Modern Acinetobacter baumannii clinical isolates replicate inside spacious vacuoles and egress from macrophages.

Multidrug-resistant Acinetobacter baumannii infections are increasing at alarming rates. Therefore, novel antibiotic-sparing treatments to combat these A. baumannii infections are urgently needed. The development of these interventions would benefit from a better understanding of this bacterium’s pathobiology, which remains poorly understood. A. baumannii is regarded as an extracellular opportunistic pathogen. However, research on Acinetobacter has largely focused on common lab strains, such as ATCC 19606, that have been isolated several decades ago. These strains exhibit reduced virulence when compared to recently isolated clinical strains. In this work, we demonstrate that, unlike ATCC 19606, several modern A. baumannii clinical isolates, including the recent clinical urinary isolate UPAB1, persist and replicate inside macrophages within spacious vacuoles. We show that intracellular replication of UPAB1 is dependent on a functional type I secretion system (T1SS) and pAB5, a large conjugative plasmid that controls the expression of several chromosomally-encoded genes. Finally, we show that UPAB1 escapes from the infected macrophages by a lytic process. To our knowledge, this is the first report of intracellular growth and replication of A. baumannii. We suggest that intracellular replication within macrophages may contribute to evasion of the immune response, dissemination, and antibiotic tolerance of A. baumannii. Acinetobacter baumannii is a nosocomial pathogen that causes multiple types of infection. This bacterium has an alarming predisposition to acquire multi-drug resistance, and infections associated with these strains are linked to greater morbidity and mortality. Therefore, novel antibiotic-sparing treatments to combat these A. baumannii infections are urgently needed. The development of these interventions would benefit from a better understanding of the mechanisms employed by A. baumannii to cause infection. A. baumannii is regarded as an extracellular opportunistic pathogen. However, research on Acinetobacter has largely focused on common lab strains that have been isolated several decades ago. These strains exhibit reduced virulence when compared to recently isolated clinical strains. In this work, we demonstrate that a subset of modern A. baumannii clinical isolates persist and replicate inside macrophages within spacious vacuoles, and identify bacterial factors involved in the process. We propose that replication inside macrophages may contribute to the evasion of the immune response and bacterial dissemination. Antibiotics cannot penetrate the macrophages, and therefore our findings contribute to explain the extremely high tolerance of A. baumannii to antibiotics. Our work opens new avenues for the development of novel therapeutic interventions against this worrisome human pathogen.

多重耐药鲍曼不动杆菌感染正以惊人的速度增长。因此，迫切需要新的节省抗生素的治疗方法来对抗这些鲍曼不动杆菌感染。这些干预措施的研发将受益于对这种细菌病理生物学的更好理解，而目前对其了解还很不足。鲍曼不动杆菌被视为一种胞外机会致病菌。然而，对不动杆菌的研究主要集中在几十年前分离的常见实验室菌株，如ATCC 19606。与近期分离的临床菌株相比，这些菌株的毒力有所降低。在这项工作中，我们证明，与ATCC 19606不同，几种现代鲍曼不动杆菌临床分离株，包括近期从尿液中分离的临床菌株UPAB1，在巨噬细胞内的宽敞液泡中持续存在并复制。我们表明UPAB1在细胞内的复制依赖于功能性I型分泌系统（T1SS）和pAB5，这是一种大型接合质粒，它控制着几种染色体编码基因的表达。最后，我们表明UPAB1通过裂解过程从受感染的巨噬细胞中逃逸。据我们所知，这是关于鲍曼不动杆菌在细胞内生长和复制的首次报道。我们认为在巨噬细胞内的复制可能有助于鲍曼不动杆菌逃避免疫反应、传播以及对抗生素的耐受性。 鲍曼不动杆菌是一种医院内病原体，可导致多种类型的感染。这种细菌极易获得多重耐药性，与这些菌株相关的感染与更高的发病率和死亡率有关。因此，迫切需要新的节省抗生素的治疗方法来对抗这些鲍曼不动杆菌感染。这些干预措施的研发将受益于更好地理解鲍曼不动杆菌引起感染所采用的机制。鲍曼不动杆菌被视为一种胞外机会致病菌。然而，对不动杆菌的研究主要集中在几十年前分离的常见实验室菌株。与近期分离的临床菌株相比，这些菌株的毒力有所降低。在这项工作中，我们证明一部分现代鲍曼不动杆菌临床分离株在巨噬细胞内的宽敞液泡中持续存在并复制，并确定了该过程中涉及的细菌因素。我们提出在巨噬细胞内的复制可能有助于逃避免疫反应和细菌传播。抗生素无法穿透巨噬细胞，因此我们的发现有助于解释鲍曼不动杆菌对抗生素的极高耐受性。我们的工作为开发针对这种令人担忧的人类病原体的新型治疗干预措施开辟了新途径。