Solitary bees are subject to a variety of pressures that cause severe population declines. Currently, habitat loss, temperature shifts, agrochemical exposure, and new parasites are identified as major threats. However, knowledge about detrimental bacteria is scarce, although they may disturb natural microbiomes, disturb nest environments, or harm the larvae directly. To address this gap, we investigated 12 Osmia bicornis nests with deceased larvae and 31 nests with healthy larvae from the same localities in a 16S ribosomal RNA (rRNA) gene metabarcoding study. We sampled larvae, pollen provisions, and nest material and then contrasted bacterial community composition and diversity in healthy and deceased nests. Microbiomes of pollen provisions and larvae showed similarities for healthy larvae, whilst this was not the case for deceased individuals. We identified three bacterial taxa assigned to Paenibacillus sp. (closely related to P. pabuli/amylolyticus/xylanexedens), Sporosarcina sp., and Bacillus sp. as indicative for bacterial communities of deceased larvae, as well as Lactobacillus for corresponding pollen provisions. Furthermore, we performed a provisioning experiment, where we fed larvae with untreated and sterilized pollens, as well as sterilized pollens inoculated with a Bacillus sp. isolate from a deceased larva. Untreated larval microbiomes were consistent with that of the pollen provided. Sterilized pollen alone did not lead to acute mortality, while no microbiome was recoverable from the larvae. In the inoculation treatment, we observed that larval microbiomes were dominated by the seeded bacterium, which resulted in enhanced mortality. These results support that larval microbiomes are strongly determined by the pollen provisions. Further, they underline the need for further investigation of the impact of detrimental bacterial acquired via pollens and potential buffering by a diverse pollen provision microbiome in solitary bees.

中文翻译：

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红梅森蜂幼虫由于与进口花粉进行微生物组交换而对细菌威胁的易感性。


独居蜜蜂面临各种压力，导致种群数量严重下降。目前，栖息地丧失、温度变化、农用化学品暴露和新寄生虫被认为是主要威胁。然而，关于有害细菌的知识却很少，尽管它们可能会扰乱自然微生物群、扰乱巢穴环境或直接伤害幼虫。为了解决这一差距，我们在 16S 核糖体 RNA (rRNA) 基因元条形码研究中调查了来自同一地点的 12 个含有死亡幼虫的双角壁蜂巢和 31 个含有健康幼虫的巢。我们对幼虫、花粉和巢材料进行了采样，然后对比了健康和死亡巢中的细菌群落组成和多样性。花粉和幼虫的微生物组与健康幼虫相似，而死亡个体的情况并非如此。我们确定了属于类芽孢杆菌属的三个细菌分类单元。 （与P. pabuli / amylolyticus / xylanexedens密切相关）、 Sporosarcina sp. 和Bacillus sp.。作为死亡幼虫细菌群落的指示，以及相应花粉供应的乳酸菌。此外，我们还进行了一项供应实验，用未经处理和灭菌的花粉以及接种芽孢杆菌的灭菌花粉喂养幼虫。从死亡幼虫中分离出来。未经处理的幼虫微生物组与提供的花粉的微生物组一致。单独灭菌的花粉不会导致急性死亡，同时也无法从幼虫中恢复微生物组。 在接种处理中，我们观察到幼虫微生物组以种子细菌为主，这导致死亡率增加。这些结果支持幼虫微生物群在很大程度上是由花粉供给决定的。此外，他们强调需要进一步研究通过花粉获得的有害细菌的影响以及独居蜜蜂中多种花粉提供微生物组的潜在缓冲作用。