Transit through the Flea Vector Induces a Pretransmission Innate Immunity Resistance Phenotype in Yersinia pestis

Abstract

Yersinia pestis, the agent of plague, is transmitted to mammals by infected fleas. Y. pestis exhibits a distinct life stage in the flea, where it grows in the form of a cohesive biofilm that promotes transmission. After transmission, the temperature shift to 37°C induces many known virulence factors of Y. pestis that confer resistance to innate immunity. These factors are not produced in the low-temperature environment of the flea, however, suggesting that Y. pestis is vulnerable to the initial encounter with innate immune cells at the flea bite site. In this study, we used whole-genome microarrays to compare the Y. pestis in vivo transcriptome in infective fleas to in vitro transcriptomes in temperature-matched biofilm and planktonic cultures, and to the previously characterized in vivo gene expression profile in the rat bubo. In addition to genes involved in metabolic adaptation to the flea gut and biofilm formation, several genes with known or predicted roles in resistance to innate immunity and pathogenicity in the mammal were upregulated in the flea. Y. pestis from infected fleas were more resistant to phagocytosis by macrophages than in vitro-grown bacteria, in part attributable to a cluster of insecticidal-like toxin genes that were highly expressed only in the flea. Our results suggest that transit through the flea vector induces a phenotype that enhances survival and dissemination of Y. pestis after transmission to the mammalian host.

Author Summary

Bubonic plague cycles depend on the ability of Yersinia pestis to alternately infect two very different hosts—a mammal and a flea. Like any arthropod-borne pathogen, Y. pestis must sense host-specific environmental cues and regulate gene expression accordingly to produce a transmissible infection in the flea after being taken up in a blood meal, and again when it exits the flea and enters the mammal. We examined the Y. pestis phenotype at the point of transmission by in vivo gene expression analyses, the first description of the transcriptome of an arthropod-borne bacterium in its vector. In addition to genes associated with physiological adaptation to the flea gut, several Y. pestis virulence factors required for resistance to innate immunity and dissemination in the mammal were induced in the flea, suggesting that the arthropod life stage primes Y. pestis for successful infection of the mammal.