Progression of primary pneumonic plague: a mouse model of infection, pathology, and bacterial transcriptional activity

Abstract

Although pneumonic plague is the deadliest manifestation of disease caused by the bacterium Yersinia pestis, there is surprisingly little information on the cellular and molecular mechanisms responsible for Y. pestis-triggered pathology in the lung. Therefore, to understand the progression of this unique disease, we characterized an intranasal mouse model of primary pneumonic plague. Mice succumbed to a purulent multifocal severe exudative bronchopneumonia that closely resembles the disease observed in humans. Analyses revealed a strikingly biphasic syndrome, in which the infection begins with an antiinflammatory state in the first 24-36 h that rapidly progresses to a highly proinflammatory state by 48 h and death by 3 days. To assess the adaptation of Y. pestis to a mammalian environment, we used DNA microarray technology to analyze the transcriptional responses of the bacteria during interaction with the mouse lung. Included among the genes up-regulated in vivo are those comprising the yop-ysc type III secretion system and genes contained within the chromosomal pigmentation locus, validating the use of this technology to identify loci essential to the virulence of Y. pestis.

Fig. 1.

Kinetics of infection with Y. pestis strain CO92 or CO92 pCD1^-. Bacteria (1 × 10⁴) were introduced intranasally into C57BL/6 mice, and cfu per organ in the lungs (A and C) and spleen (B and D) were determined every 12 h for 3 days (CO92, A and B) or every 24 h for 4 days (CO92 pCD1^-, C and D). The limit of detection is indicated by a dashed line. Each point indicates cfu recovered from a single animal. Symbols below the limit of detection represent mice that survived but did not have detectable numbers of bacteria; “X” indicates mice that succumbed to the infection. A solid line indicates the median of cfu recovered.

Fig. 2.

Histology of lungs after intranasal infection with Y. pestis strain CO92. Lungs were inflated and fixed with 10% formalin, and 5-μm sections were stained with hematoxylin/eosin. Lungs were examined from uninfected mice or mice infected with 1 × 10⁴ cfu Y. pestis for 24, 48, or 72 h. (Scale bars, 50 μm.)

Fig. 3.

Functional classification of differentially regulated Y. pestis genes during primary pneumonic plague. Genes differentially regulated (48-h lungs vs. 37°C BHI broth) 2-fold or more in both RNA amplifications were divided into 20 categories based on the CO92 chromosomal annotation; genes differentially regulated on the three plasmids are indicated separately.