Fergusonia shigelloides

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 humans, cattle, goats, swine, cats, dogs, monkeys, vultures, snakes, and toads. P. shigelloides has been isolated from a wide variety of human clinical specimens including both intestinal (usually feces or rectal swabs) and extra-intestinal. It has been isolated from the feces of humans, both with and without diarrhea, and/or vomiting (gastroenteritis). Many literature reports state or imply that Plesiomonas shigelloides actually caused the diarrhea/gastroenteritis. However, isolation from the feces of a case with diarrhea should not lead to the conclusion that the strain of P. shigelloides actually caused the diarrhea in the case; i.e. a temporal association does not prove causation. However, some strains of P. shigelloides may cause diarrhea in some people under certain conditions. Its causal role in diarrhea deserves additional study with the use of standard causation criteria. P. shigelloides has been isolated from a wide variety of human extra-intestinal clinical specimens, often from those with an immune deficiency. Some Plesiomonas strains share antigens with Shigella sonnei, and cross-reactions with Shigella antisera may 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 easily differentiated from Aeromonas sp. and other oxidase-positive organisms by standard biochemical tests.