Data from: Rich resource environment of fish farms facilitates phenotypic variation and virulence in an opportunistic fish pathogen

Phenotypic variation is suggested to facilitate the persistence of environmentally growing pathogens under environmental change. Here we hypothesized that the intensive farming environment induces higher phenotypic variation in microbial pathogens than natural environment, because of high stochasticity for growth and stronger survival selection compared to the natural environment. We tested the hypothesis with an opportunistic fish pathogen Flavobacterium columnare isolated either from fish farms or from natural waters. We measured growth parameters of two morphotypes from all isolates in different resource concentrations and two temperatures relevant for the occurrence of disease epidemics at farms and tested their virulence using a zebrafish (Danio rerio) infection model. According to our hypothesis, isolates originating from the fish farms had higher phenotypic variation in growth between the morphotypes than the isolates from natural waters. The difference was more pronounced in higher resource concentrations and the higher temperature, suggesting that phenotypic variation is driven by the exploitation of increased outside-host resources at farms. Phenotypic variation of virulence was not observed based on isolate origin but only based on morphotype. However, when in contact with the larger fish, the less virulent morphotype of some of the isolates also had high virulence. As the less virulent morphotype also had higher growth rate in outside-host resources, the results suggest that both morphotypes can contribute to F. columnare epidemics at fish farms, especially with current prospects of warming temperatures. Our results suggest that higher phenotypic variation per se does not lead to higher virulence, but that environmental conditions at fish farms could select isolates with high phenotypic variation in bacterial population and hence affect evolution in F. columnare at fish farms. Our results highlight the multifaceted effects of human-induced environmental alterations in shaping epidemiology and evolution in microbial pathogens.

表型变异（phenotypic variation）被认为可助力环境变化背景下在环境中增殖的病原体的存续。本研究提出假说：相较于自然环境，集约化养殖环境会使微生物病原体产生更高水平的表型变异，这是因为相较于自然环境，养殖环境中病原体生长的随机性更强，且生存选择压力更高。我们以从养鱼场及自然水域分离得到的条件致病性鱼类病原体柱状黄杆菌（Flavobacterium columnare）为研究对象，对该假说进行验证。我们针对所有分离株的两种形态型（morphotype），在不同资源浓度及与渔场疾病暴发相关的两种温度条件下，测定了其生长参数，并利用斑马鱼（Danio rerio）感染模型检测了它们的毒力。正如假说所预测，相较于来自自然水域的分离株，来自养鱼场的分离株在不同形态型间的生长表型变异水平更高。该差异在高资源浓度及较高温度条件下更为显著，表明表型变异是由渔场环境中宿主外资源的可利用度提升所驱动的。未观察到基于分离株来源的毒力表型变异，仅存在基于形态型的毒力表型差异。然而，当与体型更大的鱼类接触时，部分分离株中低毒力的形态型也可表现出高毒力。由于低毒力形态型在宿主外资源中的生长速率也更高，本研究结果表明，两种形态型均可参与柱状黄杆菌（Flavobacterium columnare）在养鱼场的疫病流行，在当前气候变暖的背景下尤为如此。本研究结果显示，高水平的表型变异本身并不会直接提升毒力，但养鱼场的环境条件可筛选出种群内表型变异程度更高的分离株，进而影响柱状黄杆菌（Flavobacterium columnare）在养鱼场中的演化进程。本研究结果凸显了人类活动诱导的环境改变在调控微生物病原体的流行病学特征与演化过程中所发挥的多维度作用。