The Genus Escherichia
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The Genus Escherichia RODNEY A. WELCH
Introduction Escherichia coli, originally called “Bacterium coli commune,” was first isolated from the feces of a child in 1885 by the Austrian pediatrician Theodor Escherich (Escherich, 1885). Escherichia coli is a common inhabitant of the gastrointestinal tract of humans and animals. There are E. coli strains that are harmless commensals of the intestinal tract and others that are major pathogens of humans and animals. The pathogenic E. coli are divided into those strains causing disease inside the intestinal tract and others capable of infection at extra-intestinal sites (Kaper et al., 2004). Escherichia coli is easily cultured in the clinical laboratory, but the identification of the different pathogenic genotypes requires virulence gene detection methods not typically available in most clinical laboratories. Escherichia coli can be found secondarily in soil and water as the result of fecal contamination. Classically, its detection has been used as an indicator of poor water quality. From biochemical, physiological and genetic perspectives, E. coli is one of the best understood and characterized living organisms, with laboratory studies on model strains such as E. coli K-12 taking place over the past sixty years (extensively reviewed in Escherichia coli and Salmonella: Cellular and Molecular Biology, third edition; available online through the http://www.asmpress.org/browse/virtual/index.
asp?SelectedItem=10075{ASM Press Web site}). Shown in Table 1 are additional websites providing general and specific E. coli portals for useful genetic and metabolic information.
Taxonomy and Phylogeny General Comments The comparative analysis of 5S and 16S ribosomal RNA sequences suggest that Escherichia and Salmonella diverged from a common ancestor between 120 and 160 million years ago, which coincides with the origin of mammals (Ochman and Wilson, 1987). Escherichia and Shigella have been historically separated into different genera within the Enterobacteriaceae. DNA sequence analysis of their genomes reveals a high degree of sequence similarity and suggests to many bacteriologists that they should be considered a single species (see Shigella genome references: Ewing et al., 1958; Kimura, 1980; Brenner, 1984; Saitou and Nei, 1987; Simmons and Romanowska, 1987; Nei and Miller, 1990; Pupo et al., 2000; Jin et al., 2002; Wei et al., 2003). Currently, the two organisms continue to be discussed as two different genera anchored in the historical perception of their disease potential and ecology. Besides E. coli, there are other species within the genus, E. adecarboxylata, E. blattae, E. fergusonii, E. hermanii and E. vuln-
Table 1. E. coli reference sources on the Internet. Name of Web site
Web site Address
Information Provided
International E. coli Alliance E. coli Database Portal
http://www.uni-giessen.de/ ~gx1052/IECA/ieca.html
General portal site for E. coli information
The E. coli Index
http://ecoli.bham.ac.uk/
General portal site for E
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