Data from: The index case is not enough: variation among individuals, groups, and social networks modify bacterial transmission dynamics

1.The traits of the index case of an infectious disease outbreak, and the circumstances for their etiology, potentially influence the trajectory of transmission dynamics. However, these dynamics likely also depend on the traits of the individuals with whom the index case interacts.

2.We used the social spider Stegodyphus dumicola to test how the traits of the index case, group phenotypic composition, and group size interact to facilitate the transmission of a GFP-labeled cuticular bacterium. We also compared bacterial transmission across experimentally generated “daisy-chain” versus “star” networks of social interactions. Finally, we compared social network structure across groups of different sizes.

3.Groups of 10 spiders experienced more bacterial transmission events compared to groups of 30 spiders, regardless of groups’ behavioral composition. Groups containing only one bold spider experienced the lowest levels of bacterial transmission regardless of group size. We found no evidence for the traits of the index case influencing any transmission dynamics. In a second experiment, bacteria were transmitted to more individuals in experimentally induced star networks than in daisy-chains, on which transmission never exceeded three steps. In both experimental network types, transmission success depended jointly on the behavioral traits of the interacting individuals, however, the behavioral traits of the index case were only important for transmission on star networks.

4.Larger social groups exhibited lower interaction density (i.e. had a low ratio of observed to possible connections) and were more modular, i.e., they had more connections between nodes within a sub-group and fewer connections across sub-groups. Thus, larger groups may restrict transmission by forming fewer interactions and by isolating sub-groups that interacted with the index case.

5.These findings suggest that accounting for the traits of single exposed hosts has less power in predicting transmission dynamics compared to the larger-scale factors of the social groups in which they reside. Factors like group size and phenotypic composition appear to alter social interaction patterns, which leads to differential transmission of microbes.

1. 传染病暴发的指示病例（index case）的性状及其病因学（etiology）相关背景，或可影响传播动力学（transmission dynamics）的发展轨迹。然而此类传播动力学大概率同时取决于该指示病例所接触个体的性状。
2. 本研究以社交蜘蛛杜氏丝蛛（Stegodyphus dumicola）为实验模型，探究指示病例的性状、种群表型组成及种群规模三者的交互作用对GFP标记体表细菌（GFP-labeled cuticular bacterium）传播的促进效应。此外，我们还对比了实验构建的“链式（daisy-chain）”与“星型（star）”社交互动网络间的细菌传播差异。最后，我们对不同规模种群的社交网络结构进行了比较分析。
3. 无论种群的行为组成如何，10只蜘蛛组成的种群其细菌传播事件发生频次均高于30只蜘蛛的种群。仅包含1只大胆型蜘蛛的种群，无论种群规模大小，其细菌传播水平均为最低。本研究未发现指示病例的性状对任何传播动力学存在影响的证据。在第二项实验中，实验诱导的星型网络内受细菌感染的个体数量多于链式网络，且链式网络内的传播链长度从未超过3个步骤。在两类实验网络中，传播成功率均由互动个体的行为性状共同决定；不过，仅在星型网络中，指示病例的行为性状才会对传播产生影响。
4. 规模更大的社交种群展现出更低的交互密度（interaction density）——即实际连接数与潜在连接数的比值更低——且模块化程度更高，具体表现为亚组（sub-group）内部节点间的连接更多，而亚组间的连接更少。由此可见，规模更大的种群可通过减少个体间互动、隔离与指示病例存在接触的亚组，来限制微生物的传播。
5. 上述研究结果表明，相较于宿主所处社交种群的宏观层面因素，仅关注单个暴露宿主的性状，其预测传播动力学的能力相对有限。诸如种群规模与表型组成这类因素，似乎会改变社交互动模式，进而导致微生物传播出现差异。