Escherichia coli

Escherichia coli which is commonly abbreviated as E. coli has its name derived from the scientist who discovered it. It is a gram negative rod shaped bacterium. The common location where this bacterium is frequently isolated is lower intestine of worm blooded animals. The bacteria belong to the kingdom of bacteria, phylum proteobacteria; class of Gamma proteobacteria, order enterobacteriales, family enterobacteriaceae, genus Escherichia, and species is coli.

It is grouped in the kingdom of bacteria because it is a single celled organism and since it is a gram negative organism with a capsule composed of lipopolysaccharides, it is placed in the phylum Protobacteria (Talaro, 2008). It is a gram negative rod shaped bacterium which is facultative anaerobe. The organism is motile by use of peritrichous flagella whose growth is favored at a temperature of 37 degrees Celsius. Other chemical characteristics of E. coli include oxidase negative, catalase positive and does reduce nitrates to nitrites. E.

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coli is an opportunistic flora which is found colonizing the intestines of worm blooded animals i. e. mammals. In the genus Escherichia there are about five species and E. coli is one of them. In addition to the above biochemical activities, E. coli ferments lactose and produce carbon dioxide and water, it also has an enzyme called lysine decarboxylase, results in formation of indole ring, and is Vogus- proskauer negative (Levine & Edelman, 1984). E. coli has distinctive features on growth cultures. On MacConkey agar and EMB agar, a sample containing E.

coli will produce deep red colonies since it ferments lactose and this lowers the PH of the medium therefore darkening. Growth on Levine EMB agar results in black colonies with greenish metallic sheen (Bartholomew, 1967). There are various strains of E. coli which exist in nature. A strain is considered as a subgroup within the species and it must possess characteristics that differentiate it from other strains. Differences among different strains are only detected at molecular level, but change in the physiology or even the life cycle of the bacteria can be suggestive.

The strains of E. coli have specific hosts and this makes it easy to know the source of fecal contamination in any sample (Talaro, 2008). New strains of E. coli are emerging through the process of mutation which involves change in genetic material. These new strains develop characteristics that can harm the host animal. The harmful traits change these strains into virulent strains which usually cause a short time diarrhea in healthy adults and are quite fetal in children. It is the major cause of diarrhea in children in the third world countries.

Among the virulent strain is strain0157:H7 which causes severe illness or death in the immuno-challenged individuals like children, the elderly, and the sick (Levine & Edelman, 1984). E. coli which colonizes the lower intestines of mammals is further classified based on their serological characteristics and virulence factors. Under this classification there are the following verotypes: enterotoxigenic, enteropathogenic, enteroinvasive, enterohemmorrhagic and enteroaggrigative E. coli. Enteroinvassive and enteroaggregative verotypes of E. coli are solely found in man.

In the verotype enterohaemorrhagic, there is a quite commonly found strain; 0157:H7 which is responsible for bloody diarrhea without fever. It can also lead to hemolytic uremic syndrome and finally kidney failure. It also contains shiga toxin which can cause serious inflammatory reaction. Enteroinvassive verotype causes a disease similar to shigellosis characterized by severe diarrhea and fever (Moder, 2008). References: Bartholomew, W. J. (1967). Laboratory textbook and exercises in microbiology. Ohio: Brown. Corry, J. E. L. , Curtis, G. D. W. & Baird, R. M. (1995).

Culture media for food microbiology. Amsterdam: Elsevier, 1995 Levine, M. M. & Edelman, R. (1984). Enteropathogenic Escherichia coli of classic serotypes associated with infant diarrhea: Epidemiology and pathogenesis. Retrieved on February 8, 2010 from: http://www. ncbi. nlm. nih. gov/pubmed/6386503 Moder, J. (2008). Escherichia coli. Retrieved on February 8, 2010 from: http://bioweb. uwlax. edu/bio203/s2008/moder_just/classification. htm Talaro, K. P. (2008). Foundations in Microbiology: Basic Principles. Houston, TX, U. S. A. : McGraw Hill.


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