Juttukonda Lillian J, Chazin Walter J, Skaar Eric P
Department of Pathology, Microbiology and Immunology, Vanderbilt University School of Medicine, Nashville, Tennessee, USA.
Departments of Biochemistry and Chemistry and Center for Structural Biology, Vanderbilt University, Nashville, Tennessee, USA.
mBio. 2016 Sep 27;7(5):e01475-16. doi: 10.1128/mBio.01475-16.
During infection, bacterial pathogens must adapt to a nutrient metal-limited environment that is imposed by the host. The innate immune protein calprotectin inhibits bacterial growth in vitro by chelating the divalent metal ions zinc (Zn, Zn) and manganese (Mn, Mn), but pathogenic bacteria are able to cause disease in the presence of this antimicrobial protein in vivo. One such pathogen is Acinetobacter baumannii, a Gram-negative bacterium that causes pneumonia and bloodstream infections that can be complicated by resistance to multiple antibiotics. A. baumannii inhibition by calprotectin is dependent on calprotectin Mn binding, but the mechanisms employed by A. baumannii to overcome Mn limitation have not been identified. This work demonstrates that A. baumannii coordinates transcription of an NRAMP family Mn transporter and a urea carboxylase to resist the antimicrobial activities of calprotectin. This NRAMP family transporter facilitates Mn accumulation and growth of A. baumannii in the presence of calprotectin. A. baumannii is found to utilize urea as a sole nitrogen source, and urea utilization requires the urea carboxylase encoded in an operon with the NRAMP family transporter. Moreover, urea carboxylase activity is essential for calprotectin resistance in A. baumannii Finally, evidence is provided that this system combats calprotectin in vivo, as deletion of the transporter impairs A. baumannii fitness in a mouse model of pneumonia, and this fitness defect is modulated by the presence of calprotectin. These findings reveal that A. baumannii has evolved mechanisms to subvert host-mediated metal sequestration and they uncover a connection between metal starvation and metabolic stress.
Acinetobacter baumannii is a bacterium that causes bloodstream, wound, urinary tract, and pneumonia infections, with a high disease burden in intensive care units. Treatment of A. baumannii infection is complicated by resistance to most antibiotics in use today, and resistance to last-resort therapies has become commonplace. New treatments for A. baumannii infection are desperately needed, but our current understanding of the bacterial factors required to cause infection is limited. We previously found that the abundant innate immune protein calprotectin inhibits the growth of A. baumannii by withholding essential metals. Despite this, A. baumannii is still able to infect wild-type mice, which produce calprotectin during infection. Here, we identify factors employed by A. baumannii during infection to overcome calprotectin-mediated metal sequestration. Moreover, we expose a connection between metal starvation and metabolism that may be a "chink in the armor" of A. baumannii and lead to new treatment options.
在感染过程中，细菌病原体必须适应宿主施加的营养金属限制环境。先天性免疫蛋白钙卫蛋白通过螯合二价金属离子锌（Zn）和锰（Mn）在体外抑制细菌生长，但病原菌在体内存在这种抗菌蛋白的情况下仍能致病。鲍曼不动杆菌就是这样一种病原体，它是一种革兰氏阴性菌，可引起肺炎和血流感染，且可能因对多种抗生素耐药而使病情复杂化。钙卫蛋白对鲍曼不动杆菌的抑制作用取决于钙卫蛋白与锰的结合，但鲍曼不动杆菌克服锰限制所采用的机制尚未明确。这项研究表明，鲍曼不动杆菌通过协调一个天然抗性相关巨噬蛋白（NRAMP）家族的锰转运蛋白和一个尿素羧化酶的转录来抵抗钙卫蛋白的抗菌活性。在有钙卫蛋白存在的情况下，这个NRAMP家族转运蛋白促进了鲍曼不动杆菌的锰积累和生长。研究发现鲍曼不动杆菌利用尿素作为唯一氮源，且尿素利用需要与NRAMP家族转运蛋白在一个操纵子中编码的尿素羧化酶。此外，尿素羧化酶活性对于鲍曼不动杆菌抵抗钙卫蛋白至关重要。最后，有证据表明该系统在体内对抗钙卫蛋白，因为缺失该转运蛋白会损害鲍曼不动杆菌在肺炎小鼠模型中的适应性，而这种适应性缺陷会因钙卫蛋白的存在而受到调节。这些发现揭示了鲍曼不动杆菌已经进化出了颠覆宿主介导的金属螯合的机制，并且发现了金属饥饿与代谢应激之间的联系。
鲍曼不动杆菌是一种可引起血流、伤口、泌尿道和肺部感染的细菌，在重症监护病房中疾病负担很高。由于对目前使用的大多数抗生素耐药，鲍曼不动杆菌感染的治疗变得复杂，对最后手段疗法的耐药也已变得很常见。迫切需要针对鲍曼不动杆菌感染的新治疗方法，但我们目前对导致感染所需的细菌因素的了解有限。我们之前发现，丰富的先天性免疫蛋白钙卫蛋白通过扣留必需金属来抑制鲍曼不动杆菌的生长。尽管如此，鲍曼不动杆菌仍然能够感染野生型小鼠，野生型小鼠在感染期间会产生钙卫蛋白。在这里，我们确定了鲍曼不动杆菌在感染过程中用于克服钙卫蛋白介导的金属螯合的因素。此外，我们揭示了金属饥饿与代谢之间的联系，这可能是鲍曼不动杆菌的“弱点”，并可能带来新的治疗选择。
