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.

病原体物种通常由遗传上不同的菌株组成，它们可以在宿主中建立混合感染或合并感染。在合并感染中，病原体菌株之间的相互作用可能对其传播成功产生重要影响。我们使用蜱传细菌阿氏疏螺旋体（ Borrelia afzelii）作为多菌株病原体模型，这是欧洲莱姆病的最常见病因，以研究合并感染、菌株之间的竞争和菌株特异性传播成功之间的关系。Mus musculus小鼠被一种或两种B. afzelii菌株感染, 通过在小鼠身上喂食蜱虫来衡量菌株传播成功与否，并使用 qPCR 测量每种菌株在六个不同小鼠器官和蜱虫中的分布。共感染和竞争降低了两种菌株的组织感染流行率，并改变了它们在某些组织中的细菌丰度。共感染和竞争也降低了B. afzelii菌株从受感染宿主向饲养蜱的传播成功率。B. afzelii菌株在宿主组织中建立感染的能力与其向蜱虫载体的传播成功密切相关。我们的研究表明，宿主组织内病原体菌株之间的共感染和竞争可能对其传播成功产生重大影响。