Transmission of Yersinia pestis by Fleas: Molecular Mechanisms

National Institute Of Allergy And Infectious Diseases

Investigators

Linked publications & trials

Abstract

Plague is a zoonosis that is present in wild rodent populations worldwide and is transmitted primarily by fleas. Yersinia pestis, the plague bacillus, is unique among the enteric group of gram-negative bacteria in having adopted an arthropod-borne route of transmission. Y. pestis has evolved in such a way as to be transmitted during the brief encounter between a feeding flea and a host. A transmissible infection primarily depends on the ability of Y. pestis to grow in the flea as a biofilm that is embedded in a complex extracellular matrix. Bacteria in the biofilm phenotype are deposited into the dermis together with flea saliva, elements which cannot be satisfactorily mimicked by needle- injection of Y. pestis from laboratory cultures. This project focuses on the interactions of Y. pestis with its flea vector that lead to transmission. One goal is to identify and determine the function of Y. pestis genes that mediate flea-borne transmission and the initial encounter with the host innate immune system at the infection site in the skin. A second goal is to characterize the flea response to infection, including the flea immune response and how Y. pestis resists it. Detailed understanding of the bacterial-flea interaction may lead to novel strategies to interrupt the transmission cycle. We have also established systems to compare the relative importance of the two modes of transmission and the relative vector competence of different flea species. Our studies of flea vector competence and vectorial capacity will be useful to develop more realistic mathematical modeling of the epidemiology of plague transmission and the conditions that lead to plague epizootics. During FY2021, we completed four projects, one of which is described in the Scientific Advances section below. The following three are in various stages of data analysis, manuscript preparation and submission for publication. 1. We monitored infection status and transmission efficiency of individual fleas at different times after infection. The data were used to estimate values for important parameters such as the probability of flea vectors developing a transmissible infection after feeding on a bacteremic host and the transmission efficiency during a four-week period after infection. Limited data are currently available for these values, which are needed for understanding plague epidemiology. In collaboration with Dr. Angela Luis at the University of Montana, we developed mathematical models to better comprehend the key conditions that give rise to periodic plague epizootics, and used our experimentally derived data in these models to compare the relative importance of biofilm-independent (early phase) and biofilm-dependent transmission mechanisms. 2. With the help of colleagues at the USGS, we evaluated the vector competence of the prairie dog flea Oropsylla hirsuta. Prairie dog colonies are subject to periodic explosive plague epizootics which pose a public danger to rural communities and hinder efforts to reintroduce the black-footed ferret, an endangered species. This study applied standardized methods that we developed to more rigorously quantify vector competence parameters (broadly including infectivity, flea foregut blockage rate, transmission rate, and transmission efficiency) throughout a 4-week period following a single infectious blood meal). O. hirsuta had been claimed to rarely become blocked or transmit beyond the early phase, and recently published models argued that early-phase transmission was the driving force behind plague epizootics in those rodents. However, we found that O. hirsuta proved to be a much more efficient plague vector than previously recognized. Reliable vector competence data regarding will enable more realistic modeling of epizootiologic/epidemiologic scenarios in the western U.S. 3. Another study completed in FY2021 explored the role of a Y. pestis phospholipase D enzyme that we previously showed was required for bacterial survival in the flea digestive tract. We found that this requirement is mammalian blood-source dependent. The enzyme is essential if fleas feed on infected mouse or human blood, but not if rat blood is used for the infectious blood meal. Thus, acquisition of this plasmid-encoded gene served to expand the host range of Y. pestis from rats to many other animals. During FY2021 work progressed on four other ongoing studies. 1. With Dr. Jose Ribeiro and Dr. Stephen Lu (a shared Rocky-Beth postdoc) of the LMVR, we are following up a characterization of proteins produced by midgut tissue and salivary glands of the rat flea, X. cheopis, to blood feeding and infection with Y. pestis. Two novel flea salivary gland peptides previously identified were functionally characterized as being inhibitors of mammalian thrombin and thus potent anticoagulants. 2. We are using proteomics and lipidomics to characterize the biofilm phenotype of Y. pestis collected from infected fleas, and the predominant biomolecules of the extracellular matrix (both bacterial- and flea-derived) of the biofilm that Y. pestis forms in the flea foregut. This will provide insight into the mechanisms by which Y. pestis produces a transmissible infection in its arthropod vector, and reveal the antigenic makeup of the bacteria as they are first encountered by the mammalian immune system after transmission from the flea. 3. In collaboration with the RML Genomics Unit, we have initiated a project to sequence the genome of the rat flea X. cheopis, the principal vector of plague to humans. 4. We have successfully established a laboratory colony of the human body louse Pediculus humanus. This ectoparasite has been proposed to have been a vector of human-to- human plague transmission during the Black Death pandemic, and our objective is to systematically evaluate that hypothesis experimentally using the methods we have developed to quantitate flea-borne transmission efficiency.