BACKGROUND The holistic view of bacterial symbiosis, incorporating both host and microbial environment, constitutes a major conceptual shift in studies deciphering host-microbe interactions. Interactions between Steinernema entomopathogenic nematodes and their bacterial symbionts, Xenorhabdus, have long been considered monoxenic two partner associations responsible for the killing of the insects and therefore widely used in insect pest biocontrol. We investigated this "monoxenic paradigm" by profiling the microbiota of infective juveniles (IJs), the soil-dwelling form responsible for transmitting Steinernema-Xenorhabdus between insect hosts in the parasitic lifecycle. RESULTS Multigenic metabarcoding (16S and rpoB markers) showed that the bacterial community associated with laboratory-reared IJs from Steinernema carpocapsae, S. feltiae, S. glaseri and S. weiseri species consisted of several Proteobacteria. The association with Xenorhabdus was never monoxenic. We showed that the laboratory-reared IJs of S. carpocapsae bore a bacterial community composed of the core symbiont (Xenorhabdus nematophila) together with a frequently associated microbiota (FAM) consisting of about a dozen of Proteobacteria (Pseudomonas, Stenotrophomonas, Alcaligenes, Achromobacter, Pseudochrobactrum, Ochrobactrum, Brevundimonas, Deftia, etc.). We validated this set of bacteria by metabarcoding analysis on freshly sampled IJs from natural conditions. We isolated diverse bacterial taxa, validating the profile of the Steinernema FAM. We explored the functions of the FAM members potentially involved in the parasitic lifecycle of Steinernema. Two species, Pseudomonas protegens and P. chlororaphis, displayed entomopathogenic properties suggestive of a role in Steinernema virulence and membership of the Steinernema pathobiome. CONCLUSIONS Our study validates a shift from monoxenic paradigm to pathobiome view in the case of the Steinernema ecology. The microbial communities of low complexity associated with EPNs will permit future microbiota manipulation experiments to decipher overall microbiota functioning in the infectious process triggered by EPN in insects and, more generally, in EPN ecology.

中文翻译：

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昆虫病原线虫相关微生物群：从单线性范例到病原生物组。

背景技术细菌共生的整体观点，包括宿主和微生物环境，构成了解释宿主-微生物相互作用的研究的主要概念转变。长期以来，一直认为Steinernema昆虫病原线虫与其细菌共生体Xenorhabdus之间的相互作用是导致昆虫杀死的一氧化二氮伙伴，因此广泛用于害虫的生物防治。我们通过剖析感染性幼体（IJs）的微生物群来研究这种“一氧化二氮范式”，这是一种土壤寄居形式，负责在寄生生命周期中的昆虫宿主之间传播Steinernema-Xenorhabdus。结果多基因元条形码编码（16S和rpoB标记）显示，细菌群落与实验室培养的Steinernema carpocapsae，S。feeliae，glaseri和S. weiseri物种由几种变形杆菌组成。与Xenorhabdus的联系从来都不是一味的。我们发现，实验室饲养的炭疽链球菌具有一个细菌群落，该细菌群落由核心共生菌（线虫嗜线虫）和常见的微生物群（FAM）组成，该菌群由大约十二种变形杆菌（假单胞菌，嗜单核嗜单胞菌，产碱杆菌，无色杆菌，假ch皮，O骨，短绒毛纲，Deftia等）。我们通过对来自自然条件的新鲜采样IJ进行metabarcode分析来验证这组细菌。我们分离了不同的细菌类群，验证了Steinernema FAM的概况。我们探讨了可能参与Steinernema寄生生命周期的FAM成员的功能。两种假单胞菌蛋白和P. chlororaphis，显示的致病性暗示在Steinernema毒力和Steinernema病原体组成员中起作用。结论在Steinernema生态学的情况下，我们的研究证实了从一氧化二氮范式向病原生物组观点的转变。与EPNs相关的低复杂性微生物群落将使未来的微生物群操纵实验能够破译整个微生物群在由昆虫EPN引发的感染过程中，更普遍地在EPN生态学中的功能。