Lipid A is the innermost component of the lipopolysaccharide (LPS) molecules that occupy the outer leaflet of the outer membrane in Gram-negative bacteria. Lipid A is recognized by the host immune system and targeted by cationic antimicrobial compounds. In Salmonella enterica serovar Typhimurium, the phosphates of lipid A are chemically modified by enzymes encoded by targets of the transcriptional regulator PmrA. These modifications increase resistance to the cationic peptide antibiotic polymyxin B by reducing the negative charge of the LPS. We report the mechanism by which Salmonella produces different lipid A profiles when PmrA is activated by low Mg²⁺ versus a mildly acidic pH. Low Mg²⁺ favored modification of the lipid A phosphates with 4-amino-4-deoxy-l-aminoarabinose (l-Ara4N) by activating the regulatory protein PhoP, which initially increased the LPS negative charge by promoting transcription of lpxT, encoding an enzyme that adds an additional phosphate group to lipid A. Later, PhoP activated PmrA posttranslationally, resulting in expression of PmrA-activated genes, including those encoding the LpxT inhibitor PmrR and enzymes responsible for the incorporation of l-Ara4N. By contrast, a mildly acidic pH favored modification of the lipid A phosphates with a mixture of l-Ara4N and phosphoethanolamine (pEtN) by simultaneously inducing the PhoP-activated lpxT and PmrA-activated pmrR genes. Although l-Ara4N reduces the LPS negative charge more than does pEtN, modification of lipid A phosphates solely with l-Ara4N required a prior transient increase in lipid A negative charge. Our findings demonstrate how bacteria tailor their cell surface to different stresses, such as those faced inside phagocytes.

脂质A是脂多糖（LPS）分子的最内层成分，占据了革兰氏阴性细菌的外膜外叶。脂质A被宿主免疫系统识别，并被阳离子抗微生物化合物靶向。在肠炎沙门氏菌血清型鼠伤寒沙门氏菌中，脂质A的磷酸酯被转录调节因子PmrA靶标编码的酶化学修饰。这些修饰通过减少LPS的负电荷而增加了对阳离子肽抗生素多粘菌素B的抗性。我们报告了机制，当PmrA被低Mg ²⁺相对于弱酸性pH激活时，沙门氏菌产生不同的脂质A谱。低镁²⁺通过激活调节蛋白PhoP促进了4-氨基-4-脱氧-1-氨基阿拉伯糖（1- Ara4N）对脂质A磷酸酯的修饰，该蛋白最初通过促进lpxT的转录而增加了LPS负电荷，该酶编码一种添加后来，PhoP在翻译后激活了PmrA，导致PmrA激活的基因表达，包括编码LpxT抑制剂PmrR的基因和负责整合I- Ara4N的酶。相反，通过同时诱导PhoP激活的lpxT，弱酸性pH值有利于用1- Ara4N和磷酸乙醇胺（pEtN）的混合物对脂质A磷酸酯进行修饰。和PmrA激活的pmrR基因。虽然升-Ara4N降低了LPS负电荷超过确实PETN，脂质A磷酸盐单独与修饰升-Ara4N需要在脂质A负电荷的现有瞬时增加。我们的发现表明细菌如何使细胞表面适应不同的压力，例如吞噬细胞内部面临的压力。