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革蓝氏阴性细菌为一层通透性屏障即外膜所包围。亲水性化合物能通过膜孔蛋白的充水性通道跨越外膜。例如,对大肠杆菌的研究表明,所有β-内酰胺抗生素均能充分利用膜孔蛋白通道。膜孔蛋白通道属非特异性的,故可以推测低亲水性化合物如四环素类、氨基糖苷类及氯霉素也能利用这类通道。例外的是某些抗生素能作为特异性自然通道的拟配体(见后)。疏水性的喹诺酮类先与二价阳离子络合,再扩散通过脂双层结构。氨基糖苷类则通过挠乱某些细菌带高阴电荷的脂双层结构进行扩散。膜孔蛋白通道非常狭窄,能对大分子及疏水性化合物的穿透形成有效屏障。外膜屏
Gram-negative bacteria as a layer of permeability barrier that is surrounded by the outer membrane. Hydrophilic compounds can cross the outer membrane through the water-filled channels of membrane porins. For example, studies on E. coli show that all β-lactam antibiotics take full advantage of the membrane porin pathway. Membrane pore channels are nonspecific, so it can be speculated that low hydrophilic compounds such as tetracyclines, aminoglycosides and chloramphenicol can also use such channels. The exception is some antibiotics that act as ligands for specific natural channels (see below). Hydrophobic quinolones complex with divalent cations and then diffuse through the lipid bilayer structure. Aminoglycosides diffuse through lipid-bilayer structures that defy certain bacteria with high negative charge. Membrane pore channels are very narrow and provide an effective barrier to the penetration of macromolecules and hydrophobic compounds. Outer membrane screen