Numerous bacteria have been isolated from within plants, and many reported from xylem, but only three species of xylem-limited bacteria (XLB) that are fastidious in cultural requirements, are plant pathogens, and exclusively occupy xylem, have been well characterized. Two XLB, Xylella fastidiosa and Pseudomonas syzygii, are transmitted by sucking insects that feed on xylem sap but are not transmitted mechanically from plant to plant. In contrast, Clavibacter xyli is mechanically transmitted to plants by cutting tools. All of these XLB occupy a highly specialized yet diverse ecological niche: the water-conducting systems of an extremely wide range of plant hosts. A variety of detection methods are available as diagnostic aids; each method has advantages and disadvantages; no single method is best for all uses. Molecular and genetic comparisons of strains of XLB lag behind progress being made for many other plant-pathogenic bacteria, but such studies are needed to answer important questions: (a) How do XLB move from cell to cell within plants? (b) What are the physiological and genetic bases of plant host specificity for XLB? (c) Why are only xylem-feeding specialists vectors of X. fastidiosa (and probably P. syzygii), when many leafhoppers feed regularly (but not continuously) on xylem?

中文翻译：

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严格的木质部限制细菌植物病原体。

已经从植物内部分离出许多细菌，并且从木质部中报道了许多细菌，但是已经很好地表征了在培养要求上苛刻的仅有三种木质部限制细菌（XLB），它们是植物病原体并且仅占据木质部。两种XLB，即Xylella fastidiosa和Pseudomonas syzygii，是通过吸食以木质部汁液为食的昆虫传播的，而不是在植物之间机械传播的。相比之下，木糖杆菌通过切割工具机械地传递到植物。所有这些XLB都占据着高度专业化但又多样化的生态位：众多植物宿主的导水系统。可以使用多种检测方法作为诊断辅助工具。每种方法都有优点和缺点；没有一种方法能最好地适用于所有用途。XLB菌株的分子和遗传比较落后于许多其他植物致病细菌的研究进展，但需要进行此类研究来回答重要问题：（a）XLB如何在植物内的细胞间移动？（b）XLB植物宿主特异性的生理和遗传基础是什么？（c）当许多叶蝉定期（但不连续地）以木质部为食时，为什么只用木质部为食的专长的X. fastidiosa（可能是P.syzygii）媒介？