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）指出，致病个体（包括细胞、孢子、病毒颗粒等）彼此间独立行动，即每个个体引发全身感染或致死的概率均相互独立。IAH不仅具有基础科学研究价值，更是当前人类传染病风险评估的核心依据。尽管IAH在食源及水源性病原体的疾病防控干预中发挥着关键作用，但针对细菌性病原体的IAH实验验证充其量仅为间接证据。此外，自IAH首次被提出以来，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。合作行为的核心原则之一，是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独立行动假设相悖。在本研究中，我们以苏云金芽孢杆菌（Bacillus thuringiensis, 简称B.t）为研究对象验证IAH——这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，并可感染小菜蛾（Plutella xylostella）。我们通过实验将B.t孢子与其毒素分离，结果证实IAH并不成立：毒素与孢子对宿主死亡率存在交互作用，其中毒素效应呈现协同性，无法通过独立假设进行拟合。最后，我们发现，由于病原体间存在协同交互作用，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率风险。本研究结果表明，合作分泌行为可轻易推翻IAH的假设，相关机制细节应当被纳入病原体风险分析框架之中。