A little gram negative rod (aka coccobacillus) that can have bipolar staining. Y. enterocolitica, Y. frederiksenii, Y. intermedia, Y. kristensenii, Y. pestis, Y. pseudotuberculosis.
It may be misidentified by some automated systems and delay the diagnosis (Pubmed).
Y. pestis is found in burrowing rodents. From the CDC web site: "In North America, plague is found from the Pacific Coast eastward to the western Great Plains and from British Columbia and Alberta, Canada southward to Mexico. Most of the human cases occur in two regions; one in northern New Mexico, northern Arizona, and southern Colorado, another in California, southern Oregon, and far western Nevada."
Also found in SE Asia, Africa (esp Madagascar) and China. In Africa, increased rainfall, presages plague. Rain means plants means food for rodents means a booming rodent population and with it comes death. There was a cluster of pneumonic plague in China when a dog gave it to its master, who then gave it to 12 others.
Dogs and cats are common vectors in the US. They kill a rodent, get plaque and pass it on to their owners. Cats are more likely to get and give pneumonic plague (PubMed).
Nice historial review. This organism has been plaguing humans for millenia. Get it? Plaguing? I am soooooo clever.
Y. enterocolitica, Y. pseudotuberculosis: a wide variety of domestic and wild animals.
Y. pestis: plague (review). Three clinical presentations:
1) Bubonic plague: enlarged, tender lymph nodes, fever, chills and prostration.
2) Septicemic plague: fever, chills, prostration, abdominal pain, shock and bleeding into skin and other organs, a purpura fulminans syndrome.
3) Pneumonic plague: fever, chills, cough and difficulty breathing; rapid shock and 100% fatal in 72 hours if antibiotics are delayed more than 18 hours after the onset of respiratory symptoms. Infectious: put in respiratory isolation. (CDC web site)
Y. enterocolitica can cause transfusion sepsis, usually from old pints (blood, not beer). It multiplies nicely in cold and needs iron, which occurs after a couple weeks as the red cells start to break down (PubMed).
Y. pseudotuberculosis: mesenteric adenitis; can mimic acute appendicitis. And in the Far East a Scarlet-Like Fever syndrome (PubMed).
Y. pestis: for historical reasons, in order of preference, gentamicin, streptomycin, doxycycline /tetracycline, OR chloramphenicol are the recommended agents, however, I bet all aminoglycosides, piperacillin, quinolones and third generation cephalosporins will work (although not suggested), one of which most people will get for empiric sepsis anyway.
There was a naturally occur strain with multiple resistance (chloramphenicol, streptomycin, and tetracycline, sulfonamides and tetracycline) in Madagascar (PubMed), and bioterrorism strains will likely be resistant as well.
There is an interesting book "The Biology of Plagues" that makes a compelling argument that the black death of the middle ages was NOT due to Y. pestis. However, when they have dug up patients from plague pits, it is the DNA of Y. pestis they find in the pulp of teeth.
On the basis of dental pulp it was responsible for Plague of Justinian and "Y pestis lineages that caused the Plague of Justinian and the Black Death 800 years later were independent emergences from rodents into human beings." (PubMed)