Pasteurella, Yersinia, and Francisella

Excerpt

The genus Pasteurella was originally proposed and described by Trevisan in 1887. It consisted of a group of nonmotile, small (0. 7 μm by 0. 5 μm), Gram-negative coccobacilli often exhibiting a characteristic type of bipolar staining (Fig. 29-1). Most members of this genus are associated with severe, life-threatening systemic diseases involving both hemorrhagic pneumonia and septicemia. The first pathogen to be studied (called at that time Pasteurella septica) was shown to be responsible for hemorrhagic septicemia in cattle and sheep, and fowl cholera in chickens. This organism, used by Pasteur for his milestone vaccination studies in 1880, is now called Pasteurella multocida. Adult animals may carry this organism as part of their normal nasopharyngeal or gingival microflora and may infect young, susceptible animals which develop a fulminating, rapidly lethal hemorrhagic pneumonia. The incubation period for this disease may be as short as 12 hours with very high mortality rates (80 to 100 percent). The disease can spread explosively through an apparently normal herd or flock.

Two other important pathogens were initially included in this genus. The first was Pasteurella pestis (the plague bacillus), which was isolated and described almost simultaneously by Kitasato and by Yersin in 1894. This organism is primarily a pathogen of the rat (one of a select group of acute bacterial pathogens for this host). For taxonomic reasons, it was decided in 1971 to place the plague bacillus in a new genus as Yersinia pestis, together with Y pseudotuberculosis and Y enterocolitica. The latter infect a variety of rodent species but can cause severe intestinal disease in humans. Finally, a third genus was created for an organism originally grouped with P pestis, but now known as Francisella tularensis, the agent of tularemia in rodents and humans. The various diseases caused by these three genera, together with the vectors responsible for their spread to humans, are summarized in Table 29-1.

Metabolically, these organisms are facultative anaerobes which grow best on nutrient media enriched with blood, hematin, or catalase. Most members show a restricted fermentative capacity. Although they grow well when incubated at 37°C, they can also multiply at room temperature, when some species produce putative virulence factors that help to establish the pathogen within the tissues. Vaccines are of limited value and aggressive treatment with broad-spectrum antibiotics is required to control human infections.