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Yersinia pestis (formerly Pasteurella pestis) is a Gram-negative rod-shaped bacterium belonging to the family Enterobacteriaceae. It is a facultative anaerobe that can infect humans and other animals.
Human Y. pestis infection takes three main forms: pneumonic, septicemic, and the notorious bubonic plagues.[1] All three forms have been responsible for high mortality rates in epidemics throughout human history, including the Black Death (a bubonic plague) that accounted for the death of at least one-third of the European population in 1347 to 1353.
Recently Y. pestis has gained attention as a possible biological warfare agent and the CDC has classified it as category A pathogen requiring preparation for a possible terrorist attack. Y. pestis may be afflicting al-Qaeda in Algeria.[2]
Y. pestis was discovered in 1894 by Alexandre Yersin, a Swiss/French physician and bacteriologist from the Pasteur Institute, during an epidemic of plague in Hong Kong.[3] Yersin was a member of the Pasteur school of thought. Shibasaburo Kitasato, a German-trained Japanese bacteriologist who practiced Koch’s methodology was also engaged at the time in finding the causative agent of plague.[4] However, it was Yersin who actually linked plague with Yersinia pestis. Originally named Pasteurella pestis, the organism was renamed in 1967.
Three biovars of Y. pestis are known, each thought to correspond to one of the historical pandemics of bubonic plague.[5] Biovar Antiqua is thought to correspond to the Plague of Justinian; it is not known whether this biovar also corresponds to earlier, smaller epidemics of bubonic plague, or whether these were even truly bubonic plague.[6] Biovar Medievalis is thought to correspond to the Black Death. Biovar Orientalis is thought to correspond to the Third Pandemic and the majority of modern outbreaks of plague. Y. pestis was transmitted by fleas infesting rats.
DNA from Y. pestis has been found in the teeth of an individual who supposedly died from the Black Death, and medieval corpses who died from other causes have tested positive for Y. pestis.[7][8] This suggests that Y. pestis was, at the very least, a contributing factor in some (though possibly not all) of the European plagues. It is possible that the selective pressures induced by the plague might have changed how the pathogen manifests in humans, selecting against the individuals or populations which were the most susceptible.
Y. pestis is a rod-shaped facultative anaerobe with bipolar staining (giving it a safety pin appearance).[9] Similar to other Yersinia members, it tests negative for urease, lactose fermentation, and indole.[10] The closest relative is the gastrointestinal pathogen Yersinia pseudotuberculosis, and more distantly Yersinia enterocolitica.
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