Bubonic Plague - Yersinia pestis

Yersinia pestis is always a fun little organism to see under the microscope. It’s a Gram-negative, rod-shaped bacteria, but it looks more like a safety-pin than a “rod” because of the natural bi-polar staining pattern of the organism. The species was found to be the causative agent of bubonic plague during an 1894 epidemic in Hong Kong, by Alexandre Yersin. Until 1967, however, it was categorized with the Pasteurella genus, and was known as Pasteurella pestis.

There are several strains of Y. pestis, and three different manifestations of the plague:

  • Bubonic plague - Incubation period of 2-6 days with few symptoms, while bacteria multiply within lymph nodes. Sudden fever and headache at end of incubation period, with complete loss of energy. The characteristic buboes (lymph swellings) appear at this point, as the lymph nodes swell to enormous proportions thanks to the bacteria within them. The inguinal (groin) nodes generally are the first to show signs of infection.

  • Septicemic plague - Same bacteria, different strain of Y. pestis, and way worse. From what we know, primary septicemic plague is generally caused by one unique strain, or by any strain in immuno-compromised patients. When the other manifestations of the disease cause overwhelming sepsis prior to death, this is known as secondary septicemic plague. Primary septicemic plague is characterized by hypotension, shock, hepatosplenomegaly (swollen spleen and liver), and death. Sometimes very few or even no outward symptoms develop before the patient is killed by the bacteria’s internal effects.

  • Pneumonic plague - Caused by direct inhalation of bacteria (often person-to-person), with initial site of infection being the lungs. Different strains have different degrees of ability to transfer in this manner, but it generally requires prolonged contact with infected persons or animals. Causes tracheal and bronchial hemorrhaging, large amounts of alveolar exudate, congestion of the lungs, and pleural edema. Often quickly spreads to other organs, much like bubonic plague.

While all three manifestations of the disease can be deadly, the incidence of death is greatly reduced by IV antibiotics, and thanks to modern sanitation standards, outbreaks in developed countries are unheard of.

Still, Yersinia pestis isn’t, and probably never will be, completely exterminated. Wild animals such as rodents, prairie dogs, and some marsupials and primates are known to both be affected by and serve as reservoirs for the bacteria. This means that even if humans somehow stopped acquiring the plague for a while, the bacteria itself would still be around, and we would still be able to contract it.

Interestingly, a 2011 study in the journal Nature showed that the strain of Y. pestis which caused the Black Death in both the 1st century C.E. and the early Middle Ages may no longer be extant. The genome of the bacteria analyzed from victims of those plagues showed a more ancient form of Y. pestis that lacked a number of the mutations that exist in current-day strains, which are known to have caused all epidemics beyond the Renaissance.

Have I gone on about the plague enough? If not, check out way more information than you’ll ever use about the pathogen at CIDRAP Bioterrorism and PLoS Pathogens!


  • Bacillus of Bubonic Plague - Elementary Bacteriology and Protozoology, for the use of Nurses. Herbert Fox, 1919.
  • Swelling of inguinal bubo in U.S. soldier - From the U.S. Centers for Disease Control and Prevention, ca. 1970.
  • Plague victims being blessed by priest - Omne Bonum. James le Palmer, 1360.
  • Mass grave of plague victims- From Martiques, France, dated to the last pandemic of plague in Europe, between 1720 and 1722.
  • Plague Riot of Moscow - Depicts the rioting during and after the 1770s Moscow epidemic.

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    I studied the Black Death as my Special Subject as an undergrad, so basically it was half my life in my final year. Love...
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    every year. //www.azdhs.gov/phs/oids/vector/plague/
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