Dissemination of a Highly Virulent Pathogen: Tracking The Early Events That Define Infection

Abstract

The series of events that occurs immediately after pathogen entrance into the body is largely speculative. Key aspects of these events are pathogen dissemination and pathogen interactions with the immune response as the invader moves into deeper tissues. We sought to define major events that occur early during infection of a highly virulent pathogen. To this end, we tracked early dissemination of Yersinia pestis, a highly pathogenic bacterium that causes bubonic plague in mammals. Specifically, we addressed two fundamental questions: (1) do the bacteria encounter barriers in disseminating to draining lymph nodes (LN), and (2) what mechanism does this nonmotile bacterium use to reach the LN compartment, as the prevailing model predicts trafficking in association with host cells. Infection was followed through microscopy imaging in addition to assessing bacterial population dynamics during dissemination from the skin. We found and characterized an unexpected bottleneck that severely restricts bacterial dissemination to LNs. The bacteria that do not pass through this bottleneck are confined to the skin, where large numbers of neutrophils arrive and efficiently control bacterial proliferation. Notably, bottleneck formation is route dependent, as it is abrogated after subcutaneous inoculation. Using a combination of approaches, including microscopy imaging, we tested the prevailing model of bacterial dissemination from the skin into LNs and found no evidence of involvement of migrating phagocytes in dissemination. Thus, early stages of infection are defined by a bottleneck that restricts bacterial dissemination and by neutrophil-dependent control of bacterial proliferation in the skin. Furthermore, and as opposed to current models, our data indicate an intracellular stage is not required by Y. pestis to disseminate from the skin to draining LNs. Because our findings address events that occur during early encounters of pathogen with the immune response, this work can inform efforts to prevent or control infection.

Influenza A Virus Coinfection through Transmission Can Support High Levels of Reassortment

“…For this reason, any reductions in diversity seen are most likely due to bottleneck events or possibly to guinea pig-adaptive drift mutations arising in a wt or var gene segment. Intrahost bottlenecks are more commonly associated with pathogens that spread across a physical barrier, such as the mucosa, rather than those that replicate within one tissue (45,46). If indeed the losses of diversity apparent in our data sets are due to bottlenecks rather than adaptive evolution, these bottlenecks could arise if virus replication takes place within discrete foci and clearance of virus from those foci occurs in a temporally heterogeneous manner.…”

Comparison of Models for Bubonic Plague Reveals Unique Pathogen Adaptations to the Dermis

“…The ⌬psaA strain was constructed using the Red recombination system, as described elsewhere (20,21). The procedures employed to construct the isogenic set of 10 tagged strains used in the dissemination assays were previously described (11,22). These 10 Y. pestis tagged strains have growth and virulence characteristics similar to those of untagged bacteria (11).…”

“…Bubonic plague is the most prevalent form of the disease and occurs after bacterial inoculation into the skin, typically by a flea vector (10). From the skin, Y. pestis disseminates to draining lymph nodes (LNs) via lymphatic vessels (11) and then to deeper organs through the bloodstream (12). Systemic dissemination results in septic shock, severe organ failure, and death.…”

“…In this study, we further characterize the i.d. model we used in a previous study (11) and compare i.d. and s.c. inoculations to investigate whether infection model impacts progression of bubonic plague.…”

Identification of host factors required for Yersinia pestis macrophage intracellular survival and their impact on vacuole maturation, acidification and trafficking.

“…During this transitional period, Y. pestis is highly susceptible to phagocytosis by macrophages and neutrophils [51,54]. Initial colonization of Y. pestis induces a rapid and early influx of neutrophils to the site of infection [51,184]. Upon phagocytosis by neutrophils, Y. pestis is readily killed by these professional phagocytes [53,58,80].…”