Pathogen species often consist of genetically distinct strains, which can establish mixed infections or coinfections in the host. In coinfections, interactions between pathogen strains can have important consequences for their transmission success. We used the tick-borne bacterium Borrelia afzelii, which is the most common cause of Lyme disease in Europe, as a model multi-strain pathogen to investigate the relationship between coinfection, competition between strains, and strain-specific transmission success. Mus musculus mice were infected with one or two strains of B. afzelii, strain transmission success was measured by feeding ticks on mice, and the distribution of each strain in six different mouse organs and the ticks was measured using qPCR. Coinfection and competition reduced the tissue infection prevalence of both strains and changed their bacterial abundance in some tissues. Coinfection and competition also reduced the transmission success of the B. afzelii strains from the infected hosts to feeding ticks. The ability of the B. afzelii strains to establish infection in the host tissues was strongly correlated with their transmission success to the tick vector. Our study demonstrates that coinfection and competition between pathogen strains inside the host tissues can have major consequences for their transmission success.

Biology Centre Institute of Parasitology Czech Academy of Sciences České Budějovice Czechia

Department of Veterinary Microbiology Western College of Veterinary Medicine University of Saskatchewan Saskatoon Canada

Faculty of Science University of South Bohemia České Budějovice Czechia

Laboratory of Ecology and Epidemiology of Parasites Institute of Biology University of Neuchâtel Neuchâtel Switzerland

Laboratory of Ecology and Evolution of Parasites Institute of Biology University of Neuchâtel Neuchâtel Switzerland

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