Data from: Higher resource level promotes virulence in an environmentally transmitted bacterial fish pathogen

Diseases have become a primary constraint to sustainable aquaculture, but remarkably little attention has been paid to a broad class of pathogens: the opportunists. Opportunists often persist in the environment outside the host and their pathogenic features are influenced by changes in the environment. To test how environmental nutrient levels influence virulence, we used strains of Flavobacterium columnare, an environmentally transmitted fish pathogen, to infect rainbow trout and zebra fish in two different nutrient concentrations. To separate the effects of dose and nutrients, we used three infective doses and studied the growth of bacteria in vitro. High nutrient concentration promoted both the virulence and the outside-host growth of the pathogen, most notably in a low-virulence strain. The increase in virulence could not be exhaustively explained by the increased dose under higher nutrient supply, suggesting virulence factor activation. In aquaculture settings, accumulation of organic material in rearing units can locally increase water nutrient concentration and therefore increase disease risk as a response to elevated bacterial density and virulence factor activation. Our results highlight the role of increased nutrients in outside-host environment as a selective agent for higher virulence and faster evolutionary rate in opportunistic pathogens.病害已成为制约可持续水产养殖业发展的核心瓶颈之一，但目前针对一类广泛存在的病原体——机会致病菌（opportunists）的研究关注却极为匮乏。机会致病菌通常可在宿主外的环境中存活，其致病特性会随环境变化发生改变。为探究环境营养水平对致病菌毒力的影响，本研究以环境传播性鱼类病原体柱状黄杆菌（Flavobacterium columnare）的不同菌株为研究对象，在两种不同营养浓度条件下分别感染虹鳟与斑马鱼。为区分感染剂量与营养水平各自的影响，我们设置了三种感染剂量，并开展了细菌体外生长实验。高营养浓度可同时提升致病菌的毒力与宿主外增殖能力，这一效应在低毒力菌株中尤为显著。在高营养供给条件下，毒力的提升无法完全由感染剂量的增加所解释，这提示致病菌存在毒力因子激活现象。在水产养殖场景中，养殖单元内的有机物积累可局部提升水体营养浓度，进而通过提高细菌密度与激活毒力因子来增加病害发生风险。本研究结果凸显了宿主外环境中营养水平提升的作用：其可作为选择压力，推动机会致病菌向更高毒力与更快进化速率的方向演化。