Data from: Bacterial cooperation causes systematic errors in pathogen risk assessment due to the failure of the independent action hypothesis

The Independent Action Hypothesis (IAH) states that pathogenic individuals (cells, spores, virus particles etc.) behave independently of each other, so that each has an independent probability of causing systemic infection or death. The IAH is not just of basic scientific interest; it forms the basis of our current estimates of infectious disease risk in humans. Despite the important role of the IAH in managing disease interventions for food and water-borne pathogens, experimental support for the IAH in bacterial pathogens is indirect at best. Moreover since the IAH was first proposed, cooperative behaviors have been discovered in a wide range of microorganisms, including many pathogens. A fundamental principle of cooperation is that the fitness of individuals is affected by the presence and behaviors of others, which is contrary to the assumption of independent action. In this paper, we test the IAH in Bacillus thuringiensis (B.t), a widely occurring insect pathogen that releases toxins that benefit others in the inoculum, infecting the diamondback moth, Plutella xylostella. By experimentally separating B.t. spores from their toxins, we demonstrate that the IAH fails because there is an interaction between toxin and spore effects on mortality, where the toxin effect is synergistic and cannot be accommodated by independence assumptions. Finally, we show that applying recommended IAH dose-response models to high dose data leads to systematic overestimation of mortality risks at low doses, due to the presence of synergistic pathogen interactions. Our results show that cooperative secretions can easily invalidate the IAH, and that such mechanistic details should be incorporated into pathogen risk analysis.独立作用假说（Independent Action Hypothesis，IAH）提出，致病个体（细胞、孢子、病毒颗粒等）的行为彼此独立，即每个个体引发系统性感染或死亡的概率互不相关。 该假说不仅具备基础科学研究价值，更是当前人类传染病风险评估的核心理论依据。尽管IAH在管理食源与水源传播病原体的疾病干预措施中扮演着关键角色，但目前针对细菌性病原体的IAH实验验证仅能提供间接支撑。 此外，自该假说首次提出以来，学界已在包括诸多致病菌在内的多种微生物中发现了合作行为。合作的核心原则为个体的适合度会受到其他个体的存在与行为的影响，这与独立作用假说的前提假设相悖。 在本研究中，我们针对苏云金芽孢杆菌（Bacillus thuringiensis，B.t）验证了IAH：该菌是一种广泛分布的昆虫致病菌，会释放可使接种物中其他个体获益的毒素，其感染宿主为小菜蛾（Plutella xylostella）。 我们通过实验分离B.t孢子与其毒素的作用效应，证实IAH并不成立——毒素与孢子对宿主死亡率存在交互效应，其中毒素效应呈协同作用，无法通过独立作用假设进行拟合。 最后我们发现，由于存在协同性的病原体交互作用，将推荐的IAH剂量-反应模型应用于高剂量数据时，会系统性高估低剂量下的死亡风险。 本研究结果表明，合作性分泌物质可轻易打破IAH的前提假设，相关机制细节应当被纳入病原体风险分析框架之中。