The genus Edwardsiella differs from the genus Escherichia by its ability to produce hydrogen sulfide. The genus Edwardsiella includesEdwardsiella tarda, the only pathogenic species for humans. E. tarda inhabits the intestines of snakes and other cold-blooded animals. The name tarda refers to slow or weak fer-mentation of sugars by the bacteria. E. tarda is a Gram-negative, noncapsulated, motile bacillus with weak fermentative powers.
It ferments only glucose and maltose with production of acid and some gas. It is indole, H2S, and citrate positive, and it decar-boxylates lysine and ornithine.
E. tarda is an occasional human pathogen isolated fromwounds, blood, and CSF in cases of fatal meningitis. The bacteria have also been isolated from stool of normal healthy people and that of patients with diarrhea. Nevertheless, the pathogenic role of the bacteria in causation of diarrhea is yet to be established.