Department of Ecology & Evolutionary Biology, University of Michigan, Ann Arbor, Michigan, USA.
mBio. 2024 Oct 16;15(10):e0058324. doi: 10.1128/mbio.00583-24. Epub 2024 Aug 28.
Infection outcomes can be strongly context dependent, shifting a host-symbiont relationship along a parasitism-mutualism continuum. Numerous studies show that under stressful conditions, symbionts that are typically mutualistic can become parasitic. The reverse possibility, a parasite becoming mutualistic, has received much less study. We investigated whether the parasitic microsporidium can become beneficial for its host in the presence of the more virulent fungal pathogen . We found that, even though infection with reduces the frequency of penetration of spores into the host body cavity, it does not improve the survival or reproduction of the host; conversely, coinfection increased the mortality of . This shorter lifespan of coinfected hosts disrupted the life cycle of , greatly reducing its fitness. Thus, coinfection with both pathogens was detrimental to the host at the individual level but might be beneficial for the host population as a result of greatly reduced production of spores. If so, this would mean that outbreaks should delay or prevent outbreaks. In support of this, in an analysis of dynamics of naturally occurring outbreaks in two lakes where these pathogens co-occur, we found a time lag in occurrence between and , with epidemics only occurring after the collapse of epidemics. Thus, these results suggest that the interaction between co-occurring symbionts, and the net impact of a symbiont on a host, might be qualitatively different at different scales.IMPORTANCEUnderstanding the factors that modify infection probability and virulence is crucial for identifying the drivers of infection outbreaks and modeling disease epidemic progression, and increases our ability to control diseases and reduce the harm they cause. One factor that can strongly influence infection probability and virulence is the presence of other pathogens. However, while coexposures and coinfections are incredibly common, we still have only a limited understanding of how pathogen interactions alter infection outcomes or whether their impacts are scale dependent. We used a system of one host and two pathogens to show that sequential coinfection can have a tremendous impact on the host and the infecting pathogens and that the outcome of (co-)infection can be negative or positive depending on the focal organization level.
感染结果可能强烈依赖于环境，使宿主-共生体关系在寄生-共生连续体上发生转变。许多研究表明，在压力条件下，原本互利共生的共生体可能会变成寄生的。寄生虫变成互利共生的可能性则相反，受到的研究要少得多。我们研究了在更具毒性的真菌病原体存在的情况下，寄生性微孢子虫是否会对其宿主变得有益。我们发现，尽管感染 会降低 孢子穿透宿主体腔的频率，但它并不会改善宿主的生存或繁殖；相反，共感染会增加 的死亡率。这种共感染宿主的较短寿命会破坏 的生命周期，极大地降低其适应性。因此，两种病原体的共感染对个体宿主是有害的，但由于 孢子的产生大大减少，这可能对宿主种群有利。如果是这样，这意味着 爆发应该会延迟或阻止 爆发。在对这两种病原体共同存在的两个湖泊中自然发生的疫情动态进行的分析中，我们发现 和 的发生存在时间滞后，只有在 疫情崩溃后才会出现 疫情。因此，这些结果表明，共生体的共存以及共生体对宿主的净影响，在不同的尺度上可能具有本质上的不同。
重要性
了解改变感染概率和毒力的因素对于确定感染爆发的驱动因素和模拟疾病流行进展至关重要，并且提高了我们控制疾病和减少其造成危害的能力。一个可以强烈影响感染概率和毒力的因素是其他病原体的存在。然而，虽然共暴露和共感染非常常见，但我们对病原体相互作用如何改变感染结果或它们的影响是否取决于规模仍知之甚少。我们使用一个宿主和两种病原体的系统表明，顺序共感染会对宿主和感染病原体产生巨大影响，并且（共）感染的结果可能是负面的，也可能是正面的，具体取决于焦点组织水平。
