Electron Microscopic Observation and Ultrastructural Analysis of Outer Membrane Vesicles Released by the Endophyte within Azolla microphylla
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摘要: 前人研究表明，自然界中独立生活或纯培养的细菌等原核微生物会分泌外膜泡囊，这些泡囊在多个生物学过程中有着重要的作用。本研究用扫描和透射电镜对水生植物满江红体内的共生蓝细菌和内生细菌（合称内生菌）产生的泡囊进行了观察和超微结构特征分析。结果显示，这些内生菌不仅在活跃的营养生长期，而且在有性繁殖阶段均会释放泡囊，甚至在濒临死亡时也会排出泡囊。根据这些外膜泡囊的超微结构特征，将之分成5种类型，即中透型、内膜型、单核型、多核型和多泡型。通过对泡囊产生过程中超微结构变化的分析，认为本研究材料的内生菌周质层中的肽聚糖参与外膜泡囊的生成与释放。这一结果对阐明原核生物泡囊的发生机制具有普遍意义。本研究还对电镜图像的判读进行了讨论，以避免由于样品前处理等人为干预产生的假象而导致误判。Abstract: Previous studies have revealed that the free-living or purely cultured prokaryotes produce outer membrane vesicles (OMVs) which play a critical role in biological processes. In this study, the OMVs secreted by the symbiotic cyanobacterium and other endophytic bacteria (together referred to as endophyte) within a small water fern, Azolla-cyanobacterium association, were investigated by using scanning and transmission electron microscopies (EM). The results demonstrated that the vesiculation of both the cyanobacterium and the bacteria occurs not only at the stage of active vegetative growth, but their reproductive stages and even in dying process. Based on the ultrastructural characteristics, the OMVs discovered in the materials we used could be divided to five types:bilayer-couple with a central transparent area, outer-inner membranes, bilayer-couple with a core, bilayer-couple with multi-cores and bilayer-couple with multi-bulbs. Analyzing the changes of the cellular ultrastructure has led to a suggestion that peptidoglycan in periplasmic space is involved in formation and discharge of OMVs, which seems to have a universal significance to illuminate the mechanism for biogenesis of OMVs. In this paper, the interpretation of the EM images is also discussed, so as to avoid the misinterpretation caused by the artifacts due to the sample pretreatment.
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Key words:
- outer membrane vesicle /
- cyanobacterium /
- bacterium /
- electron microscopy /
- ultrastructure
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图 1 共生蓝细菌细胞表面的膜外泡囊
注：图中长白箭头指早期泡囊；粗白箭头指中期泡囊；黑箭头指成熟泡囊。
Figure 1. OMVs on surface of cyanobacterial vegetative cells
图 2 蓝细菌细胞表面的类似细菌的圆球状结构
Figure 2. Spherical bacterium-like structure on surface of a cyanobacterial cell
图 4 共生蓝细菌营养细胞(局部)释放的膜外泡囊
Figure 4. OMVs released by a portion of cyanobacterial vegetative cell
图 5 蓝细菌厚垣孢子和内生细菌释放的泡囊
Figure 5. OMVs discharged by cyanobacterial akinets and endophytic bacteria
图 6 正在分泌泡囊的细菌的超薄切片观察
Figure 6. TEM photography of ultrathin slice of vesicle-secreting bacterium
图 7 正在分泌泡囊的细菌的负染观察
Figure 7. TEM photography of negative-stained vesicle-secreting bacterium
图 9 一濒临类似细胞程序性死亡的蓝细菌细胞正向胞外释放大量泡囊
Figure 9. A programmed near-normal-death cyanobacterial cell releasing numerous vesicles
图 10 蓝细菌细胞的肽聚糖层出现的裂口
Figure 10. Ruptured holes appeared on peptidoglycan layer of dying cyanobacterial cell
图 11 被厚层夹膜包裹仍向外分泌泡囊的细菌
Figure 11. Vesicle-secreting bacterium covered by thick capsule
图 12 叶腔内濒死细菌释放的泡囊
Figure 12. OMVs produced by dead and dying bacterial cells inside leaf cavity
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