Quantification of Brucella abortus population structure in a natural host

Authors: FIEBIG, ARETHA - Michigan State University; Vrentas, Catherine; LE, THIEN - Michigan State University; HUEBNER, MARIANNE - Michigan State University; Boggiatto, Paola; Olsen, Steven; CROSSON, SEAN - Michigan State University

Submitted to: Proceedings of the National Academy of Sciences (PNAS)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/28/2021
Publication Date: 3/16/2021
Citation: Fiebig, A., Vrentas, C.E., Le,T, Huebner, M., Boggiatto, P.M., Olsen, S.C., Crosson, S. 2021. Quantification of Brucella abortus population structure in a natural host. Proceedings of the National Academy of Sciences(PNAS). 118. https://doi.org/10.1073/pnas.2023500118.
DOI: https://doi.org/10.1073/pnas.2023500118

Interpretive Summary: Brucellosis is a disease in livestock that causes reproductive failure that can also cause clinical disease in humans. Development of intervention strategies to reduce disease in animal hosts is the most efficient way to reduce infection of humans. In this study we evaluated a Brucella abortus mutant library in which individual genes had been inactivated to identify genes that may play an essential role in establishing infection in cattle. Our data demonstrates that natural infection does cause a severe bottleneck in the number of strains that establish colonization in lymph nodes. This data provides information on how bacterial genes may play a role in the infectious process. The work will be of interest to researchers working on bacterial pathogens and personnel involved in control of brucellosis in livestock and wildlife reservoirs.

Technical Abstract: Cattle are natural hosts of the intracellular pathogen, Brucella abortus, which inflicts a significant burden of the health and reproduction of these important livestock. The genetic structure of the Brucella genus and the population dynamics of B. abortus in field settings are reasonably well described, but it is not known how infection of a natural host shapes the structure of B. abortus populations. We developed a Tn-himar mutant library containing approximately 10 to the 6 power uniquely barcoded B. abortus strains and used this resource to quantify colonization of cattle through a natural route of infection at the single strain level. Introducing 10 to the 8 power bacteria from this library to the right and left ocular conjunctiva of six animals resulted in expected levels of lymph node colonization. Strain diversity in these infected tissues relative to the infecting pool provided evidence for a severe population bottleneck between the ocular site of infection and the lymph nodes. Thus, cattle restrict the overwhelming majority of individual bacteria introduced via the conjunctiva at this infectious dose. Strains identified on the right and left side of each animal are distinct, which is consistent with stochastic colonization by a small number of strains through a narrow bottleneck. The genetic identity of strains present in retropharyngeal lymph nodes reflect the population observed in adjacent parotid lymph nodes, corroborating a model in which parotid and retropharyngeal nodes are colonized via a shared vascular route. We further observed a high level of strain heterogeneity between four dissected quadrants of the parotid nodes and large differences in strain abundance within each parotid quadrant. In many cases, less than 10 strains dominate the populations in each lymph node. These results support a model in which a small number of spatially isolated granulomas of varying sizes have arisen from unique individual founder cells at one-week post infection. Our experiments demonstrate the power of barcoded Tn-seq studies to quantify infection bottlenecks and to assess B. abortus population structure and colonization dynamics in the tissues of a bovine host.