Etudiantre en master

Laboratoire d'écologie et évolution des parasites
Institut de biologie
Rue Emile-Argand 11
2000 Neuchâtel

marika.defabritiis@unine.ch
 

For vector-borne pathogens, a critical step in their life history is the transmission to the next host or to a vector. In the system I will study, the bacteria must pass from the tick vector to the vertebrate host and vice versa.
Borrelia afzelii is a bacterial pathogen that causes Lyme borreliosis in humans. Lyme borreliosis is the most common tick-borne disease in northern Europe. The main vector is the hard tick Ixodes ricinus and the vertebrate reservoir hosts include rodents and insectivores. Borrelia afzelii is composed of different strains that can be differentiated by the highly polymorphic ospC gene (Wang et al., 1999). The ospC gene codes for outer surface protein C (OspC), and this lipoprotein triggers a strong immune response in the rodent host. Competition between strains of B. afzelii in the rodent host has been shown in previous studies (Strandh & Råberg, 2015, Råberg et al., 2017). Mixed infections in rodents are very common in nature, 54% of wild rodents carry multiple strains, and on average one host carries 2.17 strains (Andersson et al. 2013, Strandh & Råberg, 2015).
The aim of my Master project is to test whether there is competition between two strains of Borrelia afzelii (A3 and A10) in different organs of laboratory mice (Mus musculus). Mice had been previously infected with one of the two strains or with both strains (co-infection treatment). I will measure the abundance of the spirochetes of each strain in the doubly infected mice and compare it to the singly infected mice. The hypothesis is that there will be competition inside the mouse organs. I predict that the strain-specific spirochete load will be lower in the organs of the co-infected mice compared to the mice infected with single strains. I will also determine the strain-specific abundance in different tissues to see if a map of the strains can be developed in the mouse.
For the second part of my Master project, I will study the relationship between inter-strain competition in the mice and the strain-specific transmission success from the mouse to the ticks. I predict that the strain that is suffering from competition inside the mouse will have reduced mouse-to-tick transmission success. In the ticks that fed on the co-infected mice, I expect to find the strain that won the competition inside the mouse.

References:
Andersson, M., Scherman, K., & Råberg, L. (2013). Multiple-strain infections of Borrelia afzelii: a role for within-host interactions in the maintenance of antigenic diversity?. The American Naturalist, 181(4), 545-554.
Råberg, L., Hagström, Å., Andersson, M., Bartkova, S., Scherman, K., Strandh, M., & Tschirren, B. (2017). Evolution of antigenic diversity in the tick‐transmitted bacterium Borrelia afzelii: a role for host specialization?. Journal of Evolutionary Biology, 30(5), 1034-1041.
Strandh, M., & Råberg, L. (2015). Within-host competition between Borrelia afzelii ospC strains in wild hosts as revealed by massively parallel amplicon sequencing. Phil. Trans. R. Soc. B, 370(1675), 20140293.
Wang, N., Dykhuizen, D. E., Qiu, W., Dunn, J. J., Bosler, E. M., & Luft, B. J. (1999). Genetic diversity of ospC in a local population of Borrelia burgdorferi sensu stricto. Genetics, 151(1), 15-30.