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The organisms discussed in this chapter are short, pleomorphic Gram-negative rods that can exhibit bipolar staining. They are catalase positive, oxidase negative, and microaerophilic or facultatively anaerobic. Most have animals as their natural hosts, but they can produce serious disease in humans.

The genus Yersinia includes Yersinia pestis, the cause of plague; Yersinia enterocolitica, an important cause of human diarrheal disease; and several others considered nonpathogenic for humans. Pasteurella are primarily animal pathogens but Pasteurella multocida can also produce human disease.


Plague is an infection of wild rodents transmitted from one rodent to another and occasionally from rodents to humans by the bites of fleas. Serious infection often results, which in previous centuries produced pandemics of “black death” with millions of fatalities. The ability of this organism to be transmitted by aerosol and the severity and high mortality associated with pneumonic plague make Y pestis a potential biological weapon.

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. Growth is more rapid in media containing blood or tissue fluids and fastest at 30°C. In cultures on blood agar at 37°C, colonies may be very small at 24 hours. A virulent inoculum, derived from infected tissue, produces gray and viscous colonies, but after passage in the laboratory, the colonies become irregular and rough. The organism has little biochemical activity, and this is somewhat variable.


Yersinia pestis (arrows) in blood, Wright-Giemsa stain. Some of the Yersinia pestis have bipolar staining, which gives them a hairpin-like appearance. Original magnification ×1000. (Courtesy of K Gage, Plague Section, Centers for Disease Control and Prevention, Ft. Collins, CO.)

Antigenic Structure

All yersiniae possess lipopolysaccharides that have endotoxic activity when released. Y pestis and Y enterocolitica also produce antigens and toxins that act as virulence factors. They have type III secretion systems that consist of a membrane-spanning complex that allows the bacteria to inject proteins directly into cytoplasm of the host cells. The virulent yersiniae produce V and W antigens, which are encoded by genes on a plasmid of approximately 70 kb. This is essential for virulence; the V and W antigens yield the requirement for calcium for growth at 37°C. Compared with the other pathogenic yersiniae, Y pestis has gained additional plasmids. pPCP1 is a 9.5-kb plasmid that contains genes that yield plasminogen-activating protease that has temperature-dependent coagulase activity (20°–28°C, the temperature of the flea) and fibrinolytic activity (35°–37°C, the temperature of the host). This factor is involved in dissemination of the organism from the ...

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