Escherichia coli


BIOLOGY

E.coli is facultative anaerobic Gram-negative non-sporing rod-like bacterium that ferments lactose. It is catalase positive, oxidase negative and indole positive bacillus. They can grow at a temperature ranging from 7 to 46°C and can grow at a minimum water activity of 0.95. They are fairly acid tolerant and can grow at pH values ranging from 4.4 to 10.

Escherichia coli (Migula 1895) is a bacterium that is commonly found in the lower intestine of warm-blooded animals. Most strains are harmless, but some, such as serotype O157:H7, can cause serious food poisoning in humans. The harmless strains are part of the normal flora of the gut and can benefit their hosts by producing vitamin K2 and B12 or by preventing the establishment of pathogenic bacteria within the intestine. E. coli has the ability to survive for brief periods outside the body which makes them an ideal organism for fecal contamination.

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

 There is a particularly strain called E. coli O157:H7 that causes food poisoning.

The complete sequence of the genome of a harmless laboratory strain of E. coli(K-12) was reported in 5 September 1997 issue of Science, whose genome consists of a single molecule of DNA containing 4,639,221 base pairs. These encode 4288 proteins and 89 RNAs. Many of the genes were already known but the function of 38% of the genes (ORFs) remains to be discovered.

More than 700 serotypes of E. coli have been identified. The different E. coli serotypes are distinguished by their “O” and “H” antigens on their bodies and flagella, respectively. The E. coli serotypes that are responsible for the numerous reports of contaminated foods and beverages are those that produce Shiga toxin (Stx), so called because the toxin is virtually identical to that produced by another bacterium known as Shigella dysenteria type 1.

SPECIAL FEATURES

The bacteria can be grown easily and its genetics is comparatively simple and easily-manipulated, making it one of the best-studied prokaryotic model organisms, and an important species used in biotechnology.

Bacteria make useful tools for genetic research because of their relatively small genome size compared to eukaryotes. E. coli cells only have about 4,400 genes whereas humans contain approximately 30,000 genes. Also, bacteria, including E. coli, live their entire lifetime in a haploid state, with no second allele to mask the effects of mutations.

Bacteria typically grow much faster than more complex organisms. E. coli grows rapidly at a rate of one generation per twenty minutes under typical growth conditions. This allows for preparation of large cultures overnight and genetic experimental results can be obtained in hours instead of days, months or years.

Most gene cloning techniques were developed using this bacterium. E. coli is readily transformed with plasmids and other vectors, and preparation of competent cells is not complicated. Transformations with other microorganisms are often less successful.

CONTRIBUTIONS TO RESEARCH STUDIES

Because of its long history of laboratory culture and ease of manipulation, E. coli also plays an important role in modern biological engineering and industrial microbiology. The work of Stanley Norman Cohen and Herbert Boyer in E. coli, using plasmids and restriction enzymes to create recombinantDNA, became a foundation of biotechnology.

One of the first useful applications of recombinant DNA technology was the manipulation of E. coli to produce human insulin. Modified E. coli have been used in vaccine development, bioremediation, and production of immobilized enzymes. E. coli cannot, however, be used to produce some of the more large, complex proteins which contain multiple disulfide bonds and, in particular, unpaired thiols, or proteins that also require post-translational modification for activity.

E. coli is frequently used as a model organism in microbiology studies. Cultivated strains (E. coli K12) are well-adapted to the laboratory environment and, unlike wild strains, have lost their ability to live in intestine. Many lab strains lose their ability to form biofilms.

In 1946, Joshua Lederberg and Edward Tatum first described the phenomenon known as bacterial conjugation using E. coli as a model bacterium, and it remains the primary model to study conjugation. E. coli was an integral part of the first experiments to understand phagegenetics, and early researchers, such as Seymour Benzer, used E. coli and phage T4 to understand the topography of gene structure. Prior to Benzer’s research, it was not known whether the gene was a linear structure, or if it had a branching pattern.




Top