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The organisms discussed in this chapter are short, pleomorphic Gram-negative rods that often exhibit bipolar staining. They are catalase positive and microaerophilic or facultatively anaerobic. While most of the organisms discussed here have animals as their natural hosts, they are known to cause zoonotic infections and produce at times serious disease in humans.

The genus Yersinia includes 17 different species; however, only three species are pathogenic to humans, whereas the other 14 are considered to be environmental species and nonpathogenic. The three human pathogens include Yersinia pestis, Yersinia enterocolitica, and Yersinia pseudotuberculosis. These three Yersinia species generally cause disease in domestic and wild animals (eg, pigs, rodents, and birds) and humans are usually considered to be incidental hosts. Y. pestis is the cause of plague; Y. enterocolitica and Y. pseudotuberculosis are zoonotic food-borne pathogens, both typically causing a mild diarrheal disease, following ingestion of contaminated food and/or water. Pasteurella species are primarily commensals and/or pathogens in a variety of wild and domestic animals; however, Pasteurella multocida can also produce human disease.


While the epizootic cycle of plague has not been completely understood, infection with Y. pestis is fundamentally a disease of rodents, and plague is found in various endemic foci around the world. Humans are incidental, “dead-end” hosts that become infected when the plague bacillus is transmitted via flea bite or by exposure to fluids and tissues from an infected animal. As a result of such exposure, a serious infection will develop, often with a high mortality (40–100%). Plague has caused at least three major pandemics in previous centuries. The first pandemic occurred during the time of the Byzantine Empire in the 6th century, and the second pandemic, often called the “Black Death,” began in Central Asia and subsequently spread along ancient trade routes and reached Europe in 1346, where it spread quickly between the years 1347 and 1354, killing an estimated one-third of the population. During the 300 years following the “Black Death,” plague caused numerous smaller epidemics in various European countries. The third pandemic began during the 1850s in China, from where it spread via trade routes and steam ships to many countries, worldwide. While Y. pestis is an organism of considerable historic importance and interest, its potential use as an agent of Biowarfare has also been well recognized and documented. The ability of this organism to be easily transmitted by aerosol and the severity and high mortality associated with pneumonic plague make Y. pestis quite suitable for being a potential agent of biowarfare.

Morphology and Identification

Y. pestis is a Gram-negative rod that exhibits striking bipolar staining with special stains such as Wright, Giemsa, Wayson, or methylene blue (Figure 19-1). It is nonmotile. It grows as a facultative anaerobe on many bacteriologic media and can be readily isolated when sterile specimens such ...

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