Insects are often involved in endosymbiosis, that is, the housing of symbiotic microbes within their tissues or within their cells. Endosymbionts are a major driving force in insects’ evolution, because they dramatically affect their host physiology and allow them to adapt to new niches, for example, by complementing their diet or by protecting them against pathogens. Endosymbiotic bacteria are, however, fastidious and therefore difficult to manipulate outside of their hosts, especially intracellular species. The coevolution between hosts and endosymbionts leads to alterations in the genomes of endosymbionts, limiting their ability to cope with changing environments. Consequently, few insect endosymbionts are culturable in vitro and genetically tractable, making functional genetics studies impracticable on most endosymbiotic bacteria. However, recently, major progress has been made in manipulating several intracellular endosymbiont species in vitro, leading to astonishing discoveries on their physiology and the way they interact with their host. This review establishes a comprehensive picture of the in vitro tractability of insect endosymbiotic bacteria and addresses the reason why most species are not culturable. By compiling and discussing the latest developments in the design of custom media and genetic manipulation protocols, it aims at providing new leads to expand the range of tractable endosymbionts and foster genetic research on these models.

中文翻译：

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培养不可生长的细菌：昆虫共生体培养的概述和展望

昆虫经常参与内共生，即共生微生物在其组织或细胞内的居住。内共生体是昆虫进化的主要驱动力，因为它们极大地影响宿主的生理机能，并使它们能够适应新的生态位，例如，通过补充它们的饮食或保护它们免受病原体的侵害。然而，内共生细菌非常挑剔，因此很难在宿主之外进行操纵，尤其是细胞内物种。宿主和内共生体之间的共同进化导致内共生体基因组的改变，限制了它们应对不断变化的环境的能力。因此，很少有昆虫内共生菌可以在体外培养并且在遗传上易于处理，这使得对大多数内共生细菌进行功能遗传学研究变得不切实际。然而，最近，在体外操纵几种细胞内内共生体物种方面取得了重大进展，在它们的生理学以及它们与宿主相互作用的方式方面取得了惊人的发现。这篇综述全面介绍了昆虫内共生细菌的体外易处理性，并解决了大多数物种不可培养的原因。通过汇编和讨论定制培养基和基因操作方案设计的最新进展，旨在提供新的线索以扩大易处理的内共生体的范围并促进对这些模型的遗传研究。