Escherichia coli (; commonly abbreviated E. coli) is a Gram-negative gammaproteobacterium commonly found in the lower intestine of warm-blooded organisms (endotherms). The descendants of two isolates, K-12 and B strain, are used routinely in molecular biology as both a tool and a model organism. Diversity Escherichia coli is one of the most diverse bacterial species, with several pathogenic strains with different symptoms and with only 20% of the genome common to all strains. Furthermore, 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). History In 1885, Theodor Escherich, a German pediatrician, first discovered this species in the feces of healthy individuals and called it Bacterium coli commune because it is found in the colon and early classifications of Prokaryotes placed these in a handful of genera based on their shape and motility (at that time Ernst Haeckel's classification of Bacteria in the kingdom Monera was in place). Following a revision of Bacteria it was reclassified as Bacillus coli by Migula in 1895 and later reclassified as Escherichia coli. Due to its ease of culture and fast doubling, it was used in the early microbiology experiments; however, bacteria were considered primitive and pre-cellular and received little attention before 1944, when Avery, Macleod and McCarty demonstrated that DNA was the genetic material using Salmonella typhimurium, following which Escherichia coli was used for linkage mapping studies. Strains Four of the many E. coli strains (K-12, B, C, and W) are thought of as model organism strains. These are classified in Risk Group 1 in biosafety guidelines. Escherich's isolate The first isolate of Escherich was deposited in NCTC in 1920 by the Lister Institute in London (NCTC 86 ). K-12 A strain was isolated from a stool sample of a patient convalescent from diphtheria and was labelled K-12 (not an antigen) in 1922 at Stanford University. This isolate was used in 1940s by Charles E. Clifton to study nitrogen metabolism, who deposited it in ATCC (strain ATCC 10798 ) and lent it to Edward Tatum for his tryptophan biosynthesis experiments, despite its idiosyncrasies due to the F+ λ+ phenotype. In the course of the passages it lost its O antigen and in 1953 was cured first of its lambda phage (strain W1485 by UV by Joshua Lederberg and colleagues) and then in 1985