Colistin-resistant, lipopolysaccharide-deficient Acinetobacter baumannii responds to lipopolysaccharide loss through increased expression of genes involved in the synthesis and transport of lipoproteins, phospholipids and poly-beta-1,6-N-acetylglucosamine. Acinetobacter baumannii

We recently demonstrated that colistin resistance in Acinetobacter baumannii can result from mutational inactivation of genes essential for lipid A biosynthesis. Consequently, strains harboring these mutations are unable to produce the major Gram negative bacterial surface component, lipopolysaccharide (LPS). To understand how A. baumannii compensates for the lack of LPS, we compared the transcriptional profile of the A. baumannii type strain ATCC19606, to that of an isogenic, LPS-deficient, lpxA mutant strain. Analysis of the expression profiles indicated that the LPS-deficient strain showed increased expression of many genes involved in cell envelope and membrane biogenesis. In particular, up-regulated genes included those involved in the Lol lipoprotein transport system and the Mla-retrograde phospholipid transport system. In addition, genes involved in the synthesis and transport of poly-beta-1,6-N-acetylglucosamine (PNAG) were also up-regulated and a corresponding increase in PNAG production was observed. The LPS-deficient strain also exhibited reduced expression of genes predicted to encode the fimbrial subunit FimA and a type VI secretion system (T6SS). The reduced expression of genes involved in T6SS correlated with the detection of the T6SS-effector protein, AssC, in culture supernatants of the A. baumannii wild-type strain, but not in the LPS-deficient strain. Taken together, these data show that, in response to total LPS loss, A. baumannii alters the expression of critical transport and biosynthesis systems associated with modulating the composition and structure of the bacterial surface. Overall design: Comparison of a gene expression in biological duplicate samples derived from parent bacterial strain to an isogenic mutant strain.

我们近期的研究证实，鲍曼不动杆菌（Acinetobacter baumannii）的多粘菌素抗性可由脂质A生物合成必需基因的突变失活所导致。携带此类突变的菌株无法合成革兰氏阴性菌的主要表面组分——脂多糖（lipopolysaccharide, LPS）。为探究鲍曼不动杆菌如何代偿脂多糖缺失带来的影响，我们比较了鲍曼不动杆菌模式菌株ATCC19606与同基因脂多糖缺陷型lpxA突变株的转录谱。表达谱分析结果显示，脂多糖缺陷型菌株中大量参与细胞被膜与膜生物发生的基因表达上调。其中，上调基因涵盖Lol脂蛋白转运系统与Mla逆行磷脂转运系统相关基因。此外，参与聚β-1,6-N-乙酰葡糖胺（poly-beta-1,6-N-acetylglucosamine, PNAG）合成与转运的基因同样出现表达上调，且对应检测到PNAG产量的提升。脂多糖缺陷型菌株还表现出编码菌毛亚基FimA以及VI型分泌系统（type VI secretion system, T6SS）的基因表达下调。T6SS相关基因的表达下调与以下检测结果相符：在鲍曼不动杆菌野生型菌株的培养上清中可检测到T6SS效应蛋白AssC，但在脂多糖缺陷型菌株的培养上清中未检测到该蛋白。综上，上述数据表明，在完全缺失脂多糖的情况下，鲍曼不动杆菌会改变关键转运与生物合成系统的表达，以调控细菌表面的组成与结构。实验设计：对亲本菌株与同基因缺失突变株的生物学重复样本进行基因表达比较。