Escherichia coli

Escherichia coli, /ˌɛʃəˈrɪkiə ˈkoʊlaɪ/; commonly abbreviated E. coli; named after Theodor Escherich) is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms (endotherms). Most E. coli strains are harmless, but some, such as serotype O157:H7, can cause serious food poisoning in humans, and are occasionally responsible for product recalls. The harmless strains are part of the normal flora of the gut, and can benefit their hosts by producing vitamin K2, and by preventing the establishment of pathogenic bacteria within the intestine.

E. coli bacteria are not always confined to the intestine, and their ability to survive for brief periods outside the body makes them ideal indicator organisms to test environmental samples for fecal contamination. The bacterium can also be grown easily, and its genetics are comparatively simple and easily manipulated or duplicated through a process of metagenics, making it one of the best-studied prokaryotic model organisms, and an important species in biotechnology and microbiology.

E. coli was discovered by German pediatrician and bacteriologist Theodor Escherich in 1885, and is now classified as part of the Enterobacteriaceae family of gamma-proteobacteria.

Diversity

As more is known about certain organisms, such as genetic information, the taxonomic classification of species is changed to reflect the advance in knowledge, however in the case of Escherichia coli due to its medical importance, this has not occurred (namely split into several genera/species) and remains one of the most diverse bacterial species: only 20% of the genome is common to all strains. In fact, from the evolutionary point of view, the members of genus Shigella (dysenteriae, flexneri, boydii, sonnei) are actually E. coli strains "in disguise" (i.e. E.coli is paraphyletic to the genus).

A strain of E. coli is a sub-group within the species that has unique characteristics that distinguish it from other E. coli strains. These differences are often detectable only at the molecular level; however, they may result in changes to the physiology or lifecycle of the bacterium. For example, a strain may gain pathogenic capacity, the ability to use a unique carbon source, the ability to take upon a particular ecological niche or the ability to resist antimicrobial agents. Different strains of E. coli are often host-specific, making it possible to determine the source of faecal contamination in environmental samples. For example, knowing which E. coli strains are present in a water sample allows researchers to make assumptions about whether the contamination originated from a human, another mammal or a bird.

A common subdivison system of E.coli, but not based on evolutionary relatedness, is by serotype, which is based on major surface antigens (O antigen: part of lipopolysaccharide layer; H: flagellin; K antigen: capsule), e.g. O157:H7) (NB: K-12, the common laboratory strain is not a serotype.)

New strains of E. coli evolve through the natural biological process of mutation and through horizontal gene transfer. Some strains develop traits that can be harmful to a host animal. These virulent strains typically cause a bout of diarrhoea that is unpleasant in healthy adults and is often lethal to children in the developing world. More virulent strains, such as O157:H7 cause serious illness or death in the elderly, the very young or the immunocompromised.

E. coli is the type species of the genus and the neotype strain is ATCC 11775, also known as NCTC 9001,[22] which is pathogenic to chickens and has a O1:K1:H7 serotype. However, in most studies either O157:H7 or K-12 MG1655 or K-12 W3110 are used as a representative E.coli.

Biology and biochemistry

E. coli is Gram-negative, facultative anaerobic and non-sporulating. Cells are typically rod-shaped, and are about 2.0 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. E. coli uses mixed-acid fermentation in anaerobic conditions, producing lactate, succinate, ethanol, acetate and carbon dioxide. Since many pathways in mixed-acid fermentation produce hydrogen gas, these pathways require the levels of hydrogen to be low, as is the case when E. coli lives together with hydrogen-consuming organisms, such as methanogens or sulphate-reducing bacteria.

Optimal growth of E. coli occurs at 37°C (98.6°F) but some laboratory strains can multiply at temperatures of up to 49°C (120.2°F). Growth can be driven by aerobic or anaerobic respiration, using a large variety of redox pairs, including the oxidation of pyruvic acid, formic acid, hydrogen and amino acids, and the reduction of substrates such as oxygen, nitrate, dimethyl sulfoxide and trimethylamine N-oxide.

