Plesiomonas shigelloides is a species of bacteria that was formerly classified in the family Vibrionaceae, but now most microbiologists agree that a better classification is in the family Enterobacteriaceae (see box on the right). It is a Gram-negative, rod-shaped bacterium which has been isolated from freshwater, freshwater fish, and shellfish and from many types of animals including humas, cattle, goats, swine, cats, dogs, monkeys, vultures, snakes, and toads.