Little is known about factors which enable Salmonella serotypes to circulate within populations of livestock and domestic fowl. We have identified a DNA region which is present in Salmonella serotypes commonly isolated from livestock and domestic fowl (S. enterica subspecies I) but absent from reptile-associated Salmonella serotypes (S. bongori and S. enterica subspecies II to VII). This DNA region was cloned from Salmonella serotype Typhimurium and sequence analysis revealed the presence of a 6,105-bp open reading frame, designated shdA, whose product's deduced amino acid sequence displayed homology to that of AIDA-I from diarrheagenic Escherichia coli, MisL of serotype Typhimurium, and IcsA of Shigella flexneri. The shdA gene was located adjacent to xseA at 52 min, in a 30-kb DNA region which is not present in Escherichia coli K-12. A serotype Typhimurium shdA mutant was shed with the feces in reduced numbers and for a shorter period of time compared to its isogenic parent. A possible role for the shdA gene during the expansion in host range of S. enterica subspecies I to include warm-blooded vertebrates is discussed.Salmonella serotypes are a frequent constituent of the intestinal flora of poikilothermic animals. The percentage of apparently healthy, cold-blooded vertebrates which harbor Salmonella serotypes ranges from 74 to 94% (20,28,32,34,59), and these bacteria could thus be considered part of the normal intestinal flora (23,48). Salmonella serotypes are also commonly isolated from a fraction (usually Ͻ20%) of warmblooded animal hosts (15,31,49,54). Although chronic carriers, which appear healthy, are observed within the human population and among warm-blooded animals (22,27,35,40), Salmonella serotypes are commonly associated with illness in these hosts (55). Consequently Salmonella serotypes are regarded as pathogens rather than part of the normal intestinal flora of homeothermic animals.On the basis of multilocus enzyme electrophoresis and comparative sequence analysis of orthologous genes, two species, S. enterica and S. bongori, have been assigned to the genus Salmonella (18,46). S. enterica is further subdivided into seven subspecies designated with roman numerals (18, 44). While S. bongori and S. enterica subspecies II, IIIa, IIIb, IV, VI, and VII are mainly associated with cold-blooded vertebrates, members of S. enterica subspecies I are frequently isolated from avian and mammalian hosts (44). For instance, of the 90,201 Salmonella isolates collected between 1977 and 1992 by the German National Reference Center for Enteric Pathogens from humans and warm-blooded animals, 89,798 isolates (99.55%) belonged to S. enterica subspecies I (1). Currently it is not clear which virulence mechanisms are responsible for the apparent adaptation of S. enterica subspecies I to circulation within populations of warm-blooded animals.S. bongori or S. enterica subspecies II to VII are able to infect humans, colonize the intestine and cause disease (1). Human infections with serotypes of S. bongori and S. enterica su...