Escherichia is a genus of Gram-negative, non-spore forming, facultatively anaerobic, rod-shaped bacteria from the family Enterobacteriaceae. Inhabitants of the gastrointestinal tracts of warm-blooded animals, Escherichia species provide a portion of the microbially-derived vitamin K for their host. Escherichia coli are the most numerous aerobic commensal inhabitants of the large intestine in humans.