Y. ruckeri is a Gram-negative, rod-shaped bacterium that shows a variable motility due to the presence of flagella. These flagella are not always observed. ruckeri is the causative agent of enteric redmouth disease (ERM) which is characterized by a chronic or acute enterosepticemia with high morbidity and mortality rates in salmonids and non-salmonids species. Six serovars have been described according to whole-cell serological typing. Yersinia is a genus of bacteria in the family Enterobacteriaceae. Yersinia are Gram-negative rod shaped bacteria, a few micrometers long and fractions of a micrometer in diameter, and are facultative anaerobes. Some members of Yersinia are pathogenic in humans; in particular, Y. pestis is the causative agent of the plague. Rodents are the natural reservoirs of Yersinia; less frequently other mammals serve as the host. Infection may occur either through blood (in the case of Y. pestis) or in an alimentary fashion, occasionally via consumption of food products (especially vegetables, milk-derived products and meat) contaminated with infected urine or feces. Speculations exist as to whether or not certain Yersinia can also be spread via protozoonotic mechanisms, since Yersinia are known to be facultative intracellular parasites; studies and discussions of the possibility of amoeba-vectored (through the cyst form of the protozoan) Yersinia propagation and proliferation are now in progress.