Delineating genetic determinants of polymyxin resistance in Serratia marcescens

本研究旨在阐明粘质沙雷氏菌对多黏菌素耐药的分子机制。 这种医院内病原体中碳青霉烯类耐药性的出现使治疗陷入困境， 存在治疗选择。S.长期以来，粘质虫被认为是对多粘菌素具有内在抗性， 主要用于治疗碳青霉烯类耐药肠杆菌。然而，我们意外地注意到， 相当大比例的碳青霉烯类耐药S. marcescens（CR-SM）对多粘菌素完全敏感。在 此外，抗性菌株表现出两种不同的表型（低水平（R1）和高水平抗性（R2））。 多粘菌素是靶向细菌外膜的带负电荷组分的阳离子肽， 特别是脂质A。通过酶促L-Ara 4 N转移到脂质A的脂质A的化学修饰是主要的 它是多粘菌素抗性的贡献者，并由氨基阿拉伯糖转移酶ArnT催化。脂质A分析 的代表3个表型组的分离物证明在L-Ara 4 N修饰中完全缺乏， 敏感分离株和R1和R2表型之间不同的脂质A谱。通过全基因组 测序我们已经开始建立多粘菌素易感性的推定遗传决定因素。这包括 一个独特的arnC样基因只存在于敏感菌株和一个翻转酶有关的阿恩只编码在 高抗性R2菌株。此外，R1和R2分离株具有独特的arnD和eptA等位基因， 敏感的分离物。在这里，我们的目标是详细定义多粘菌素易感性的机制， 提供关于多粘菌素作为CR-SM感染治疗选择的潜在适用性的关键信息。 我们已经组建了一支高技能的团队，在微生物学，细菌基因组学， 和膜蛋白生物化学。在目标1中，我们将确定多粘菌素敏感性的患病率， 全面收集临床多药耐药（MDR）和非MDR S.粘质虫分离物。通过 全基因组测序，我们将完善PR表型的推定遗传标记，并通过系统发育 重建可以确定丢失或PR是否重复发生或代表稳定的亚系。然后我们将 通过互补、基因编辑和 删除实验在目标2中，我们将评估同基因突变体的LPS结构和脂质A修饰 使用放射性标记技术和质谱法测定目标1中生成的化合物。我们将评估基因表达 脂质A修饰酶和跨表型转录谱的总体差异。最后我们 将研究PR突变体对细菌适应性和毒力的影响。我们的研究结果将填补 在内在PR的基因组和分子决定因素的知识方面存在重大差距， 对脂质A修饰的破坏机制，提供分子标记物，用于快速筛选 CR-SM中的多粘菌素易感性表型;并提高我们对进化权衡的理解 潜在的内在PR表型的损失。

This proposal aims to define the molecular mechanisms of resistance to polymyxins in Serratia marcescens. The emergence of carbapenem resistance in this nosocomial pathogen poses a treatment dilemma as few treatment options exist. S. marcescens has long been considered intrinsically resistant to polymxyins, a mainstay for treatment of carbapenem-resistant Enterobacteriales. However, we unexpectedly noted that a substantial proportion of carbapenem-resistant S. marcescens (CR-SM) are fully susceptible to polymyxin. In addition, resistant isolates displayed two distinct phenotypes (low-level (R1) and high-level resistance (R2). Polymyxins are cationic peptides targeting negatively charged components of the bacterial outer membrane, notably Lipid A. Chemical modification of Lipid A through enzymatic transfer of L-Ara4N to Lipid A is the major contributor to polymyxin resistance and is catalyzed by the aminoarabinose transferase ArnT. Lipid A analysis of isolates representing the 3 phenotypic groups demonstrated a complete lack of L-Ara4N modification in susceptible isolates and distinct lipid A profiles between R1 and R2 phenotypes. Through whole genome sequencing we have begun to establish putative genetic determinants of polymyxin susceptibility. This includes a unique arnC-like gene only present in susceptible isolates and a flippase related to arnE only encoded in highly-resistant R2 isolates. In addition, R1 and R2 isolates harbored unique arnD and eptA alleles compared to susceptible isolates. Here, we aim to define the mechanisms underlying polymyxin susceptibility in detail to provide critical information on the potential suitability of polymyxins as a treatment option for CR-SM infections. We have assembled a highly skilled team with complementary expertise in microbiology, bacterial genomics and membrane protein biochemistry. In Aim 1, we will determine the prevalence of polymyxin susceptibility in a comprehensive collection of clinical multi-drug resistant (MDR) and non-MDR S. marcescens isolates. Through whole genome sequencing we will refine putative genetic markers of PR phenotypes and through phylogenetic reconstruction determine if the loss or PR occurred repeatedly or represents a stable sublineage. We will then validate candidate genetic markers on PR through a combination of complementation, gene editing and deletion experiments. In Aim 2, we will assess LPS structure and lipid A modifications of isogenic mutants generated in Aim 1 using radiolabeling techniques and mass spectrometry. We will evaluate gene expression of lipid A modifying enzymes and overall differences in transcriptional profiles across phenotypes. Lastly, we will examine the impact of PR mutants on bacterial fitness and virulence. Findings from our study will fill significant gaps in knowledge of the genomic and molecular determinants of intrinsic PR, provide information on the mechanisms of disruption of lipid A modifications, deliver molecular markers for rapid screening of polymyxin susceptibility phenotypes in CR-SM; and improve our understanding of the evolutionary trade-offs underlying the loss of the intrinsic PR phenotype.