ABSTRACT
In contrast to the common misconception that enterobacteria are solely inhabitants of the gastrointestinal tract of vertebrates, the family Enterobacteriaceae is rather widely dispersed in nature and many species exist in free-living states in a variety of niches in the biosphere. These ecological niches can be broadly broken down into four major groups: strains (1) principally associated in commensal or saprophytic states in the alimentary tract of humans and other vertebrates or at extraintestinal sites; (2) intimately associated with plants or plant diseases; (3) chiey found in water, soil, invertebrate species, and industrial processes; and (4) obligate endosymbionts or commensals of insects.2 Some species have developed such stable symbiotic relationships with their hosts that no genomic rearrangements or gene acquisitions have occurred over the past 50 million years.3