Strains that possess flagella can swim and are motile. The flagella have a peritrichous arrangement.

E. coli and related bacteria possess the ability to transfer DNA via bacterial conjugation, transduction or transformation, which allows genetic material to spread horizontally through an existing population. This process led to the spread of the gene encoding shiga toxin from Shigella to E. coli O157:H7, carried by a bacteriophage.

Role in disease

Virulent strains of E. coli can cause gastroenteritis, urinary tract infections, and neonatal meningitis. In rarer cases, virulent strains are also responsible for haemolytic-uremic syndrome, peritonitis, mastitis, septicaemia and Gram-negative pneumonia.

Laboratory diagnosis

In stool samples microscopy will show Gram negative rods, with no particular cell arrangement. Then, either MacConkey agar or EMB agar (or both) are inoculated with the stool. On MacConkey agar, deep red colonies are produced as the organism is lactose-positive, and fermentation of this sugar will cause the medium's pH to drop, leading to darkening of the medium. Growth on Levine EMB agar produces black colonies with greenish-black metallic sheen. This is diagnostic of E. coli. The organism is also lysine positive, and grows on TSI slant with a (A/A/g+/H2S-) profile. Also, IMViC is {+ + – -} for E. coli; as it's indole-positive (red ring) and methyl red-positive (bright red), but VP-negative (no change-colourless) and citrate-negative (no change-green colour). Tests for toxin production can use mammalian cells in tissue culture, which are rapidly killed by shiga toxin. Although sensitive and very specific, this method is slow and expensive.

Typically diagnosis has been done by culturing on sorbitol-MacConkey medium and then using typing antiserum. However, current latex assays and some typing antisera have shown cross reactions with non-E. coli O157 colonies. Furthermore, not all E. coli O157 strains associated with HUS are nonsorbitol fermentors.

The Council of State and Territorial Epidemiologists recommend that clinical laboratories screen at least all bloody stools for this pathogen. The American Gastroenterological Association Foundation (AGAF) recommended in July 1994 that all stool specimens should be routinely tested for E. coli O157:H7.[citation needed] It is recommended that the clinician check with their state health department or the Centers for Disease Control and Prevention to determine which specimens should be tested and whether the results are reportable.

Other methods for detecting E. coli O157 in stool include ELISA tests, colony immunoblots, direct immunofluorescence microscopy of filters, as well as immunocapture techniques using magnetic beads. These assays are designed as screening tool to allow rapid testing for the presence of E. coli O157 without prior culturing of the stool specimen.

Vaccination

Researchers have actively been working to develop safe, effective vaccines to lower the worldwide incidence of E. coli infection. In March 2006, a vaccine eliciting an immune response against the E. coli O157:H7 O-specific polysaccharide conjugated to recombinant exotoxin A of Pseudomonas aeruginosa (O157-rEPA) was reported to be safe in children two to five years old. Previous work had already indicated that it was safe for adults.A phase III clinical trial to verify the large-scale efficacy of the treatment is planned.

In 2006 Fort Dodge Animal Health (Wyeth) introduced an effective live attenuated vaccine to control airsacculitis and peritonitis in chickens. The vaccine is a genetically modified avirulent vaccine that has demonstrated protection against O78 and untypeable strains.

In January 2007 the Canadian bio-pharmaceutical company Bioniche announced it has developed a cattle vaccine which reduces the number of O157:H7 shed in manure by a factor of 1000, to about 1000 pathogenic bacteria per gram of manure.

In April 2009 a Michigan State University researcher announced that he has developed a working vaccine for a strain of E. coli. Mahdi Saeed, professor of epidemiology and infectious disease in MSU's colleges of Veterinary Medicine and Human Medicine, has applied for a patent for his discovery and has made contact with pharmaceutical companies for commercial production.