The outer membrane of Gram-negative bacteria is essential for their survival in harsh environments and provides intrinsic resistance to many antibiotics. This membrane is remarkable; it is a highly asymmetric lipid bilayer. The inner leaflet of the outer membrane contains phospholipids, whereas the fatty acyl chains attached to lipopolysaccharide (LPS) comprise the hydrophobic portion of the outer leaflet. This lipid asymmetry, and in particular the exclusion of phospholipids from the outer leaflet, is key to creating an almost impenetrable barrier to hydrophobic molecules that can otherwise pass through phospholipid bilayers. It has long been known that these lipids are not made in the outer membrane. It is now believed that conserved multisubunit protein machines extract these lipids after their synthesis is completed at the inner membrane and transport them to the outer membrane. A longstanding question is how the cell builds and maintains this asymmetric lipid bilayer in coordination with the assembly of the other components of the cell envelope. This Review describes the trans-envelope lipid transport systems that have been identified to participate in outer-membrane biogenesis: LPS transport via the Lpt machine, and phospholipid transport via the Mla pathway and several recently proposed transporters.

中文翻译：

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细菌外膜脂质的组装和维护

革兰氏阴性细菌的外膜对于它们在恶劣环境中的生存至关重要，并提供对许多抗生素的内在抗性。这种膜是非凡的；它是一种高度不对称的脂质双层。外膜的内叶含有磷脂，而与脂多糖 (LPS) 相连的脂肪酰基链构成外叶的疏水部分。这种脂质不对称，特别是从外叶中排除磷脂，是对疏水分子形成几乎不可穿透的屏障的关键，否则这些疏水分子可以通过磷脂双层。人们早就知道这些脂质不是在外膜中产生的。现在认为，保守的多亚基蛋白质机器在内膜完成合成后提取这些脂质并将它们运输到外膜。一个长期存在的问题是细胞如何构建和维持这种不对称的脂质双层，与细胞包膜其他成分的组装相协调。本综述描述了已被确定参与外膜生物发生的跨包膜脂质转运系统：通过 Lpt 机器进行的 LPS 转运，以及通过 Mla 途径和几种最近提出的转运蛋白转运的磷脂。