Fengyi Tang and Milton H. Saier Jr
Escherichia coli K12 are currently the best-understood organism on Earth; a larger fraction of its genes have been functionally characterized than for any other organisms. However, organisms given this species designation are extremely diverse, genetically and phenotypically. E. coli proves to be a valuable model system for understanding bacterial physiology, metabolism, genetics and pathogenesis. Why has E. coli been so much more extensively studied than any other bacterium on Earth? It all began with the German pediatrician and bacteriologist, Theodor Escherich, who was dismayed to find that so many babies were dying of diarrhea diseases. In fact, we now know that E. coli is one of the top causes of infant mortality, adult diarrhea and urinary tract infections worldwide. It was because of the organism's broad host specificity and ability to cause many different diseases in large numbers of animals, including humans, that it was first selected as an important bacterium for study. More recently, it has been found that certain strains of E. coli are probiotic, having beneficial effects on host animals, counteracting the detrimental consequences of their pathogenic counterparts. Genetic analyses have revealed that the tremendous phenotypic diversity of the hundreds of currently recognized E. coli strains resulted from the frequent occurrence of horizontal gene transfer during the evolutionary divergence of the Escherichia genus. In fact, the pan genome of E. coli far exceeds the core genome, by up to 20-fold or more. Thus E. coli should not be thought of as an entity or species; it represents an entire spectrum of related organisms sharing only the core housekeeping genes.
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