The genus Yersinia is composed of Gram negative, bipolar staining coccobacilli. Like other Enterobacteriaceae , their metabolism is fermentative. Y. pestis produces a thick anti-phagocytic slime layer, while Y. enterocolitica is motile at 28 . Plague is a disease that is primarily maintained among rodent populations and transmitted by infected fleas. Urban plague involves rats and has been the major source for human epidemics.
Sylvatic plague exists in wild rodent populations.
Pneumonic plague occurs under crowded conditions when contaminated respiratory droplets expelled by infected persons are directly inhaled by another person. This might explain why the bacteria are not virulent in their alternate host, the flea, which has a body temperature near 25 . Moreover, the bacteria are capable of surviving and multiplying within monocytes, but not PMNs, and upon emerging from the monocytic host, the bacteria possess their F1 and VW antigens. Most disease produced by Y. enterocolitica is a typical gastroenteritis characterized by fever, abdominal pain, and diarrhea. Illness generally lasts from 1 to 2 weeks but chronic cases may persist for up to a year.