Plesiomonas shigelloides is a species of bacteria. 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 cattle, goats, swine, cats, dogs, monkeys, vultures, snakes, and toads.
Infections from this organism cause gastroenteritis, followed by septicemia in immune deficient patients. It is placed among the Enterobacteriaceae. Some Plesiomonas strains share antigens with Shigella sonnei, and cross-reactions with Shigella antisera occur. Plesiomonas can be distinguished from Shigella in diarrheal stools by an oxidase test: Plesiomonas is oxidase positive and Shigella is oxidase negative. Plesiomonas is positive for DNAse; this and other biochemical tests distinguish it from Aeromonas sp.