Eutrophication of aquatic ecosystems is associated with an increased risk of pathogen infection via increased pathogen growth and host exposure via increased pathogen doses. Here we studied the effect of nutrients on the virulence of an opportunistic bacterial pathogen of fish, Flavobacterium columnare, in challenge experiments with rainbow trout fingerlings. We hypothesized that removing all nutrients by washing the bacteria would reduce virulence as compared to unwashed bacteria, but adding nutrients to the tank water would increase the virulence of the bacterium. Nutrient addition and increase in bacterial dose increased virulence for both unwashed and washed bacteria. For unwashed bacteria, the addition of nutrients reduced the survival probability of fish challenged with low bacterial doses more than for fish challenged with higher bacterial doses, suggesting activation of bacterial virulence factors. Washing and centrifugation reduced viable bacterial counts, and the addition of washed bacteria alone did not lead to fish mortality. However, a small addition of nutrient medium, 0.05% of the total water volume, added separately to the fish container, restored the virulence of the washed bacteria. Our results show that human-induced eutrophication could trigger epidemics of aquatic pathogens at the limits of their survival and affect their ecology and evolution by altering the dynamics between strains that differ in their growth characteristics.

中文翻译：

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营养富集增加了机会性环境病原体的毒力，在低细菌剂量下效果更大

水生生态系统的富营养化与病原体生长增加和病原体剂量增加导致宿主暴露增加病原体感染风险相关。在这里，我们在虹鳟鱼种的攻击实验中研究了营养物质对鱼类机会性细菌病原体柱状黄杆菌毒力的影响。我们假设，与未清洗的细菌相比，通过清洗细菌去除所有营养物质会降低细菌的毒力，但向水箱水中添加营养物质会增加细菌的毒力。营养物的添加和细菌剂量的增加增加了未洗涤和洗涤细菌的毒力。对于未清洗的细菌，添加营养物质降低了接受低细菌剂量挑战的鱼的生存概率，而不是接受高细菌剂量挑战的鱼，这表明细菌毒力因子被激活。洗涤和离心减少了活细菌数量，并且单独添加洗涤的细菌不会导致鱼类死亡。然而，单独添加到鱼容器中的少量营养培养基（总水量的0.05%）恢复了洗涤后的细菌的毒力。我们的研究结果表明，人类引起的富营养化可能会在其生存极限下引发水生病原体的流行，并通过改变生长特性不同的菌株之间的动态来影响其生态和进化。