Department of Microbiology, The ADA Forsyth Institute, Boston, MA 02142, United States.
Department of Oral Medicine, Infection, and Immunity, Harvard School of Dental Medicine, Boston, MA 02115, United States.
ISME J. 2024 Jan 8;18(1). doi: 10.1093/ismejo/wrad034.
Saccharibacteria (formerly TM7) are a group of widespread and genetically diverse ultrasmall bacteria with highly reduced genomes that belong to Candidate Phyla Radiation, a large monophyletic lineage with poorly understood biology. Nanosynbacter lyticus type strain TM7x is the first Saccharibacteria member isolated from the human oral microbiome. With restrained metabolic capacities, TM7x lives on the surface of, and forms an obligate episymbiotic relationship with its bacterial host, Schaalia odontolytica strain XH001. The symbiosis allows TM7x to propagate but presents a burden to host bacteria by inducing stress response. Here, we employed super-resolution fluorescence imaging to investigate the physical association between TM7x and XH001. We showed that the binding with TM7x led to a substantial alteration in the membrane fluidity of XH001. We also revealed the formation of intracellular lipid droplets in XH001 when forming episymbiosis with TM7x, a feature that has not been reported in oral bacteria. The TM7x-induced lipid droplets accumulation in XH001 was confirmed by label-free Raman spectroscopy, which also unveiled additional phenotypical features when XH001 cells are physically associated with TM7x. Further exploration through culturing XH001 under various stress conditions showed that lipid droplets accumulation was a general response to stress. A survival assay demonstrated that the presence of lipid droplets plays a protective role in XH001, enhancing its survival under adverse conditions. In conclusion, our study sheds new light on the intricate interaction between Saccharibacteria and their host bacteria, highlighting the potential benefit conferred by TM7x to its host and further emphasizing the context-dependent nature of symbiotic relationships.
Saccharibacteria（原 TM7）是一组广泛存在且遗传多样性极高的超小型细菌，基因组高度简化，属于Candidate Phyla Radiation（未培养生物的门），这是一个具有未被充分理解的生物学特征的大型单系谱系。Nanosynbacter lyticus 模式菌株 TM7x 是第一个从人类口腔微生物组中分离出来的 Saccharibacteria 成员。由于代谢能力有限，TM7x 生活在其细菌宿主 Schaalia odontolytica 菌株 XH001 的表面，并与之形成专性外共生关系。这种共生关系使 TM7x 得以繁殖，但同时也通过诱导应激反应给宿主细菌带来负担。在这里，我们采用超分辨率荧光成像技术研究了 TM7x 和 XH001 之间的物理关联。我们发现，与 TM7x 结合导致 XH001 的膜流动性发生显著变化。我们还揭示了当 TM7x 与 XH001 形成外共生关系时，XH001 中会形成细胞内脂滴，这一特征在口腔细菌中尚未报道过。无标记拉曼光谱证实了 TM7x 诱导的 XH001 中脂滴的积累，当 XH001 细胞与 TM7x 物理结合时，该技术还揭示了其他表型特征。通过在各种应激条件下培养 XH001 进行进一步探索表明，脂滴的积累是对应激的普遍反应。生存试验表明，脂滴的存在对 XH001 具有保护作用，增强了其在不利条件下的生存能力。总之，我们的研究揭示了 Saccharibacteria 与其宿主细菌之间复杂的相互作用，强调了 TM7x 对其宿主的潜在益处，并进一步强调了共生关系的上下文依赖性。
