfer.

Some strains can be harmful to a host animal. The strains typically cause a bout of diarrhea that is unpleasant in healthy adults and is often lethal to children in the developing world. More virulent strains can cause severe illness or death in the elderly.

E. Coli normally colonizes an infant’s gastrointestinal tract within 40 hours of birth where it adheres to the mucus of the large intestine. Without genetic elements that encode for virulence factors they remain benign commensals. Some strains produce lethal toxins and food poisoning usually happens due to consumption of unwashed food or undercooked meats. If the bacteria escape the intestinal tract and enter the abdomen they usually cause peritonitis which can be fatal.

Cooking food properly, preventing cross-contamination, instituting barriers such gloves, pasteurization of juice or dairy products are all ways to disrupt the transmission of the fecal-oral cycle.
Microscopy will show Gram negative rods, with no particular cell arrangement. Typical diagnosis has been done by culturing on sorbitol-MacConkey medium and then using an antiserum. Other methods include ELISA tests, colony immunoblots, and direct immunofluorescence microscopy of filters.

Bacterial infections usually treated with antibiotics. Antibiotic sensitivities of different strains of E. coli vary widely. Some are resistant to many antibiotics that are effective against Gram-positive organisms. Amoxicillin and other semi-synthetic penicillins are often used to treat. Antibiotic resistance is a growing problem. This is partly due to overuse of antibiotics and partly due to the use of antibiotics as growth promoters in food of animals.

Escherichia coli