Biovar-related differences apparent in the flea foregut colonization phenotype of distinct Yersinia pestis strains do not impact transmission efficiency

Athena Lemon,Viveka Vadyvaloo,Janelle Sagawa,Kameron Gravelle

doi:10.1186/s13071-020-04207-x

Abstract

BackgroundYersinia pestis is the flea-transmitted etiological agent of bubonic plague. Sylvatic plague consists of complex tripartite interactions between diverse flea and wild rodent species, and pathogen strains. Transmission by flea bite occurs primarily by the Y. pestis biofilm-mediated foregut blockage and regurgitation mechanism, which has been largely detailed by studies in the model interaction between Y. pestis KIM6+ and Xenopsylla cheopis. Here, we test if pathogen-specific traits influence this interaction by determining the dynamics of foregut blockage development in X. cheopis fleas among extant avirulent pCD1-Y. pestis strains, KIM6+ and CO92, belonging to distinct biovars, and a non-circulating mutant CO92 strain (CO92gly), restored for glycerol fermentation; a key biochemical difference between the two biovars.MethodsSeparate flea cohorts infected with distinct strains were evaluated for (i) blockage development, bacterial burdens and flea foregut blockage pathology, and (ii) for the number of bacteria transmitted by regurgitation during membrane feeding. Strain burdens per flea was determined for fleas co-infected with CO92 and KIM6+ strains at a ratio of 1:1.ResultsStrains KIM6+ and CO92 developed foregut blockage at similar rates and peak temporal incidences, but the CO92gly strain showed significantly greater blockage rates that peak earlier post-infection. The KIM6+ strain, however, exhibited a distinctive foregut pathology wherein bacterial colonization extended the length of the esophagus up to the feeding mouthparts in ~65% of blocked fleas; in contrast to 32% and 26%, respectively, in fleas blocked with CO92 and CO92gly. The proximity of KIM6+ to the flea mouthparts in blocked fleas did not result in higher regurgitative transmission efficiencies as all strains transmitted variable numbers of Y. pestis, albeit slightly lower for CO92gly. During competitive co-infection, strains KIM6+ and CO92 were equally fit maintaining equivalent infection proportions in fleas over time.ConclusionsWe demonstrate that disparate foregut blockage pathologies exhibited by distinct extant Y. pestis strain genotypes do not influence transmission efficiency from X. cheopis fleas. In fact, distinct extant Y. pestis genotypes maintain equivalently effective blockage and transmission efficiencies which is likely advantageous to maintaining continued successful plague spread and establishment of new plague foci.

Highlights

Yersinia pestis is the flea-transmitted etiological agent of bubonic plague
KIM6+ and CO92 infected X. cheopis fleas have similar blockage rates To determine if Y. pestis strain specific distinctions can be made in the interactions of Y. pestis and its prototype flea vector, X. cheopis, blockage rates and flea colonization dynamics were directly compared between CO92 and KIM6+ strains
To simultaneously determine the role of glycerol utilization in potential differences that may be uncovered in flea infection dynamics between CO92 and KIM6+, we tested a previously reported CO92 strain [32] that was restored in ability to metabolize glycerol

Summary

Introduction

Yersinia pestis is the flea-transmitted etiological agent of bubonic plague. Sylvatic plague consists of complex tripartite interactions between diverse flea and wild rodent species, and pathogen strains. The etiologic agent of bubonic plague, can be transmitted by a flea bite. In this model, the flea siphons blood into the midgut using muscles in its head that act as a peristaltic pump during feeding. When the flea acquires an infectious Y. pestis blood meal, the bacteria forms a biofilm in the PV that partially or completely obstructs function of this organ and blood passage to the midgut. This is referred to as biofilm-mediated blockage of the flea foregut [2]. High blockage rates are synonymous with effective transmission, and both partial and complete blockage can cause blockage-mediated regurgitative transmission of Y. pestis [2, 4]

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